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4,000 years of environmental change in central Colorado

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Title:
4,000 years of environmental change in central Colorado a paleocological perspective
Alternate title:
Four thousand years of environmental change in Colorado
Creator:
Del Priore, Tera Marie ( author )
Place of Publication:
Denver, Colo.
Publisher:
University of Colorado Denver
Publication Date:
Language:
English
Physical Description:
1 electronic file (117 pages). : ;

Thesis/Dissertation Information

Degree:
Master of Science
Degree Grantor:
University of Colorado Denver
Degree Divisions:
Department of Geography and Environmental Sciences, CU Denver
Degree Disciplines:
Environmental Sciences
Committee Chair:
Briles, Christy E.
Committee Members:
Simon, Gregory
Fornwalt, Paula J.

Subjects

Subjects / Keywords:
Forest management -- Colorado ( lcsh )
Mountain ecology -- Colorado ( lcsh )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Review:
Colorado forests have experienced severe drought and fire in recent history. This paper examines how past climate change has affected Colorado subalpine forests and disturbance events. High-resolution pollen and charcoal preserved in lake sediments were analyzed to reconstruct a 4,000-yr-long environmental and disturbance (i.e. fire, avalanches, and insects) history from the Taylor Basin in central Colorado. A subalpine forest of Picea and Abies existed during the late Holocene, but the abundances of these taxa along with Pinus fluctuated in response to climate change. For example, Pinus was abundant in the forest after 2300 cal yr BP, suggesting a more open forest canopy than before, and the forest supported fewer fires. This corresponds with a shift initiated ca. 3000 cal yr BP from a summer-to winter-dominate precipitation regime (less effective summer moisture) as reflected in the Bison Lake oxygen-isotope record from Colorado and the Pink Panther speleothem record from New Mexico. Prior to ~2300, between ~850-1700, and ~450 cal yr BP to present, fires were more prevalent, corresponding with more-closed forests. The lowest fire frequency occurred ca 850 to 450 cal yr BP, corresponding with periods of drought and possibly less convective thunderstorms. Multiple avalanche events, evidenced by pulses of course-grain sediment and macrofossils, occurred between 1700 cal yr BP and present, suggesting avalanches increased during periods with more winter precipitation (i.e. snowpack). Therefore, changes in seasonal precipitation in Colorado drive forest structure and disturbance frequency, such as fires and avalanches. However, no evidence for insect outbreaks is recorded.
Thesis:
Thesis (M.S.)--University of Colorado Denver. Environmental sciences
Bibliography:
Includes bibliographic references.
System Details:
System requirements: Adobe Reader.
General Note:
Department of Geography and Environmental Sciences
Statement of Responsibility:
by Tera Marie Del Priore.

Record Information

Source Institution:
University of Colorado Denver
Holding Location:
Auraria Library
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
911184204 ( OCLC )
ocn911184204

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Full Text
4,000 YEARS OF ENVIRONMENTAL CHANGE IN CENTRAL COLORADO:
A PALEOECOLOGICAL PERSPECTIVE
by
TERA MARIE DEL PRIORE
B.A., University of Colorado Boulder, 2007
A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado Denver in partial fulfillment
of the requirements for the degree of
Master of Science
Environmental Sciences
2015


11
This thesis for the Master of Science degree by
Tera Marie Del Priore
has been approved for the
Department of Geography and Environmental Sciences
by
Christy E Briles, Chair
Gregory Simon
Paula J Fomwalt
April 19, 2015


Ill
Del Priore, Tera Marie (M.S., Environmental Sciences)
4,000 Years of Environmental Change in Central Colorado: A Paleoecological
Perspective
Thesis directed by Assistant Professor Christy E Briles
ABSTRACT
Colorado forests have experienced severe drought and fire in recent history. This
paper examines how past climate change has affected Colorado subalpine forests and
disturbance events. High-resolution pollen and charcoal preserved in lake sediments
were analyzed to reconstruct a 4,000-yr-long environmental and disturbance (i.e. fire,
avalanches, and insects) history from the Taylor Basin in central Colorado. A subalpine
forest of Picea and Abies existed during the late Holocene, but the abundances of these
taxa along with Pinus fluctuated in response to climate change. For example, Pinus was
abundant in the forest after 2300 cal yr BP, suggesting a more open forest canopy than
before, and the forest supported fewer fires. This corresponds with a shift initiated ca.
3000 cal yr BP from a summer-to winter-dominate precipitation regime (less effective
summer moisture) as reflected in the Bison Lake oxygen-isotope record from Colorado
and the Pink Panther speleothem record from New Mexico. Prior to -2300, between
-850-1700, and -450 cal yr BP to present, fires were more prevalent, corresponding with
more-closed forests. The lowest fire frequency occurred ca 850 to 450 cal yr BP,
corresponding with periods of drought and possibly less convective thunderstorms.
Multiple avalanche events, evidenced by pulses of course-grain sediment and


macrofossils, occurred between 1700 cal yrBP and present, suggesting avalanches
increased during periods with more winter precipitation (i.e. snowpack). Therefore,
changes in seasonal precipitation in Colorado drive forest structure and disturbance
frequency, such as fires and avalanches. However, no evidence for insect outbreaks is
recorded.
The form and content of this abstract are approved. I recommend its publication.
Approved: Christy E Briles


V
DEDICATION
I would like to dedicate this thesis to my loving husband. His support and
understanding was imperative in my success and I am forever grateful to him. I would
also like to thank my parents. They instilled my love of nature as a child and always
encouraged me to pursue my dreams.


VI
ACKNOWLEDGEMENTS
I would like to thank my advisor, Christy Briles for her guidance, time, insight,
and for introducing me to the world of paleoecology. Her enthusiasm to teach others has
resonated well with me.


vii
TABLE OF CONTENTS
CHAPTER
I. INTRODUCTION............................................................. 1
II. BACKGROUND INFORMATION .................................................. 5
Physical environment .............................................. 5
Geology ........................................................... 5
Climate .......................................................... 6
Vegetation ........................................................ 7
Disturbance ....................................................... 8
Proxy data and natural archives ................................. 11
Regional environmental history ............................................ 12
Ecosystem resilience and range of natural variability ............ 16
III. SITE DESCRIPTION ....................................................... 19
IV. METHODS AND DATA ANALYSIS .............................................. 24
Field methods .................................................... 24
Laboratory methods................................................ 24
Core preparation, lithology, and chronology ................. 24
Magnetic susceptibility & loss-on-ignition ................... 25
Pollen ..................................................................... 25
Charcoal...................................................... 27
Data analysis.................................................... 27
Pollen........................................................ 27
Charcoal ..................................................... 29
V. RESULTS ................................................................ 31
Chronol ogy...................................................... 31
Lithology........................................................ 33
Magnetic susceptibility & loss-on-ignition........................ 35
Pollen........................................................... 35
Charcoal ......................................................... 39
VI. DISCUSSION ............................................................. 43
The climate of the late Holocene (4000 yr BP to present)
43


Vlll
Ecosystem response to late Holocene climatic conditions 49
Summer-dominant precipitation regime................. 49
Winter-dominant precipitation regime................. 50
Insect outbreaks..................................... 56
Avalanches........................................... 57
Management implications ................................. 59
VII CONCLUSION ................................................... 64
REFERENCES .......................................................... 67
APPENDIX ............................................................ 75
A. KEY TO ABBREVIATIONS USED IN APPENDICES
B, C, D, AND E BY ORDER OF APPEARANCE .................. 75
B. MAGNECTIC SUSCEPTIBILITY FOR MIRROR LAKE ..................... 78
C. LOSS-ON-IGNITION FOR MIRROR LAKE ............................. 84
D POLLEN COUNTS AND SUMS FOR MIRROR LAKE ....................... 88
E. CHARCOAL SAMPLE VOLUME, CHARCOAL COUNT,
CHARCOAL CONCENTRATION, AND DEPOSITION
RATE FOR MIRROR LAKE.......................................... 97


IX
LIST OF TABLES
TABLE
1. Plant species list within Mirror Lake
watershed and Taylor Park .......................................... 23
2. Uncalibrated radiocarbon dates and calibrated
14C ages for Mirror Lake ........................................... 32
3. Regional site descriptions .......................................... 45


X
LIST OF FIGURES
FIGURE
1. Site map .............................................................. 20
2. Age-versus-depth curve based on 14C dates.............................. 32
3. Core lithology, loss-on-ignition, and magnetic susceptibility.......... 34
4. Pollen percentages, pollen accumulation rates (PAR), pollen zones,
(green line) and constrained cluster analysis output (CONISS)......... 36
5. Mean fire return interval, peak magnitude, charcoal accumulation
rates (CHAR) (gray line is background charcoal accumulation
rates BCHAR), and pollen zones (green line)........................... 40
6. Climate history for the southern Rocky Mountains and
Mirror Lake environmental history .................................... 44


CHAPTERI
INTRODUCTION
Colorado has experienced notable and widespread disturbance, primarily from fire
and beetle outbreaks, over the last two centuries. For example, wildfires have burned
over one million acres in Colorado in the last decade, with the 2002 Hayman fire
spreading over 138,000 acres in the largest wildfire in Colorados recorded history
(Berwyn, 2014). The fires have had substantial economic impacts, including damages
and restoration costs exceeding 40 million dollars for the Hayman fire alone, with an
additional 39 million dollars spent on suppression (Kent et al., 2003). Since 1996, over
4.5 million acres of land were subject to beetle outbreaks in Colorado, leading to severe
economic damages to tourism, property values, recreation, and increasing the amount of
dead fuel in the forests (USFS, 2013). Additionally, avalanches, which occur routinely in
Colorado, can alter local forest structure and result in death and significant damages to
property or infrastructure when occurring in a populated or well-traveled area (CO
Geological Survey, 2015).
Forest managers and the general public often question if such widespread
disturbance events are unprecedented or a common occurrence. However, limited
historical information exists to examine if these disturbances are within the historical
range for our current climate and ecosystem conditions. Some important historical data
come from tree-rings and proxy indicators found in lake sediments. Dendrochronological
(tree-ring) records provide seasonal data on tree response to climate variations and fire


2
events. However, tree-ring data have a limited temporal resolution (-300 years for
Colorado) and are most useful in low-severity fire regimes of montane forests (Sherriff et
al., 2001; Whitlock et al., 2003). Paleoecological studies on lake sediments use pollen,
macro botanicals, charcoal, insects, diatoms, isotopes, and a range of sediment properties
to provide detailed information on forest and ecosystem processes extending back
millennia, beyond the capability of tree-ring studies (Willis and Bhagwat, 2010). For
example, in coniferous forests of western North America, dendrochronological fire
histories have been coupled with longer sedimentary charcoal records to reconstruct
historic fire regimes (Froyd and Willis, 2008). These reconstructions have provided
practical recommendations for managers, including directing the reintroduction of fire
and manipulation of forest structure at a landscape scale in ponderosa pine ecosystems in
northern Arizona to mimic the beneficial impacts of the natural fire regime (Moore et al.,
1999).
In central Colorado, the region of study for this thesis, paleoecological studies
have been conducted in the montane, subalpine, and tundra plant communities. The
studies focus on the millennial-scale trends in vegetation and fire, and extend through the
Holocene, but lack high-resolution data for the late Holocene, a 4000-year period
typically examined to reconstruct the historical range of disturbance (Fall 1997a,b).
However, there are higher resolution records of both fire and vegetation outside the study
region (Jimenez-Moreno and Anderson, 2012; Jimenez-Moreno et al., 2011; Johnson et
al., 2013; Toney and Anderson, 2006; Anderson et al., 2008a; Higuera et al., 2014).
Therefore, the main objective of this research is to determine the late Holocene (4000
years to present) environmental conditions that have influenced forest development in


3
subalpine forests of central Colorado. Lake sediments were selected as the natural archive
as they provide millennia-long records of ecosystem dynamics, including vegetation
change and disturbance events including fire, beetle outbreaks, and avalanches. A high-
resolution study of Mirror Lake was conducted to expand on existing paleoenvironmental
research in central Colorado subalpine forests where dendrochronological records are
limited, and to address the following questions:
1. What was the late Holocene vegetation and disturbance history of subalpine
forests in central Colorado, and how does climate change influence this history?
2. How does the late Holocene disturbance and forest reconstruction inform
current and future management?
This thesis is developed around six additional chapters. Chapter two describes
Colorados present-day physical environment, geology, climate, vegetation, and
disturbance regimes. It further discusses the nature of proxy data and the regional
environmental history for Colorado based on paleoenvironmental records. Finally,
concepts of ecosystem resiliency and the natural range of variability are presented and
how paleoecological records are critical for informing management in regards to these
concepts. Chapter three describes the Mirror Lake study area. Chapter four presents the
field and laboratory methods and how the sediment, pollen and charcoal data were
analyzed. Chapter five presents the data, including: the chronology built using
radiocarbon dating, lithology of the core, magnetic susceptibility, loss-on-ignition, pollen,
and charcoal, and the appropriate interpretation. Chapter six discusses the climate history
of Colorado during the last 4000 years using proxy records from the region and globally,
and the ecosystem response (vegetation distributions and disturbance regimes) to


4
changing climate conditions at Mirror Lake and from other regional paleoenvironmental
records. Finally, forest management implications based on the research are discussed.
Chapter seven concludes the thesis.


5
CHAPTER II
BACKGROUND INFORMATION
Physical environment
The Sawatch Mountains of central Colorado encompass the southern portion of
the Rocky Mountain chain, which extends from northern New Mexico to northern
Alaska. In Colorado, the Rocky Mountains rise sharply from the plains of Denver and
extend several hundred miles to the western edge of the state where rugged plateaus, or
high mesas, with interspersed canyons, define the landscape. There are over fifty peaks
measuring at least 4,250 meters (m) in height and several large, open, and flat parks
interspersed within the multiple sub-ranges throughout the state (Fall, 92a). The
Continental Divide is a hydrological boundary that runs along the crest of the Rockies
and separates rivers draining to the Atlantic and Gulf of Mexico in the east and those
draining to the Pacific Ocean in the west.
Geology
The Rocky Mountains formed as a result of millions of years of intense geologic
activity in the western portion of the United States, culminating with the uplift of the
mountain chain during the Laramide orogeny (70 to 40 million years ago). The rocks that
make up the Southern Rockies are up to 1.7 billion years old and are remnants of an
ancestral Rocky Mountain chain formed 300 million years ago. It is thought that tectonic
plate activity at the plate boundary of the North American plate and the Pacific Ocean
crust are responsible for the mountain chain formation, but geologically they are very


6
complex. An interior mountain range that spans such enormous distances is unique
geologically. It is believed that oceanic crust subducting beneath the continental plate at
a shallow angle created interior buckling and mountain-building hundreds of kilometers
inland from the actual plate boundary (USGS, 2014).
A fault-bounded block consisting of intrusive rocks uplifted during the Late
Mesozoic-Early Tertiary Laramide Orogeny forming the Sawatch Mountain range within
Taylor Park in central Colorado. Evidence exists for two periods of extensive Pleistocene
glaciation in the Sawatch Range (the Wisconsin and the Pinedale), with the most recent
ending -18,000 years ago. These periods of glaciation and subsequent deglaciation
resulted in the sharp relief of the range (Brugger, 2006).
Climate
The climate of Colorado is strongly dictated by its interior geographic location,
high elevations statewide, and the presence of the Rocky Mountains. Colorados inter-
continental location, within the middle latitudes, with no close proximity to any major
oceans, results in low humidity and aridity, with an average precipitation of only 43 cm
per year. Prevailing winds are primarily westerly, but highly dependent on topography.
Due to the highly varied topography of the mountains, steep climatic gradients exist
(Doesken et al., 2003).
Moist air masses from the Pacific Ocean bring wintertime snow west of the
Continental Divide (western slope), however they rarely influence precipitation east of
the Continental Divide (eastern slope) (Doesken et al., 2003). Winter precipitation,
mostly as snow, is higher on the western slope (Fall, 1997a). Cold and dry arctic air
masses in the winter often result in dramatic temperature declines on the eastern slope,


and if they collide with warm moist air masses from the Gulf of Mexico, large snowfall
events can occur as blizzards. The foothills block these storms and, therefore, the
7
western slope rarely experiences similar systems (Doesken et al., 2003).
Summer precipitation (April through September) occurring on the eastern slope
results from southern warm moist air masses, and along with surface heating, the
development of afternoon convective thunderstorms. For southwestern Colorado,
frequent rain showers result from intrusions of moist air accompanied by winds, often
called the southern monsoon, and usually occurs from July through September
(Doesken et al., 2003). The amount of precipitation the Rockies receive is influenced by
variations in Pacific Ocean surface temperatures, such as the inter-annual El Nino
Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) (Kitzberger et
al., 2007). El Nino conditions (warmer tropical Pacific ocean temperatures) or a positive
PDO result in increased winter precipitation in the southern Rockies, while La Nina
(cooler tropical Pacific ocean temperatures) or a negative PDO results in precipitation
below normal (Anderson, 2012; NOAA, 2014).
Vegetation
Topography plays a major role in the distribution of plant species along elevation
gradients in the Rockies. On average, increasing elevation results in increased
precipitation and decreased temperatures. There are four major vegetation zones
represented in the central Rockies and from low to high elevation include: (1) shrub-
steppe and grassland, indicative of intermountain valleys (2,300 to 2,900 m elevation),
(2) the montane, with drought resistant trees (2,800 to 3,000 m elevation), (3) the
subalpine, primarily spruce and fir (2,900 to 3,400 m elevation), and (4) the alpine, above


8
timberline (limit of erect trees at least 4 meters high), with low lying shrubs and herbs
(>3,500 m elevation) (Fall, 1992b).
Disturbance
Disturbances, such as fire and beetle outbreaks alter subalpine forest structure and
composition on different temporal and spatial scales, and can influence future
disturbances for decades to centuries (Higuera et al., 2014; Whitlock et al., 2010). Fire is
considered the dominant and most significant of these, however beetle outbreaks, such as
spruce and mountain pine beetle, and avalanches can also be significant disturbances to a
forest ecosystem (Kulakowski and Veblen, 2007).
Under modern climate conditions, the subalpine forest in Colorado is subject to
high-severity, stand replacing fires every -100 to 300+ years (Fall, 1997a; Calder et al.,
2014). This data has been primarily obtained from proxies, such as lake sediments, since
fire histories based upon the tree-ring record, in particular fire-scars, is limited in
Colorado subalpine ecosystems due to the stand replacing fires (Fall, 1997a; Calder et al.,
2014; Sherriff et al., 2001; Veblen et al., 1994).
Climate variability is a significant driver of fire in the subalpine zone, and in
particular during a strong La Nina and a negative PDO, that results in low effective
moisture and droughts (Higuera et al., 2014; Schoennagel et al., 2005; Sherriff et al.,
2001). Abundance of fuels (sufficient biomass) through the forest and canopy also
influences the likelihood of fire ignition, severity, and spread. Typically, the catalyst for
fire ignition is lightning strikes; however, human caused fires have been prevalent in
recorded history as well. Due to the mesic conditions in the subalpine, fire is limited,


9
which allows fuels to build up and facilitates high-severity and intensity crown fires
during droughts (Whitlock et al., 2010; Fall, 1997a).
The stand-replacing high-severity fires of the subalpine zone are capable of
affecting forest structure and composition, dictating forest succession, cycling nutrients,
degrading water quality, promoting mass-wasting events, and altering wildlife habitat
(Sherriff et al., 2001). Anthropocentrically, the affects of wildfires are negative, such as
the loss of property or life, loss in tourism and recreation opportunities, the cost of
suppression and restoration/rebuilding, health affects from smoke inhalation, and the
negative aesthetic connotation (Kent et al., 2003).
Dendroctonus rufipennis (spruce beetle) outbreaks have been well documented
from historical records and tree-ring records in the 19th and 20th centuries in subalpine
forests of Colorado, particularly in northern Colorado (Morris, J. 2013). Since 1996, a
widespread outbreak throughout Colorado has affected 1,144,000 acres. Additionally,
Dendroctonusponderosae (mountain pine beetle) has affected over 3.4 million acres
throughout Colorado since 1996, primarily in lodgepole pine forests. This current
outbreak has primarily affected forests in northern Colorado. In southern Colorado, the
outbreak is concentrated to the east of the Continental Divide (USFS, 2013).
Beetle outbreaks are capable of altering forest structure and composition,
ecosystem nitrogen pools, water quality, duration of snow cover, and fuel structures
(Morris et al., 2014). For example, severe outbreaks are capable of killing more than
90% of canopy-size Engelmann spruce in a stand since beetles prefer mature, large trees
(Kulakowski et al., 2003; Veblen et al., 1994). With insect outbreaks expected to
increase in severity and frequency with a warming climate, it is anticipated that they will


10
have even more influence on ecosystem processes (Morris et al., 2014; Anderson et al.,
20120; Kulakowski et al., 2003). There are few paleoecological studies describing the
frequency and severity of beetle outbreaks through the Holocene; therefore, most
reconstructions are from tree-ring studies or observational records (Morris et al., 2014;
Veblen et al., 1991). The scarcity of paleoecological studies is due to preservation and
limited methodological studies. Beetle remains do not preserve well in sediments, as the
chitin is a food source for aquatic organisms. In addition, few methodological lake
studies have examined ways of reconstructing historical insect infestations. However, a
few studies have shown that the ratio of Picea (spruce) to Abies (fir) pollen is a useful
proxy of past spruce beetle outbreaks, and a major drop in Pinus (pine) pollen abundance
may be an indicator of mountain pine beetle outbreaks (Morris, 2013; Morris et al., 2014;
Whitlock etal., 2012).
The steepness of Colorados mountains results in avalanches. Avalanches are
only reported when there is loss of human life or property damage; therefore a remote
location will likely not have documentation (Vasskog et al., 2011). However, avalanches
can have large impacts on ecosystems within a localized area and can negatively affect
the ski/winter sports industry by closing mountain highways and creating potential
hazards for skiers (CO Geological Survey, 2015). Avalanches are most likely to occur on
steep terrain with a slope of 30 to 45 and after weather events where: (1) there was a
significant amount of precipitation the day before, (2) there were high wind speeds the
day before, and (3) there were days with above freezing temperatures (0C) leading up to
avalanche occurrence (Vasskog et al., 2011). However, it is not known how past climate


11
change has impacted the frequency of avalanche events and how it might change in the
future.
Proxy data from natural archives
Proxy data can come from many sources, and they allow reconstructions of
ecosystem change at multiple spatial and temporal scales. Lake and ocean sediments,
tree-rings, corals, ice cores, packrat middens, isotope ratios, and speleothems are a few
natural archives of paleoenvironmental data. Not all proxies provide the same level of
resolution. For example, pollen and charcoal in lake and ocean sediments provide a
snapshot of change from decades to centuries, unless they contain varved sediments that
record annual variations. Tree-rings, corals, and ice core data provide annually resolved
records of temperature and/or precipitation; however, their temporal extent is limited and
may only span a few hundred years, with the exception of corals and ice cores (Mann,
2002c; Smol and Cummings, 2000).
The use of multiple proxies provides a more coherent and robust analysis as
individual proxies only reconstruct one aspect of the environment (vegetation, fire,
precipitation, temperature, etc.) (Mann, 2002c; Shulmeister et al., 1996). For instance,
high-resolution lake sediment charcoal records can be combined with tree-ring records
when they exist, to extend the temporal resolution of historic fire regimes (Falk et al.,
2011).
In this thesis, the proxies that are utilized are pollen, charcoal, and the lithological
data preserved in lake sediments. Pollen comes primarily from wind-pollinated plants, is
abundant, preserves well, and provides information on local forest composition and
structure, as well as the occurrence and impacts of disturbance events within 0 to 6 km of


12
a site (Willis et al., 2010; Lynch, 1996). Charcoal preserved in lake sediments is from
partially combusted organic material and provides an indication of local fire occurrence
and severity within 0 to 6 km of a site (Whitlock and Larsen, 2001). Sediment
characteristics, such as poor sorting, large grain size (>lmm), and an abundance of large
macrofossils have been used to identify avalanches in the sediment record. However,
records of avalanches have been primarily restricted to Europe, and in particular, Norway
(Vasskog et al., 2011). The Mirror Lake sediment record provides us the opportunity to
explore these events in the US Rocky Mountains.
Regional environmental history
The long-term environmental history of Colorado and the surrounding region is
based on pollen, charcoal, and isotope records that allow for a regional
paleoenvironmental reconstruction and provide additional data to compare with the
Mirror Lake record. To put the Mirror Lake record into context, the regional records are
introduced here, and revisited in Chapter 6 to document and explain regional variations in
environmental conditions (See Figure 1 and Table 3).
In Colorado, the early-to-mid Holocene (11700 to -5500 yr BP (Before Present))
had a climate and forest communities that were different from today (late Holocene,
-4000 yr BP to present). The early Holocene (11700 to 9000 yr BP) was characterized
by -8% greater levels of summer insolation, while winter insolation was -9% lower than
present (Anderson, 2012). The increased summer insolation and temperatures resulted in
decreased effective moisture due to a strengthened Pacific subtropical high-pressure
system, a northward shift in the jet stream, and strengthened summer monsoons (Briles et
al., 2012; Jimenez-Moreno et al., 2011; Anderson, 2012). However, in the winter, lower


13
insolation and colder winters resulted in less snowpack, evidenced by lower lake levels in
northern Colorado (Shuman et al., 2009). Most of the precipitation fell in summer, but
high evaporation rates resulted in extensive droughts (Anderson, 2012; Shuman et al.,
2009).
Pollen studies conducted throughout Colorado during the early Holocene suggest
upper treeline was higher and trees expanded to lower elevations than present. The forest
of the subalpine zone (-3000 m) was co-dominated by Pinus (pine), Picea (spruce), and
Abies (fir) and resembled a woodland (open forest structure), with abundant Quercus
(oak) occurring below the montane zone (Briles et al., 2012; Fall, 1997a,b; Jimenez-
Moreno and Anderson, 2012; Jimenez-Moreno et al., 2011; Johnson et al., 2013).
Corresponding to the summer insolation maximum and warmer summer temperatures,
fire activity was highest in the early-to-mid Holocene -10000 to 8000 cal yr BP. The
expansion of fire adapted species such as Pinus and Quercus likely contributed to this
increase in fire activity (Briles et al., 2012).
Gradually declining summer and increasing winter insolation resulted in
significant climate changes in the mid-to-late Holocene (Anderson, 2012). Changes in
insolation resulted in enhanced ENSO variability, increased winter and decreased
summer precipitation, and cooler temperatures (Anderson, 2012; Anderson et al., 2008a;
Briles et al., 2012). Forest structure and composition of the subalpine zone during the
mid Holocene (-5500 yr BP) was more closed than before, with Picea and Abies
dominating, and fewer Pinus trees (Briles et al., 2012; Fall, 1997a,b; Jimenez-Moreno
and Anderson, 2012; Jimenez-Moreno et al., 2011). This forest structure remained for
several thousand years until the late Holocene, where there was a major transition in


14
climate from a summer-dominated to a winter-dominated precipitation regime, driven by
changing insolation, which resulted in forest structure shifting from a closed-canopy to an
open-canopy forest (Fall, 1997a,b; Toney and Anderson, 2006; Jimenez-Moreno et al.,
2008; Briles et al., 2012). At the Keystone Ironbog near Crested Butte, this abrupt
change in vegetation occurred at 2600 yr BP where a closed Picea and Abies dominated
forest with abundant shrubs and herbs was replaced by an open Pinus dominated forest
(Fall, 1997a). Similarly, high elevation records (-3300 m) from southern Colorado,
northern New Mexico, and Rocky Mountain National Park (RMNP) documented a
similar abrupt change between 2650 and 2440 cal yr BP (Toney and Anderson, 2006;
Jimenez-Moreno et al., 2008; Higuera et al., 2014). Additionally, lower elevation records
(-2800 m) from northern Colorado also indicate a similar transition from a closed
subalpine forest of Picea and Abies in the mid Holocene, to a more open forest in the late
Holocene; however, this transition was seen roughly 900 years prior, at 3500 cal yr BP
(Caffrey and Doerner, 2012; Jimenez-Moreno et al., 2011).
Another indicator of the climate transition during the mid-to-late Holocene is the
establishment of cooler temperatures, which are reflected in timberline fluctuations,
because they are temperature dependent. A reconstruction of timberline from multiple
basins in the Crested Butte and Taylor Park area experienced an upper timberline shift
that was as much as 300 meters higher than current timberline between 9000 and 4000 yr
BP, due primarily to warmer summertime temperatures. As conditions became cooler
associated with declining summer insolation, upper timberline retreated by as much as
200 meters between 4000 and 2000 yr BP ago (Fall, 1997b; Briles et al., 2012). A
treeline reconstruction near Fairplay, CO documented a 150 to 300 meter upslope


15
movement of subalpine tree species between 9500 and 3500 cal yr BP, after which
Artemisia (sagebrush) steppe expanded, coinciding with declining temperatures and
increased winter precipitation (Jimenez-Moreno and Anderson, 2012). Modern plant
communities, timberline, and climate were established approximately 2000 yr BP in
Colorado (Fall, 1997b; Vierling, 1998; Toney and Anderson, 2006; Shuman et al., 2009;
Emslie et al., 2005).
Charcoal records from Colorado and New Mexico indicate that fire activity was
highest at the onset of the Holocene followed by a gradual decline after -10000 cal yr
BP, with an increase again in the mid-Holocene (9000 to 7000 cal yr BP) (Briles et al.,
2012). The fire history from charcoal records from the Holocene suggests that fire
frequency and severity corresponded to climate and available fuels (Higuera et al., 2014;
Anderson et al., 2008a). For example, a composite record from northern Colorado
suggests there was a transition from high severity to lower severity crown fires in the
subalpine forests after 2400 cal yr BP, due to decreased forest density (Higuera et al.,
2014). Charcoal records from multiple lakes in both southern and northern Colorado
indicate that between -2000 and 1000 cal yr BP fire activity increased from before
(Toney and Anderson, 2006; Anderson et al., 2008a; Jimenez-Moreno et al., 2011).
Small-scale drivers of fire variability in the late Holocene stem from decreased
effective summer moisture, which subsequently affects the abundance and structure of
available fuels If extensive drought periods alternate with wetter periods, a buildup of
fuels result and can easily ignite and bum in subsequent drought periods (Anderson et al.,
2008a; Whitlock et al., 2010). For example, the last 1500 years were characterized by
increased fire severity in the subalpine of northern Colorado, correlating to a time period


16
with extensive decadal-scale droughts (Higuera et al., 2014; Cook et al., 2004).
However, fire frequency and severity is generally thought to be lower in the late
Holocene compared to the early-to-mid Holocene due to cooler summer temperatures,
increased snowpack, and more open forest structure (Higuera et al., 2014; Briles et al.,
2012). While it seems intuitive that wet conditions would support fewer, less severe
fires, it is likely the interplay between wet and dry periods determines the long-term fire
regime.
Ecosystem resilience and range of natural variability
The characterization of plant communities assumes a static environment, while, in
fact, they are constantly changing. Plants respond to changing environmental conditions
individualistically (Gleason, 1939; Jackson and Overpeck, 2000). Climate, geology, and
disturbance (a discrete event in time that disrupts ecosystem, community, or population
structure and changes resources, nutrient availability, and/or the physical environment)
create environmental conditions that allow plants to co-exist or not (White and Pickett,
1985). If one or more of these factors change either gradually or abruptly, there is a
reorganization of plants and new communities develop. Paleoecological studies allow us
to better understand the nature of plant dynamics in a changing environment, in particular
during abrupt events in the past (e.g., climate change).
Ecosystem resiliency refers to the level of disturbance a system can withstand
without changing its inherent processes and structures, or stable state. A forest system
can experience change or variations in its composition and structure; however, the system
often bounces back to its original state, or varies within an average state or baseline
condition (Froyd and Willis, 2008; Minckley et al., 2012). For example, in the subalpine


17
zone of the Rocky Mountains where disturbances, such as fire, are large and destructive,
forest composition has remained relatively consistent over millennia based on
paleoenvironmental studies. This suggests that subalpine ecosystems already prone to
disturbance are highly resilient to such events, and following fire, reestablishment of the
original plant species will follow successional species establishment (Minckley et al.,
2012). On the contrary, if baseline conditions are exceeded, a new stable state
establishes, without reestablishment of the former state (Morecroft et al., 2012).
Proxy data are useful for determining when, and what conditions lead to
ecosystem changes or regime shifts to new steady states (Willis and Bhagwat, 2010).
Specifically, proxy data demonstrate variability in climate and ecological ecosystems in
the past (i.e. the historical range of natural variability) that provides context for
interpreting current variability (Mann, 2002c; Shulmeister et al., 1996; Smol and
Cummings, 2000; Swetnam et al., 1999). The historical range of natural variability
describes the amount of ecological and climatological change a system can tolerate
without entering a new stable state. The historical range of natural variability often forms
the basis for ecosystem management by identifying and quantifying when an external
driver pushes an ecosystem outside what has, and what can, occur within a system (Froyd
and Willis, 2008; Keane et al., 2009). For example, large-scale climate changes impact
the frequency and severity of disturbances. If conifer species in the Rocky Mountains
have been subjected to repeated disturbance and climate changes historically without an
ecosystem shift to a new state, then they are recognized to have built up resilience to the
events and have broad climatic niches and adaptability to disturbance (e.g., Pinus trees)
(Minckley et al., 2012).


18
Historically, subalpine forests of Colorado have experienced a range of conditions
and the legacy of past climate and disturbance regimes ultimately define the present day
forest. The modem climate has only been around for the past 2000 years, but within that
time frame there has been significant change, and understanding how these forests
operate under such variability is a major goal of this thesis.


19
CHAPTER III
SITE DESCRIPTION
Mirror Lake (Lat., 3844'37.53"N, Lon., 10625'55.51"W, 3347 m elevation) is
located in the subalpine zone of central Colorado approximately 29 km west/southwest of
Buena Vista, within the Taylor Park watershed of the Sawatch Mountain Range (See
Figure 1). The lake encompasses an area ca 1 hectare and has a maximum depth of 22 m.
East Willow Creek flows into Mirror Lake from the south.
Mirror Lake is a kettle lake formed by Pleistocene ice-age glaciers that carved the
landscape. During deglaciation, glacial ice and till was left behind where Mirror Lake
presently is located and as the ice melted it left a large, deep depression filled with water.
The surrounding peaks are composed of metamorphic rock of primarily mica schist with
some garnet porphyroblasts (personal communication Richard Ashmore). A steep
mountainside near the east side of the lake has several old avalanche paths, that have
slope values between 32 and 47, and terminate at the lakes edge. The Mirror Lake
topography (steep eastern slope) and plausible weather conditions can give rise to
avalanches (See Figure 1 :D). However, due to the remote location of Mirror Lake, there
are no historical records of avalanche events, fires, or beetle infestations.


20
1 WY -105W NE
18i5 Colorado 16
40N 19 DENVER 14
UT 13 12
, l Mirror Lake 3 E]2 KS
38N 5
6 11 0 50 100 200
7nm OK
AZ 9 8 10 * TX

Subalpine spruce/fir
Mixed conifer -
lodgepole/douglas fir
Woodland pinyon/
pondero sa/j uniper
Steppe shrub/grassland
Tundra, rock, ice
Continental Divide
Roads
Tincup pass (CR 267)
12 16
Figure 1. Site map. a. Location of Mirror Lake in
Colorado. Regional study site locations (lakes): (1)
Lily Pond (2) Cottonwood Pass Pond (3) Copley,
Red Lady Fen, Red Well Fen, Splains, Splains
Gulch Meadow, Keystone Ironbog, Alkali Basin (4)
Hunters (5) Little Molas (6) De Herrera (7) Brazos
Ridge Marsh (8) Stewart Bog (9) Chihuahuenos Bog
(10) Alamo Bog (11) Cumbres Bog (12) Lost Park
(13) Kite (14) Bison (15) Tiago (16) Hidden (17)
Seven Lakes (18) Summit (19) RMNP: Thunder,
Sand Beach, Odessa, Lone Pine, Bear b. Photograph
taken at Mirror Lake in July 2014 looking from the
south towards the north, c. Taylor Park plant species
distribution and location of Mirror Lake within the
park. d. Contour lines depicting elevation (m) and
topographic features within the Mirror Lake
watershed. Red lines indicate avalanche paths and x
indicates coring location.


21
Cold winters and warm summers, with heavy snowfall in the winter and
thunderstorms in the summer, characterize the present day climate of Taylor Park.
Topography significantly influences the climate in the park, causing distinct
microclimates that vary with aspect and elevation. Generally, wet Pacific air masses
bring winter precipitation, and late summer monsoonal flows from the Gulf of Mexico
bring peak summer precipitation (Fall, 1997a). Total mean annual precipitation is 426
mm yr"1, with 120 mm yr"1 of the total occurring in the summer
(http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl7cotayl). Minimum precipitation occurs
during May and June; however, effective moisture from snowmelt provides ample
moisture for plants during the spring (Fall, 1997a). Taylor Park lies below the northern
limit of the summer monsoon dominated climate regime that extends through central
Colorado, meaning summertime precipitation averages will be higher below the boundary
as opposed to north of the boundary (Northern Colorado and beyond) (Fall, 1997b; Feiler
et al., 1997). Winter temperatures average -12 C with a range from an average
minimum of -22 C to an average maximum of -2 C. Summer temperatures average 12
C with a range from an average minimum of 3 C to an average maximum of 21 C
(http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl7cotayl).
Subalpine forest surrounds the lake and is composed primarily of Picea
engelmannii (Engelmann spruce) and Abies bifolia (subalpine fir), with Piceapungens
(Colorado blue spruce), Pinus contorta subsp. latifolia (lodgepole pine), and Pseudotsuga
menziesii (Douglas fir) occurring less frequently. Taxonomic nomenclature is based on
Weber and Wittmann (2012); with the exception that Artemisia tridentata (big sagebrush)
is retained, as opposed to the more recent name Seriphidium tridentatum. Dominant


22
riparian species consist of willow along the south/east/west aspects and include Salix
planifolia (plane-leaved willow), Salix brachycarpa (short-fruit willow), and Salix glauca
(grey-leaved willow). Several species of Juncus spp. (rush) and Carex spp. (carex) also
occur near the shoreline and along East Willow creek. An isolated, small population of
Populus tremuloides (quaking aspen) occurs 300 meters above the lake along the
northeast margin. For a complete species list of plants within the Mirror Lake watershed
and in Taylor Park see Table 1.


23
Table 1. Plant species list within Mirror Lake watershed and Taylor Park
Scientific Name Common Name Scientific Name Common Name
Abies bifolia * subalpine fir Oxytropis spp.* locoweed
Achillea millefolium* eommom yarrow Bentaphylloides floribunda * shrubby cinquefoil
Acomastylis spp.* alpine avens Phleum commutatum * alpine timothy
Alnus incana subsp. tenuifolia* alder Picea engelmannii* engelmann spruce
Amaranthaceae* amaranth family Picea pun gens* Colorado blue spruce
Ambrosia spp.* ragweed Pinus albicaulis w'hitbark pine
Antennaria spp' pussytoes Pinus aristata bristleeone pine
Apiaceae* parsley family Pinus contorta subsp. latifolia* lodgepole pine
Aquilegia coerulea * R.M. columbine Pinus flexilis limber pine
Arceuthobium americamtm * dwarf mistletoe Pinusponderosa subsp. scopulorum ponderosa pine
Arctostaphylos uva-ursi* kinnikinnick Poa spp * bluegrass
Arnica cordifolia * heart-leaved arnica Poaceae* grass family
Artemisia frigida fringed sage Populus tremuloides * quaking aspen
Artemisia tridenlala big sagebrush Pseudotsuga menziesii* douglas fir
(or Serphidium tridentatum) Psychrophila leptosepala * marsh-marigold
Aster spp.* aster Pteridophyta* ferns and fern allies
Asteraceae* aster family Quercus gambelii gambel oak
Belula glandulosa * bog birch Ouercus turbinella shrub live oak
Bislorta bistortoides* bistort Ranunculus spp.* buttercup
Boraginaceae* borage family Ribes spp.* current
Brassicaceae* mustard family Rosa woodsii* Woods' rose
Bromus spp.* brome Rubacer parviflorum * thimbleberry
Calamagrostis purpurascens * purple reedgrass Salix brachycarpa * short-fruit willow
Car ex aquatilis * sedge Salix glauca * grey-leaved willow'
Carex utriculata * sedge Salix planifolia* plane-leaved willow
Caryophyllaceae * pink family Salix spp.* willow
Castilleja spp.* paintbrush Sarcobatus spp.* greasew'ood
Chrysothamnus parryi rabbitbrush Saxifragaceae* rock breaker family
Chrysothamnus viscidiflonts yellow rabbitbrush Selaginella spp.* spike moss
Cirsium spp.* thistle Sparganium spp.* bur-reed
Cyperaceae* sedge family Taraxacum officinale * dandelion
Deschampsia cespitosa * tufted hairgrass Vaccinium caespitosum * dwarf blueberry
Dryas spp.* mountain dryad Vaccinium myrtillus subsp. oreophilum * myrtle blueberry
Eriophorum anguslifolium * cotton-sedge Valeriana spp.* valerian
Erythronium grandiflorum * avalanche lily Veratrum tenuipetalum * cornhusk lily
Isoetes spp.* quillwort Vicia spp.* vetch
Juncus spp.* rush Viola spp.* violet
Junipents communis subsp. alpina * commonjuniper
* Present within Mirror Lake watershed
Nomenclature follow's Weber and Wittmann (2012).


24
CHAPTER IV
METHODS AND DATA ANALYSIS
Field methods
In September 2013, a 2.735-m-long continuous sediment core was taken from the
center of Mirror Lake from a floating anchored platform in 11 m of water using a 5-cm-
diameter modified Livingstone square-rod piston sampler (Wright et al., 1983).
Unconsolidated sediment at the top of the core was not captured (6 cm). The cores were
measured for length and described lithologically, wrapped in plastic wrap and aluminum
foil, placed in PVC piping for safe transport to the laboratory, and kept under
refrigeration. Notes and photographs were taken regarding the geologic and hydrologic
features of the watershed. A comprehensive vegetative survey was completed in the
summer of 2014 consisting of species composition and structure (See Table 1).
Laboratory methods
Core preparation, lithology, and chronology
Cores were opened individually and field lengths were confirmed in the
laboratory. Core lithology was described to assess changes in composition and
characteristics of the sediment. Changes in appearance and texture of the sediment were
described in detail for each core. The cores were sectioned longitudinally, sampled at
continuous U cm increments, and stored in 2-oz labeled Whirl-Pak bags.
Radiocarbon dating of macrofossils determined ages through the Mirror Lake
core. Macrofossils suitable for AMS-radiocarbon dating were dried, weighed, described,


25
and sent to the Center for Accelerator Mass Spectrometry (CAMS) at the Lawrence
Livermore National Laboratory (LLNL). Four macrofossil samples (3 wood and 1
needle) were sent for dating. A chronology was determined by calibrating the
radiocarbon dates using the software package Calib (version 7) to build an age-to-depth
model, which describes the rate at which sediment was deposited through the core.
Magnetic susceptibility & loss-on-ignition
Sediment magnetic susceptibility was measured to document allochthonous
sediment pulses from possible erosional events (Gedye et al., 2000). A Bartington MS2E
magnetic susceptibility point sensor was used for measurement at V2 cm intervals on the
intact cores. The meter measures the strength of magnetic inductance of iron-bearing
clastic sediments. Measurements were recorded in units of cgs.
Loss-on-ignition (LOI) was conducted to assess changes in lake productivity and
allochthonous sediment inputs. LOI determinations were made at contiguous 2-cm
intervals. 1cm3 samples were first dried for 24 hours at 90 C to determine dry weight.
The percent organics was calculated from the weight-loss after heating the samples for 2
hours at 550 C. Finally, percent carbonates were determined from the weight-loss after
heating for 2 hours at 900 C (Dean, 1974).
Pollen
Pollen preserved in the lake sediments was used to reconstruct past vegetation
composition and forest structure using pollen percentages and accumulation rates. Pollen
was subsampled at intervals ranging from 1-4 cm with a sample resolution of 30-230
years. After general pollen trends were established through the record, sampling was
concentrated in areas of interest. Pollen samples were processed using standard


26
palynological methods described by LacCore Pollen Preparation Procedure (University of
Minnesota). Pollen preparation requires the removal of organics and silicates to isolate
the pollen. Potassium hydroxide (KOH) leaches out humic acids from organic material,
hydrofluoric acid (HF) removes silicates, and acetolysis, a mixture of sulfuric acid and
acetic anhydride, removes the remaining insoluble organics. Due to the large amount of
coarse-grain silicates in the samples, a pour-off method (a decant of sample liquid once
sediment settled) was used before the HF treatment to remove fine-grained silicates (a
method used in the Texas A&M Palynology Laboratory). Decanted material was
checked to make sure no pollen was lost. Prior to processing, Lycopodium tracer spores
were added to calculate pollen concentrations (grains cm"3). The final product was
suspended in glycerin oil for long-term storage. A small subsample of the pollen residue
was placed on a microscope slide, covered with a coverslip, and sealed with nail polish.
Pollen grains were identified under magnification of 400x and lOOOx with counts ranging
from 180-320 terrestrial grains per sample, with most >300 grains.
Pollen identification was to the lowest taxonomic level possible using reference
collections and atlases (Jones et al., 1995; Kapp et al., 2000). Taxa determinations were
based upon modem phytogeography. Pinus grains were separated into Haploxylon
(white pine) and Diploxylon (yellow pine) types, and those without a distal membrane
were identified as Undifferentiated Pinus. Pollen grains that were broken, corroded, or
hidden were identified as degraded, and unidentifiable pollen types were identified as
unknown.


27
Charcoal
Macroscopic charcoal analysis was used to reconstruct past fire history including:
biomass burned, fire events, fire event magnitude, and fire activity (frequency or mean
fire return interval). Sample processing followed procedures outlined by Whitlock and
Larsen (2001). Sediment samples of 2cm3 at every V2 cm were disaggregated and soaked
in household bleach for 24 hours and then washed through a 125-micron metal-meshed
sieve. Residual charcoal greater than 125-microns was then transferred into a gridded
petri dish for counting. Charcoal particles were counted under a stereomicroscope at
magnifications of 50x-100x. Charcoal concentration was calculated by dividing raw
charcoal counts by sample volume (particles cm"3). Charcoal accumulation rates (CHAR;
particles cm'2 year"1) were determined by dividing charcoal concentrations by deposition
rate (years cm"1). CharAnalysis software (Philip Higuera, U of Idaho) was used for
statistical analysis and graphing charcoal data and the analyses are discussed in the data
analysis section below.
Data analysis
Pollen
Pollen percentages, accumulation rates (grains cm'3yr''), and ratios between select
taxa were used to reconstruct past vegetation. Terrestrial pollen percentages consisted of
trees, shrubs, grasses, herbs, and pteridophytes. Tree pollen originated from conifers, in
particular Pinus pollen, which likely consisted of Pinus contorta subsp. latifolia
(lodgepole pine), with some possible Pinusponderosa subsp. scopulorum (ponderosa
pine) pollen. Pinus grains were overwhelmingly Diploxylon-type (yellow pine) grains
and for analysis were grouped into one category, Pinus. Picea pollen likely came from


28
Picea engelmannii (Engelmann spruce) and Piceapungens (Colorado blue spruce).
Pseudotsuga pollen was Pseudotsuga menziesii (Douglas fir). Abies pollen was Abies
bifolia (subalpine fir). Shrub pollen consisted of Cupressaceae that likely originated from
Juniperus communis subsp. alpina (common juniper), Quercus, likely from Quercus
gambelii (gambel oak) or Quercus turbinella (shrub live oak), and Artemisia, likely from
Artemisia tridentata (big sagebrush). Grass pollen likely originated from genera such as
Bromus (brome) and Poa (bluegrass), and the species Phleum commutatum (alpine
timothy) and Deschampsia cespitosa (tufted hairgrass). Herb and shrub pollen consisted
of species from the Amaranthaceae family, including the genera Sarcobatus spp.
(greasewood), the Asteraceae family, including: Artemisiafrigida (fringed sage),
Ambrosia spp. (ragweed), Aster spp. (aster), Cirsium spp. (thistle), Antennaria spp.
(pussytoes), Achillea millefolium (common yarrow), Arnica cordifolia (heart-leaved
arnica), and Taraxacum officinale (dandelion), and the Rosaceae family, including:
Pentaphylloides floribunda (shrubby cinquefoil), Ribes spp. (current), Dry as spp.
(mountain dryad), Acomastylis spp. (alpine avens), Rubacerparviflorum (thimbleberry),
and Rosa woodsii (Woods rose). The species of willow within the immediate vicinity of
the shoreline were deemed aquatics.
Pollen zones were determined using a constrained cluster analysis (CONISS) on
pollen percentages using the software package Tillia. CONISS uses the Euclidean
distance between pollen samples to determine similarity between samples. A
dendrogram was produced depicting a hierarchical clustering of the zones based on their
degree of similarity. Zones were based on groups of samples that had the greatest
dissimilarity. Pollen accumulation rates (PAR, grains cm"2 yr'1) were calculated based


29
upon concentration (grains cm"3) and deposition times (calibrated yr cm'1) to determine
changes in individual taxon abundance, independent of other pollen types (Birks and
Gordon, 1985). Pollen ratios were calculated using the percentages of the two samples of
interest and dividing by the sum i.e. A to B ratio = (A/(A+B)). Pollen analysis and
graphics utilized the software package C2 (Steven Juggins, U of Newcastle, UK).
Charcoal
Charcoal is always accumulating in sediments due to gradual deposition of
sediments in the littoral zone of the lake, charcoal coming in from afar, and local fires
within the watershed. CharAnalysis software decomposes charcoal accumulation rates
(CHAR) into a peaks component and a background component (BCHAR). The peaks
component is interpreted as individual fire episodes, or periods of increased fire activity
within the watershed, whereas the background component represents slowly varying
charcoal that is either from extra-local fires or stored and introduced into the watershed
over time (Higuera et al., 2010; Calder et al., 2014). Variations in background charcoal
(BCHAR) are attributed to changes in woody fuel biomass and deposition of charcoal
present in the watershed during non-fire years (Long et al., 1998). BCHAR was modeled
with a Lowess smoother robust to outliers using a smoothing window of 800 years. An
800-year smoothing window was chosen based upon robust signal-to-noise and
goodness-of-fit indices (Higuera et al., 2010). Subtracting BCHAR from CHAR
separated the peaks and background components. A threshold value (99%) for peak
determination on CHAR was correlated to a noise distribution model based upon a 1-
mean Gaussian distribution. If a charcoal peak exceeded the threshold than a fire-episode


30
event is registered. Peak occurrence (e.g. fire frequency and fire-return interval) was set
to a 500-year smoothing window to capture sub-millennial-to-centennial scale variability.


31
CHAPTER V
RESULTS
The results of the analyses are discussed in the following sub-sections. The
chronology section discusses the results from radiocarbon dating conducted at Lawrence
Livermore National Laboratory and age model development. The lithology section
describes the Mirror Lake core based on visual inspection of the major sedimentological
changes and from detailed measurements involving the magnetic susceptibility of the
sediments and loss-on-ignition to determine lake productivity. In the final two sections,
the pollen and charcoal data are described by zones from past to present, as determined
by shifts in plant communities.
Chronology
Calibrated radiocarbon dates were used to determine the age of the cores up to
4000 years (200.5 cm depth). Two of the four calibrated (cal) 14C AMS age
determinations were used to build an age versus depth model. The top of the core was
assigned -63 cal yr BP, the year of sediment collection. A 2nd order polynomial
regression was used to describe the age-depth relationship (Reimer et al., 2014; See
Figure 2 and Table 2). The other two radiocarbon dates were likely erroneous and were
determined to have occurred within the rapid sedimentation events (RSE) and were not
included in the age model (See Lithology below). Given the similar ages of the RSEs
(5265 30 and 5620 30 14C age BP), the dated material could have come from buried
material on the steep eastern slope during two separate instantaneous events.


32
Depth below mud-water
Figure 2. Age-versus-depth curve based on 14C dates. Blue circles represent dated
material based upon the medium probability, with the record beginning at -63 cal yr BP
(2013 CE). The line of the curve is based upon a 2nd order polynomial. See Table 2 for
age information.
Table 2. Uncalibrated radiocarbon dates and calibrated 14C for Mirror Lake
a Lab no. Core Depth (cm)1 Material dated Radiocarbon date (14C yr BP)' Upper age range (cal yr BP/ Lower age range (cal yr BP/ Calibrated age (cal yr BP; e Medium Prob.)
_ - 0 _ _ _ _ -63
165297 Dl-A 39.5 Macrofossil 380 30 426 505 449
165300 D3-A 171 Macrofossil 2995 30 3071 3251 3178
165298* D2-A 67 Macrofossil 5265 30 - - -
165299* D2-A 104 Macrofossil 5620 30 - - -
a 14C age determinations from Center for Accelerator Mass Spectrometry (CAMS) at the Lawerence Livermore National
Laboratory (LLNL).
b Depth below mud surface.
c 14C age based upon Libby half life of 5,568 years following conventions of Stuiver and Polach (1977).
4 95% confidence interval.
' 14C calibrated ages derived from Calib 7.0.1 calibration curves (Reimer et at, 2013).
* Depth to age erroneous, dates not used in age-model.


33
Lithology
Core description and analysis
The sediment from the Mirror Lake core consisted of dark gyttja with abundant
macrofossils, mica, and silicates throughout. From 200.5 to 184 cm depth (4008 to 3530
cal yr BP) the sediment was a dark fine detritus gyttja (FDG). A transition to lighter
FDG was observed from 184 to 104 cm depth (3530 to 1600 cal yr BP) and dark FDG
returned from 104 to 6 cm depth (1600 to 6 cal yr BP) (See Figure 3). The top 6 cm of
the core was not retrieved and represent the last 69 years of the record.
Poorly sorted sediment with larger than average grain size (>lmm), and abundant,
large macrofossils, primarily wood, were determined to be RSEs. Three RSEs were
present in the record totaling 23.5 cm in length. This sediment was removed from the
record for several reasons: (1) the large size of the sediment (>lmm), the presence of
large botanical remains, and unconsolidated sediment, (2) the orientation of the
macrofossils were inconsistent, (3) the width of the events were substantial in relation to
the rest of the record, (4) two radiocarbon dates at 67 and 104 cm depth were the same
age and out of sequence with the other dates, and (5) pollen concentrations were very
low. Given the evidence from the lithology, topography, and climate of the area suggests
the active avalanche paths on the east side of the lake likely supplied the material to the
lake. The similar age material that was dated likely represents larger wood debris stored
on the slopes and subsequently subject to being carried onto the lake during avalanches.
The RSEs occurred at 1600, 900 and 355 cal yr BP (104 to 33 cm depth) (See Figure 3).


Depth (cm)
34
Lithology
Loss-on-ignition
Magnetic susceptibility
20
40
60
80 H
100
120 H
140
160 -I
180 -
200 -I
RSE3
RSE2
RSE1
Unconsolidated sediment
Dark tine detritus gyttja
Fine detritus gyttja
Coarse detritus gyttja
Sand
Rapid sedimentation Event (RSE)
RSE1: 11 cm removed
RSE2: 6 cm removed
RSE3: 6.5 cm removed
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
3200
3400
3600
3800
4000
0 5 10 15 20 25 0.0 2.0 4.0 6.0 8.0
% Organic Content (cgs)
Figure 3. Core lithology, loss-on-ignition, and magnetic susceptibility.
Age (cal yr BP)


35
Magnetic susceptibility & loss-on-ignition
The magnetic susceptibility of the sediments gradually increased, from an average
of 1.9 cgs at the beginning of the record to 2.4 cgs at present, suggesting an increase in
allochthonous sediment input. A possible explanation for the high readings is the input
of sediments from the inflowing stream (East Willow Creek) and the erosion off the
eastern slope. The loss-on-ignition data suggests decreased organics towards present.
Organic content does not surpass 25% at any point, and decreases from an average of
15% to 10% through the core towards present, suggesting a less productive lake in
modern times and/or greater input of inorganic sediment from the inflowing stream.
Carbonate content is relatively low (<4%) throughout the entire core, and decreases
slightly towards present, suggesting that carbonate inputs were minor (See Figure 3).
Pollen
The CONISS dendrogram, pollen percentages, pollen accumulation rates, and
ratios of select taxa are displayed in Figure 4. The dominant pollen types present in the
record in descending abundance include: Finns, Artemisia, Picea, the Amaranthaceae
family (including Sarcobatus), the Poaceae family, the Asteraceae family (including
Ambrosia), the Rosaceae family, Abies, Quercus, the Cupressaceae family, and
Psuedotsuga.


Figure 4. Pollen percentages, pollen accumulation rates (PAR), pollen ratios, pollen zones (green line), and constrained cluster
analysis output (CONISS).
G\
Depth (cm)


37
Pinus pollen dominates the record. Pinus species are prolific pollen producers
and the aerodynamics of the grains aid in efficient dispersal (Lynch, 1996). In the pollen
percentage diagrams Pinus appears to dominate the forest; however, it is likely a more
minor component of the forest than the percentages suggest based on modern pollen
percentages relative to the abundance of Pinus in the forest. However, species like
Pseudotsuga and Abies have large grains that dont transport as far and are likely under
represented in the pollen record compared to their abundances on the landscape. For
example, the top most sample from Mirror Lake (6 cal yr BP) contained 58% Pinus, 9%
Picea, \% Abies, <1% Pseudotsuga, and 10% Artemisia, while the present day
composition of forest species surveyed within the Mirror Lake watershed was <1%
Pinus, 60% Picea, 36% Abies, <1% Pseudotsuga, and <1% Artemisia. Pollen
accumulation rates (PAR) were high, -12,000 grains cm"2 yr'1. This disparity in
abundance is likely caused by differential production and dispersal of wind-pollinated
species (Fall, 1992b). Therefore, the following results and later interpretation is
compared to the modern sample.
Zone la (4038 to ca 2300 cal yr BP; 201.5 to -136 cm depth) is characterized by
low Pinus percentages (55%), suggesting fewer pine trees in the forest than today, while
Picea percentages (8%) are relatively high, suggesting an abundance of spruce trees
similar to today. Abies percentages (1.3%) are also high. A short-lived decline in
average Picea percentages (to 6%) between 2968 to 2864 cal yr BP (163 to 159 cm
depth) with a simultaneous increase in Pinus percentages (to 62%), may suggest a
possible disturbance. Artemisia percentages are high (10%) throughout this zone relative
to the rest of the record. A decline in total terrestrial PAR to 2,025 grains cm"2 yr"1


38
occurred ca 2417 cal yr BP (141 cm depth), indicating less pollen productivity. Other
than this brief drop, PAR average 6,783 grains cm"2 yr'1, which is lower than present day
PAR. The pollen data indicate a closed subalpine forest.
Zone lb (ca 2300 to 1700 cal yr BP; -136 to 110 cm depth) features a slight
increase in the percentages (57%) of Pinus from Zone la, while Picea percentages and
PAR both drop by half, suggesting more pine in the forest and less spruce. Total
terrestrial PAR increases to 7,586 grains cm"2 yr"1 from Zone la. Percentages of other
conifers decline, including Psuedotsuga, Abies, and Cupressaceae, as well as the grasses
(to 0.2%, 0.2%, 0.05%, and 1% respectively). Xerophytic and disturbance adapted
species percentages, including the Asteraceae family, the Amaranthaceae family,
Artemisia, Sarcobatus, Ambrosia, and Quercus increase slightly from Zone la, possibly
indicating drought across the landscape. The ratio of Picea to Pinus decreases to the
lowest of the record (-0.05). The pollen data suggest that mesophytic species declined in
the forest and xerophytic species were more abundant, most notably there was a decline
in Picea and an increase in Pinus in the forest from Zone 1 a, and the forest was more
open than before.
Zone 2a (ca 1700 to 500 cal yr BP; -110 to 43 cm depth) has the highest Pinus
percentages (61%) of the record, suggesting the greatest abundance of pine trees in the
forest. The percentages of Picea increased slightly (to 6%) from Zone lb, suggesting a
gradual increase of spruce trees in the forest from Zone lb. Other conifer species
(.Psuedotsuga, Abies, and Cupressaceae) and Poaceae percentages also increase to levels
similar to Zone la. Xerophytic species (Amaranthaceae, Sarcobatus, and Ambrosia)
decline in abundance similar to those seen in Zone la. Artemisia percentages (7%) also


39
decline to the lowest levels in the record. Total terrestrial PAR average 7,514 grains cm"2
yr'1, similar to Zone lb. There is one major peak (-0.16) in the otherwise stable ratio of
Picea to Pinus at 804 cal yr BP (62 cm depth), possibly indicating a short-lived
disturbance that reduced the abundance of Pinus in the forest. The pollen data suggest a
more closed forest than Zone lb, but not as closed as Zone la.
Zone 2b (ca 500 to 6 cal yr BP; -43 to 6 cm depth) features a slight decline in
Pinus percentages (58%), suggesting an even more closed forest structure than Zone 2a.
Picea percentages (7%) slightly increase from the previous two zones. Artemisia
percentages (9%) rebound from Zone 2a. Total terrestrial PAR average 9,386 grains cm'2
yr"1, the highest values of the record, suggesting the forest closed producing more pollen
than before. In the most recent 231 cal yr BP, Picea PAR average 991 grains cm"2 yr"1,
also the highest of the record. The Picea to Pinus ratio steadily increases through this
zone towards present (-0.07 to -0.2), indicating greater abundance of Picea in the forest.
The increased Picea abundance (9%) toward the top of the zone (178 to 6 cal yr BP)
marks the establishment of the modem subalpine ecosystem, a closed forest of spruce and
fir, and is comparable to Zone la, although the forest was likely more closed based on
PAR levels.
Charcoal
Charcoal accumulation rates (CHAR), background CHAR (BCHAR), mean fire-
return intervals (FRI), and peaks and peak magnitudes are presented below (See Figure
5). Given the average deposition time of the Mirror Lake core, 10 years per cm, and the
modern fire regime of similar forests today with a FRI of 100 to 350 years, it is likely that


40
the peaks represent individual fires. The summaries of fire-return intervals are reported
on sub-centennial timescales to compare with the pollen data.
0
400
800
1200;
8 1600-
2000:
S3
O
V 2400-
00
2800-
3200-
3600-
4000-1
TT
400
FRI (yr fire1)
500-yr mean
95% Cl
i i i i
10 20 30 40
peak magnitude
(pieces cm 2 yr_1)
0.5
CHAR Pollen
(pieces cm 2 yr_1) zones
Figure 5. Mean fire return interval, peak magnitude, charcoal accumulation rates
(CHAR) (gray line is background charcoal accumulation rates BCHAR), and pollen
zones (green line).
Throughout the record, BCHAR, defined by an 800-year running average of CHAR,
averaged 0.3 particles cm"2 yr'1 and ranged between 0.52 and 0.002 particles cm"2 yr"1.
There were 28 fire events recorded during the last 4000 years. A high signal-to-noise
index (SNI) will indicate good separation between peaks and BCHAR values (Higuera et
al., 2014). The mean SNI at Mirror Lake is 3.87, indicating strong likelihood that the
peaks registered are fires. FRI, or the time period between fires, using a 500-year mean,
ranged from 30 to 400 years.
Depth (cm)


41
In Zone la (4038 to ca 2300 cal yr BP; 201.5 to -136 cm depth) BCHAR was
initially low, averaging 0.05 particles cm"2 yr'1, with no fire events, suggesting that fires
initially were not prevalent. There was an increase in CHAR through the remainder of
the zone to an average of 0.2 particles cm"2 yr"1, suggesting fires burned more plant
material. Fires initially burned frequently, every 100 years between 3660 and 3180 cal yr
BP (190 to 171 cm depth), decreased to 320 years between 3180 and 2700 cal yr BP (171
to 153 cm depth), and then increased toward the top of the zone to 50 years between fire
events. There were a total of 13 fire events in this zone.
The fire record in Zone lb (ca 2300 to 1700 cal yr BP; -136 to 110 cm depth) is
marked by the lowest fire activity of the record. Two fire events were identified, one at
the beginning and one at the end of the zone. BCHAR was initially low, 0.2 particles cm"
2 yr"1, and increased during the latter half of the zone to -0.4 particles cm"2 yr"1. The FRI
ranged significantly from 50 to 400 years, reflecting a change in fire frequency in just
600 years.
The fire record in Zone 2a (ca 1700 to 500 cal yr BP; -110 to 43 cm depth) had
the highest BCHAR of the record, 0.52 particles cm"2 yr"1. Increased levels of BCHAR
suggest ample woody fuels to burn. From 850 to 500 cal yr BP (65 to 43 cm depth)
BCHAR declined to 0.14 particles cm"2 yr"1, which is also apparent in the peak
magnitudes, which were initially large, but declined through the zone, suggesting that
fires burned more biomass initially rather than later in the zone. The FRI ranged from 50
to 200 years, suggesting more frequent fire than before.
Zone 2b (ca 500 to 6 cal yr BP; -43 to 6 cm depth) continues with low BCHAR
extending from Zone 2a to the lowest values in the record, encompassing ca 400 years


42
(850 to 460 cal yr BP; 65 to 40 cm depth). This also corresponds with a fire free window
of ca 350 years between 810 and 460 cal yr BP (62 to 40 cm depth). Otherwise, the FRI
averages between 50 to 100 years, with 4 fire events recorded. BCHAR increased
towards present to values similar to those seen in Zone 2a. The increased levels of
BCHAR towards present and the increased fire activity suggest more available biomass
and woody fuels were available to burn.


43
CHAPTER VI
DISCUSSION
The following section discusses the detailed late Holocene climate history of
Colorado and the surrounding region, followed by a discussion of how climate has
influenced forests and disturbance patterns. Mirror Lake, along with other
paleoenvironmental and paleoclimatological records for the region are compared (See
Figure 6; Table 3).
The climate of the late Holocene (4000 yr BP to present)
The late Holocene is marked by the transition to modern climate conditions, with
precipitation and temperatures varying on decadal-to-centennial timescales both globally
and locally (Kitzberger et al., 2007; Jimenez-Moreno and Anderson, 2012; Anderson,
2012). Climate in the western United States (US) was influenced by large-scale
mechanisms such as insolation and oscillating Pacific and Atlantic Ocean sea surface
temperatures (SSTs) (Kitzberger et al., 2007; Sibold and Veblen, 2006). The
relationships between these drivers has been described and quantified using a
combination of instrumental records and proxy data. Paleoclimate data for Colorado and
beyond is included in Figure 6 and is compared with the Mirror Lake and regional
environmental history.


44
-50
1000
Age (cal yr BP)
2000 3000
4000
5000
Closed- \ More / canopy \ closed-canopy .* Pempy Closed-canopy
r r> 1 r>. .a. i r> More Pinus & Less Pinus* More Pinus than before *AT,a.r r ^ * NAP than before
Picea-Abies forest
RSE3-
RSE2
RSE1-
Frequent, .
Mgh-Severity|I^e,)1
fire, high
biomass
burned
-4-
-6
-10
-3 -
S -4-
o
Oh
cn
O
to
-5
-15.
-16-
-17-
-18
o-
-2-
-3-
-4-
V
50
Frequent,
high-severity fire,
high biomass burned
Frequent, high-severity fire,
moderate biomass burned
+ +-
Palmer Drought Severity Index, Grid Point 131
t
Less drought
More drought
I
Temperature anomaly, Northern Hemisphere, C
Hidden Lake, CO water level (depth below modem)
Forest structure
Forest change
Avalanches
Fire history
Fire events
t
Lake level
I
Seasonal precipitation balance^ Bison Lake, CO
Insolation anomaly, solstice, 40 N. lat.
-i-------1------1------1------1------1------1------1------1----
1000 2000 3000 4000 5000
Age (cal yr BP)
Figure 6. Climate history for the southern Rocky Mountains and Mirror Lake environm-
ental history. MCA: red box (1000 to 700 yr BP), LIA: blue box (650 to 80 yr BP), NAP:
non-arboreal pollen. Data sources: (a) Cook et al., 2008 (b) Mann et al., 2009 (c) Shuman
et ah, 2009 (d) Asmerom et ah, 2007 (e) Anderson, 2011 (f) Berger and Loutre, 1991


45
Table 3. Regional site descriptions
Lake name Nearest locality Latitude (N) Longitude (W) Elevation Vegetation Type of data (m) type Reference
Mirror Buena Vista, CO 384437 1062555 3,347 Subalpine Pollen, charcoal, MS, LOI This study
Lily Pond Buena Vista, CO 385606 1063837 3,208 Subalpine- Montane Pollen, charcoal, MS, LOI Briles et al., 2012
Cottonwood Pass Pond Buena Vista, CO 384950 1062445 3,670 Tundra Pollen, macros Fall, 1997b
Copley Crested Butte, CO 385228 107050 3,350 Subalpine Pollen, macros Fall, 1997b
Red Lady Fen Crested Butte, CO 385250 107230 3,350 Tundra- Subaloine Pollen, macros Fall, 1997b
Red Well Fen Crested Butte, CO 385340 107315 3,290 Tundra- Subalpine Pollen, macros Fall, 1997b
Splains Crested Butte, CO 38500 107430 3,160 Subalpine Pollen, macros Fall, 1997b
Splains Gulch Meadow Crested Butte, CO 38500 107430 3,150 Subalpine Pollen, macros Fall, 1997b
Keystone Ironbog Crested Butte, CO 38520 107230 2,920 Subalpine- Montane Pollen, macros Fall, 1997a,b
Alkali Basin Crested Butte, CO 38450 106500 2,750 Steppe Pollen, macros Fall, 1997a,b
Hunters Alamosa, CO 373630 1065040 3,516 Tundra- Subalpine Pollen, charcoal, MS Anderson et aL, 2008a
Little Molas Silverton, CO 374430 1074230 3,370 Tundra- Subalpine Pollen, charcoal, MS, TOC, macros Toney and Anderson, 2006
De Herrera Alamosa, CO 37o0444 1062536 3,343 Subalpine Charcoal, MS Anderson et aL, 2008a
Brazos Ridge Marsh Alamosa, CO 3655,45 10621T5 3,222 Subalpine Charcoal, MS Anderson et aL, 2008a
Stewart Bog Santa Fe, NM 35400 105450 3,100 Subalpine- Montane Pollen, MS Jimenez-Moreno et al., 2008
Chihuahuenos Bog Santa Fe, NM 360250 1063030 2,925 Montane Pollen, charcoal, MS, macros. C/N Anderson et aL, 2008a,b
Alamo Bog Santa Fe, NM 355440 1063520 2,630 Montane Charcoal, MS Anderson et aL, 2008a
Cumbres Bog Alamosa, CO 37ri8 106270 3,050 Subalpine Pollen, MS, LOI, diatoms Johnson et aL, 2013
Lost Park Fairplay, CO 391730 1053230 3,079 Subalpine- Montane Pollen, LOI Vierling, 1998
Kite Fairplay, CO 39=T947 1060747 3,665 Tundra Pollen, MS, macros Jimenez-Moreno and Anderson. 2012
Bison Glenwood Springs, CO 3945,55 1072044 3,255 Subalpine 5180 Anderson, 2012
Tiago Steamboat Springs, CO 403449 1063646 2,770 Montane Pollen, charcoal, MS, TOC. 613C Jimenez-Moreno et al., 2011
Hidden Steamboat Springs, CO 403018 1063626 2,710 Montane Lake level Shuman et al, 2009
Seven Lakes Steamboat Springs, CO 405346 1064057 3,277 Tundra- Subalpine Charcoal Calder et al, 2014
Summit Steamboat Springs, CO 403244 1064056 3,149 Tundra- Subalpine Charcoal Calder et al, 2014
Thunder RMNP, CO 401331 1053883 3,231 Subalpine Charcoal, pollen Higuera et aL, 2014
Sand Beach RMNP, CO 401312 10536T0 3,140 Subalpine Charcoal Higuera et aL, 2014
Odessa RMNP, CO 401982 10541T2 3,051 Subalpine Charcoal, pollen Higuera et al, 2014
Lone Pine RMNP, CO 40139 1054389 3,016 Subalpine Charcoal, pollen Higuera et al, 2014
Bear RMNP, CO 4018,47 1053853 2,888 Subalpine- Montane Pollen, charcoal, MS CafFrey and Doemer, 2012
Abbreviations: MS: magnetic susceptibility, LOI: loss-on-ignition, macros: macrofossils, TOC: total organic carbon, C/N: carbon to nitrogen ratio, 8180: oxygen isotope ratios, 813C: carbon isotope ratios, RMNP: Rocky Mountain National Park


46
Variations in winter and summer insolation are good indicators of changes in the
Earths energy balance, and resulting temperature. Insolation amount also drives large-
scale climate features such as the position of the westerly jet stream and strength of
pressure systems. Winter insolation at 40 N latitude increased from 173 W/m2to 181
W/m2 from 4000 yr BP to present, or ~5%. Simultaneously, summer insolation
decreased from 478 W/m2 to 462 W/m2, or ~4% (Berger and Loutre, 1991). These long-
term shifts in insolation resulted in warmer winters and cooler summers towards present
(Anderson, 2012). Greater summer insolation caused an enhanced subtropical high-
pressure system and a northward shift of the westerly jet stream. These climate
conditions resulted in moisture coming in primarily from the southern oceans in summer
due to the warmer ocean temperatures. Storms coming from the Pacific were likely
shifted further north than they are today. Through the late Holocene, as summer
insolation decreased and the subtropical high pressure weakened, summer moisture from
the southern oceans decreased. The increase in winter insolation likely resulted in
warmer temperatures and greater snowfall than before (Bartlein et al., 1998).
Temperature reconstructions for the Northern Hemisphere come from a
combination of proxy data, namely tree-rings, ice cores, marine sediments, corals, and
historical records, and describe temperature anomalies dating back to -200 CE (Mann et
al., 2008). Two time periods of noteworthy changes in Northern Hemisphere
temperatures are the Medieval Climate Anomaly (MCA; ca 1000 to ca 700 yr BP) and
the Little Ice Age (LIA; ca 650 to ca 80 yr BP). The MCA is marked by slight above
average North American temperatures. The LIA is characterized by modest cooling in
the Northern Hemisphere, with a drop in temperatures of roughly 0.6 C. The estimates


47
for temperature changes are for the entire Northern Hemisphere and vary considerably,
with some regions experiencing very little change from before (Mann, 2002a,b). During
the MCA on the Colorado plateau, tree-ring reconstructions suggest elevated
temperatures and severe extended drought periods (Meko et al., 2007). Glacial
advancement in the southern Rockies suggests cooler temperatures were the likely cause
during the LIA (Grove, 1988).
The cyclical cycle of El Nino Southern Oscillation (ENSO) is significantly
correlated with periods of either drought or enhanced precipitation. Recent advances in
modeling and data collection techniques suggest drought is more likely in the western CIS
when a La Nina event co-occurs during the cool-phase of the Pacific Decadal Oscillation,
and when warm SSTs in the North Atlantic are recorded (Atlantic Multidecadal
Oscillation) (Kitzberger et al., 2007; Sibold and Veblen, 2006). These cyclical events are
typically recorded in the rings of trees and in recorded history, which extend beyond the
instrumental data. The Palmer Drought Severity Index (PDSI) documents these
oscillations and more specifically, periods of drought, or below average moisture
availability, for the last 2000 years at some locations. The index is built from
observational data and tree-rings and distributed geographically using grid points
(Kitzberger et al., 2007; Cook et al., 2004). Drought is closely tied to wildfire, so the
PDSI is useful to compare to the Mirror Lake charcoal record. A significant time-period
of elevated aridity in the western United States based upon a reconstruction from 108 grid
points constituting the western US, occurred during the MCA, with four intervals (1014,
916, 800, and 697 cal yr BP) identified as megadroughts due to their duration and
severity. Colorado PDSI reconstructions from grid point 131 show periods of extended


48
or severe drought occurring within the LIA and centered around 480, 446, 387, 314, 276,
198, 152, 136, and 103 cal yr BP (Cook et al., 2004).
Isotope and lake level data provide centennial-to-millennial scale evidence of the
late Holocene climate of Colorado. A seasonal precipitation shift, inferred from analyses
of calcite-5180 in lake sediments from Bison Lake near Glenwood Springs, indicated an
increase in winter precipitation starting ca 3000 cal yr BP (Anderson, 2012).
Additionally, lake level trends reconstructed from Hidden Lake near Steamboat Springs
suggests more winter precipitation during the last two millennia (2000 cal yr BP to
present) as indicated by higher lake levels (>2 m) (Shuman et al., 2009). The higher lake
levels likely resulted from more snow-dominated precipitation and cooler summers. In
addition, a speleothem record of 5180 from southern New Mexicos (NM) Pink Panther
Cave indicates greater winter precipitation than in summer ca 2700 cal yr BP (Asmerom
et al., 2007). Finally, mites preserved in stalagmites from Hidden Cave (southern NM)
suggest a similar shift from a summer to a winter dominant precipitation regime ca 2800
to 2600 cal yr BP (Polyak and Asmerom, 2001).
In summary, the climate conditions of the late Holocene resulted from shifts in
insolation influencing large-scale climate drivers that determined the timing and amount
of precipitation and regional temperatures. The late Holocene climate of Colorado was
characterized by a general cooling trend in temperature, with the exception of the MCA
and the last few decades, and increased winter precipitation. These general trends were
punctuated by heightened periods of drought on yearly-to-decadal scales (Kitzberger et
al., 2007; Sibold and Veblen, 2006; Anderson, 2012; Shuman et al., 2009; Mann,
2002a,b; Trouet et al., 2013).


49
Ecosystem response to late Holocene climatic conditions
Summer-dominant precipitation regime
The Mirror Lake pollen record documents the transition from a summer-to winter-
dominant precipitation regime, which began ca 3000 cal yr BP, as evidenced by isotope
analysis from Bison Lake (Anderson, 2012). The pollen data between 4038 and 2300 cal
yr BP suggest a closed forest dominated by Picea engelmannii and Abies bifolia. This
coincides with multiple regional records indicating similar forests during this time period
(Higuera et al., 2014; Caffrey and Doerner, 2012; Fall, 1997a,b; Jimenez-Moreno and
Anderson, 2012; Jimenez-Moreno et al., 2011). Warmer and wetter summers relative to
today, and cooler winter temperatures resulted in higher effective moisture than today,
enabling tree growth that resulted in a dense forest canopy (Higuera et al., 2014).
Between 4000 and 2000 yr BP, as warmer summer temperatures gradually declined (~1
C), local pollen records suggest timberline retreated downslope by 100 to 200 meters
(Fall, 1997b; Emslie et al., 2005). The montane zone consisting of mixed conifer species
(Pinus and Pseudotsuga) was likely found at lower elevations than today, as reflected in
the lower percentages of those pollen types, and was probably due to the warmer summer
temperatures of the mid-to-late Holocene. Lower elevation regional records, including
Tiago Lake near Steamboat Springs and sites near Crested Butte, recorded higher
abundance of Pinus during this time-period (Jimenez-Moreno et al., 2011; Fall, 1997a,b).
Artemisia pollen percentages hold steady during this time period, likely suggesting the
shrub occurred at a lower elevation in an open steppe environment (Fall, 1997a,b). The
short-lived drop (-100 yr) in Picea from 2968 to 2864 cal yr BP may indicate a


50
disturbance event, although the other pollen and charcoal data do not suggest a fire,
insect, or climate had an influence.
Infrequent, high-severity, stand-replacing crown fires are characteristic of spruce
and fir forests and lake sediment records are excellent recorders of fires. In contrast,
trees, and subsequently tree-rings, are destroyed in high-severity fires, and therefore do
not make good recorders of fire in subalpine forests. Fire activity between ca 4000 and
2300 cal yr BP was frequent and individual fires burned significant levels of fuels. It is
likely that convective thunderstorms and lightning were more prevalent during this time
than today due to enhanced summer precipitation (Bartlein et al., 1998). Hidden Lake
levels declined between 3700 and ca 3000 cal yr BP, suggesting seasonal variation in
moisture during this time period led to lower water levels (Shuman et al., 2009). High
effective summer moisture during this time likely resulted in abundant forest biomass
(closed-canopy); however, periodic droughts resulted in high-severity fires.
Winter-dominant precipitation regime
The most significant shift in the forest at Mirror Lake occurred at 2300 cal yr BP,
as evidenced by the drop in Picea and Abies percentages, the lowest ratio of Picea to
Pinus (0.05), and the slight increase in Pinus and Artemisia percentages. Regionally, an
abrupt change in vegetation is described in several subalpine records (Keystone Iron Bog
near Crested Butte ca 2600 yr BP, RMNP composite record ca 2440 cal yr BP; Little
Molas Lake in San Juan Mnts. ca 2650 cal yr BP, Stewart Bog NM ca 2800 cal yr BP)
(Fall, 1997a; Higuera et al., 2014; Toney and Anderson, 2006; Jimenez-Moreno et al.,
2008). While the timing of the event is not synchronous across all sites, it falls within the
range of error of the radiocarbon dates. At lower elevations, mixed conifer forests record


51
a similar shift in vegetation to more drought tolerant Pinus and Artemisia species than
before (Bear Lake in RMNP ca 3520 cal yr BP, Tioga Lake near Steamboat Springs ca
3000 cal yr BP, Chihuahuenos Bog NM ca 2700 cal yr BP) (Caffrey and Doerner, 2012;
Jimenez-Moreno et al., 2011; Anderson et al., 2008b). The vegetation changes at Mirror
Lake, and across the region, occur after the shift in the seasonal precipitation balance
from summer to winter was well underway. Warmer winters and increased precipitation
along with cooler summers with less effective summer moisture limited the growth of
mesophytic trees like Picea and Abies, and enabled Pinus trees and Artemisia to increase
in the forest (Jimenez-Moreno et al., 2008). At Hidden Lake, an 800-year drop (2m from
present) in lake levels occurred between 2000 and 1200 cal yr BP, suggesting decreased
effective summer moisture (Shuman et al., 2009). In the Mirror Lake record, xerophytic
species such as those in the Amaranthaceae family, Artemisia, and Quercus were in
abundance between 2300 and ca 1700 cal yr BP. These trends, along with increased
Pinus and decreased Picea, Abies, and Cupressaceae (likely Juniperus communis (carpet
juniper)) suggest the forest canopy was more open than previously (Jimenez-Moreno et
al., 2008).
Fire activity between 2300 and 1700 cal yr BP was low, with a FRI of 400 years.
BCHAR and CHAR both declined from before, suggesting less available biomass to
burn. The decreased charcoal levels correspond to a more open forest canopy than before
with higher occurrence of shrubs/herbs that do not support high-severity fires. The
general lack of high-severity fires corresponds to a period of cooler summer temperatures
and a marked change into more winter precipitation. A composite charcoal record from


52
RMNP recorded a decrease in fire severity ca 2400 cal yr BP, associated with a decreased
forest density as well (Higuera et al., 2014).
The establishment of the modem precipitation regime, and temperatures
comparable to today, evidenced by Northern Hemisphere temperature reconstructions,
suggest that modem forest structure and composition was established ca 2000 yr BP, with
only minor changes in forest characteristics noted thereafter, specifically during the LIA
(Mann et al., 2008; Fall, 1997b; Vierling, 1998; Toney and Anderson, 2006; Shuman et
al., 2009; Emslie et al., 2005). From 1700 to -250 cal yr BP, an increase in diploxylon
Pinus percentages, from an average of 57% to 61%, indicates an increased abundance of
pine trees in the forest, likely Pinus contorta subsp. latifolia (lodgepole pine). Picea and
Abies also increased slightly from low abundances after 1700 cal yr BP. Due to total
terrestrial PAR averaging 7,514 grains cm"2 yr'1, the high Pinus abundance in the forest,
and the decline in species such as Artemisia, the Amaranthaceae family, Sarcobatus, and
Ambrosia, the forest was more closed than before, but not as closed as the beginning of
the record. The closed forest structure is consistent with cooler temperatures, an
established winter-dominant precipitation regime, and reduced effective summer
moisture (Anderson, 2012). Pinus trees are more drought tolerant than Picea or Abies
and the reduced effective summer moisture likely favored Pinus over the other conifers;
however, higher snowpack amounts enabled maintenance of significant amounts of Picea
and Abies in the forest as well. Regional pollen records also indicate drier summer
conditions with increased Pinus in the forests from ca 2000 to -1000 yr BP (Vierling,
1998; Fall, 1997a; Higuera et al., 2014; Toney and Anderson, 2006; Jimenez-Moreno et
al., 2008).


53
The MCA exhibited slightly warmer temperatures (1000 to 700 yr BP) and
subsequent periods of extensive drought in the western US (Mann et al., 2008; Cook et
al., 2004). The pollen record does not indicate significant sensitivity to the
megadroughts of the MCA. The lack of plant sensitivity to the sub-centennial MCA
droughts likely results from the long duration of tree growth and the inherent resiliency of
conifer species to short-term climate fluctuations; in particular, Pinus trees can occupy a
broad climate niche and can withstand extensive drought periods (Minckley et al., 2012).
The pollen record during this period maintains high percentages of Pinus, likely due to
the warm and dry climatic conditions conducive for pine tree growth.
The fire activity between 1700 and 850 cal yr BP transitioned to higher frequency
(-100 yr FRI), high-severity fires from the low fire activity present before (2300 to 1700
cal yr BP). Multiple fire events and the most abundant BCHAR of the record were
recorded. This suggests ample biomass was available to bum at relatively frequent
intervals. The more abundant Pinus pollen at this time was likely from lodgepole pine.
Lodgepole pine trees incorporate a more dense forest structure in comparison to low
elevation pine dominated forests, and require high-intensity fires to open their serotinous
cones for reproduction (Sibold et al., 2007). This, combined with the Picea and Abies
present in the forest, provided the fuel necessary to produce the high levels of charcoal in
the record. A composite record from RMNP recorded increased levels of CHAR and
inferred increased fire severity over the last 1500 years (Higuera et al., 2014).
Additionally, records from southern Colorado and northern New Mexico recorded
increased fire frequency during this time period (1700 to 800 cal yr BP), suggesting this
trend was region-wide (Anderson et al., 2008a).


54
The catalyst for fires during this period (1700 to 850 cal yr BP) was likely due to
variability in effective precipitation during the late Holocene. Evidence from speleothem
records from southern New Mexico and PDSI reconstructions as early as ~1100 yr BP,
all suggest decadal-scale droughts were a common occurrence during this time period
(See Figure 6) (Shuman et al., 2009; Polyak, and Asmerom, 2001; Asmerom et al., 2007;
Cook et al., 2004). These droughts sufficiently dried out fuels and lead to the high-
severity fires. The fire events at Mirror Lake correspond with PDSI reconstructed
droughts for the region, suggesting that while droughts did not produce significant
vegetation changes, they did influence fire occurrence.
A transition to the lowest charcoal levels of the record with a FRI of 350 years
occurred during the transition between the MCA and LIA (-850 to 450 cal yr BP),
suggesting little fire activity despite abundant fuels. Picea and Abies dominated the
forest structure during this time period; however, Pinus remained abundant. In fact, a
400-yr increase in the percentage of Pinus was recorded, to a high of 63%. The greater
percent of Pinus likely reflects even greater abundance of lodgepole pine in the forest.
The counter-intuitive decline in fire activity with a dense forest structure can be
explained in a few ways. The primary cause is likely linked to climatic variability at the
time (Sibold and Veblen, 2006). The moisture flow from the southern oceans
significantly declined since 4000 years ago and this inhibits convective thunderstorms
that produce lightning strikes, which are the primary ignition source for fires in subalpine
forests (Anderson, 2012). Regardless of the abundance of fuels present in the forest, fire
cannot occur without ignition. The reduction in summer moisture is evidenced in lower
lake levels at Hidden Lake between 850 and 490 cal yr BP, indicating lake sensitivity to


55
increased summer evaporation rates and the megadroughts indicative of the time period
(Shuman et al., 2009). Additionally, increased winter precipitation will result in more
snowpack, and due to significantly decreased summer insolation, snowpack persists much
longer than before creating a shorter fire season (Anderson, 2012).
The latter part of the LIA (450 to 80 yr BP) is reflected in the pollen record as an
increase in cold-tolerant mesophytic species. In particular, since ca 200 cal yr BP, the
percentage of Picea increased from -6% to -9% and the ratio of Picea to Pinus increased
to ~0.2. Other species indicative of forest closure increased such as Abies and
Cupressaceae (likely carpet juniper), while Pinus decreased slightly in the forest. A
similar increase in Picea and slight decline in Pinus percentages during the second half of
the LIA is described in two subalpine records from southern Colorado (Johnson et al.,
2013; Toney and Anderson, 2006). The cooler temperatures (-0.6 C cooler Northern
Hemisphere average; Mann et al., 2008) of the LIA led to prolonged and increased
snowpack that likely caused the resurgence of a more closed spruce-and fir-dominated
forest at Mirror Lake. Modem day subalpine forests in Colorado exhibit this forest
structure, which was established during the LIA, and are unusual based on previous forest
types of the last -2000 years, which had a higher abundance of Pinus trees.
Despite the wetter and cooler conditions of the LIA, the fire record indicates
increased frequency of high-severity fires following the low fire activity discussed for the
transition between the MCA and LIA. Even though the LIA exhibited slightly cooler
temperatures and increased winter precipitation than before, summer precipitation was
still variable, and moisture deficits and droughts remained routine as evidenced in the
PDSI data (Cook et al., 2004). Fire events recorded during this time period generally


56
corresponded to PDSI reconstructed droughts for central Colorado. Additionally,
charcoal reached record levels, and the FRI decreased to an average frequency of 100
years. The drought intervals alternating with wet periods likely enabled the build-up of
fuels and facilitated high-severity fires (Whitlock et al., 2010; Fall, 1997a). The forest
was more closed than before and Pinus decreased to levels seen in the early part of the
record after -250 cal yr BP, resulting in a shift back towards more frequent, high-severity
crown fires.
Insect outbreaks
Insect outbreaks such as Dendroctonus rufipennis (spruce beetle) and
Dendroctonusponderosae (mountain pine beetle) are both major forms of disturbance
today in Colorado coniferous forests. In subalpine forests, spruce beetle outbreaks are
capable of widespread tree mortality, killing large numbers of spruce trees during an
outbreak (Veblen et al., 1991; Kulakowski et al., 2003). Mountain pine beetle outbreaks
occur largely in the montane zone, with the same capability of widespread mortality of
pine trees and alteration of forest characteristics.
To understand the frequency and duration of historical insect outbreaks, several
dendrochronological and palynological studies have been conducted in the western US.
The most informative of these comes from the combination of tree-ring records from
periods of documented outbreaks with high-resolution pollen records (Morris, 2013;
Anderson et al., 2010). In the pollen data, a spruce beetle outbreak is evidenced by a
significant drop in Picea pollen followed by an increase in Abies pollen, as reflected in
the ratio of Picea to Abies. Pollen records from Colorado and Utah conducted through
the 19th century have suggested spruce abundance declines and fir increases after an


57
outbreak (the Picea to Abies ratio declined to negative values in most instances), as
confirmed from existing tree-ring records near the study areas (Morris, 2013; Anderson et
al., 2010). This trend continues until spruce abundance returns to pre-outbreak levels
(Anderson et al., 2010; Veblen et al., 1991). Theoretically, a millennial-scale pollen
record should reflect similar trends in the data if a spruce beetle outbreak were to have
occurred (Anderson et al., 2010). A mountain pine beetle outbreak is evidenced in the
pollen record by a significant drop in Pinus abundance as reflected in the percentages
(decline by roughly half), while other species are not affected. Additionally, the ratio of
Pinus to Picea declines during a mountain pine beetle outbreak (Whitlock et al., 2012).
The Mirror Lake pollen data (Picea to Abies ratio and Picea abundance) do not
suggest any prior spruce beetle outbreaks within the watershed over the last 4000 years.
There is also no evidence in the Pinus pollen abundance or the Pinus to Picea ratio to
suggest mountain pine beetle outbreaks (See Figure 4). The record suggests that insect
infestations were not common in the forest around Mirror Lake under late Holocene
climate conditions. The Mirror Lake record cannot be put into regional context due to the
absence of high-resolution paleorecords for the region that may provide evidence for past
insect outbreaks (Morris, et al., 2014). Additional records are needed to determine the
spatial history of insect infestation in subalpine and montane forests of Colorado
(Anderson et al., 2010).
Avalanches
Avalanches are a common disturbance in subalpine ecosystems that are capable of
uprooting vegetation and transporting large amounts of debris downslope, altering the
forest structure within its path (Vasskog et al., 2011). Visual inspection of the avalanche


58
terrain at Mirror Lake today indicates immature conifers and more abundant shrubs with
decreased vegetation density as compared to the surrounding slopes, suggesting recent
avalanches have influenced the local forest. However, the pollen data do not reflect any
changes in species abundance or composition immediately following an avalanche,
meaning the impacts seen across the landscape today are not registered in the pollen
record. In addition to avalanches, rapid sedimentation events may also result from
significant flood events depositing course-grained sediment into the lake; however,
without further investigation the determination between the two mechanisms remains
inconclusive, therefore the RSEs in the record were determined to be avalanches.
The Mirror Lake record documents three avalanches, identified in the sediment
record by poorly sorted, larger than average grains (>lmm), and abundant, large
macrofossils, primarily wood (see full description in results section). Additionally, the
magnetic susceptibility and loss-on-ignition data indicate more sediment input and less
lake productivity towards present, correlating with avalanche occurrence, suggesting
debris transport may have increased during these intervals. The avalanches occurred
between 1600 and 355 cal yr BP with an average frequency of every 600 years. This
frequency is less than the frequency of avalanches found in a composite record from
southern Norway, which suggests an average of two events per 100 years from 1200 to
1100 and from 400 to 100 cal yr BP, these time periods were described as high frequency
(Vasskog et al., 2011). A possible explanation for this disparity in frequency is the
spatial and climatological difference at the sites and the sampling resolution for grain-size
and grain characteristics (the record from Norway analyzed every 1 cm of sediment using
a sampling device to find sediment >125 microns and the Mirror Lake record was


59
determined from visual interpretation). Therefore, at Mirror Lake we are likely only
documenting large avalanches that transported large amounts of debris from the
surrounding hillslopes.
The ability of avalanches to transport debris is highly dependent on snow density,
with dry, powdery snow less capable of transport than wet, heavy snow (Blikra and
Nemec, 1998). Greater amounts of debris are transported during larger avalanches,
therefore it is possible that smaller avalanches may not be recorded at Mirror Lake
(Vasskog et al., 2011). All of the avalanches at Mirror Lake occurred during a period of
greater winter precipitation after 3000 cal yr BP, suggesting that the current climate and
snowpack is ideal for avalanches. Both the Mirror Lake and Norway records reflect
increased frequency of avalanches on a millennial-scale towards present, especially
during the LIA for the Norway record, suggesting climate fluctuations drive occurrence.
However, a network of regional records is necessary to determine the spatial impact of
avalanches and the impact of climate changes on avalanche occurrence (Vasskog et al.,
2011).
Management implications
Management of the western US coniferous forests in the 21st century offers a
unique challenge due to the uncertainties changing climatic conditions will have on
vegetative distributions and disturbance regimes. In particular, Colorado has experienced
an increase of ca 1 C mean annual temperature in the last several decades, with more
significant warming in subalpine environments (Ray et al., 2008). Climate models
further predict higher average summer temperatures than today (ca 2.75 C), with more
winter-dominant precipitation by mid-century (Ray et al., 2008). How forests will


60
respond to these changing climatic conditions is unknown and is an important
consideration for forest managers. The research presented in this thesis can provide
insight for future conditions in central Colorado subalpine forests by examining the
degree and nature forests have changed in the past during periods of similar climatic
conditions.
Colorados modern-day subalpine forests were established during the latter part of
the LIA (-250 years ago). Average temperatures were lower during the LIA as compared
to the period before or at present; therefore, warmer climate conditions are likely to alter
forest structure (Froyd and Willis, 2008). In fact, the conditions of the LIA were unusual
during the modem winter-dominated precipitation regime that began -3000 cal yr BP,
and so are the forests that have developed under them. In addition, we have been very
successful at suppressing fires and this has left large amounts of fuels in the forests to
burn. While fire suppression has probably had relatively little impact on subalpine
forests, due to their long FRIs, it has influenced lower elevation pine dominated forests
whose fires can move into upper elevation forests (Schoennagel et al., 2005).
Under warmer conditions, subalpine forests in central Colorado will likely
experience a greater abundance of Pinus and less Picea and Abies, resulting in a more
open-canopy forest than present. Additionally, treeline will likely increase in elevation as
trees find more suitable climate conditions (Fall, 1997b). If forest structure changes as
described, fire frequency will likely decrease and fires will bum less biomass in the
subalpine zone, resembling the fire characteristics that were seen in the MCA. However,
the severity and frequency of fires will ultimately depend on available fuels in the forest
in addition to the cyclical nature of wet and dry periods, just as they have in the past. For


61
example, if the subtropical high-pressure system strengthens in the summer, it will likely
reduce summer precipitation and convective thunderstorms, limiting ignition sources for
fires, and promoting more pine in the forest.
If more precipitation falls in winter under a warmer climate, in conjunction with
the increased likelihood of experiencing above freezing (0 C) days in the winter, than we
would expect to see increased potential for avalanches in central Colorado. Increased
avalanche occurrence would lead to more hazardous conditions for winter traveling and
backcountry recreation users. The historical rate and severity of beetle outbreaks through
the late Holocene is unknown and/or not evident from proxy data due to the lack of
studies; therefore, it is unclear whether the extensive outbreaks experienced throughout
much of Colorado during the last few centuries are unprecedented (Morris et al., 2014;
This study). However, modern-day ecological studies do suggest the frequency and
severity of beetle outbreaks are expected to increase in the future under a warming
climate, primarily due to two factors: (1) the acceleration of bark beetle fecundity, and (2)
the reduction in cold-induced mortality (Morris et al., 2014; Anderson et al., 2010).
An important aspect to 21st century management of western forests is contingent
upon understanding the historical range of natural variability. Quantifying when, and if,
the ecological threshold for ecosystem change is reached will enable understanding what
impact climate changes may have on vegetation distributions and disturbance (Froyd and
Willis, 2008). There is a certain degree of resiliency in southern Rocky Mountain
coniferous forests to long-term climate shifts and disturbance events of the Holocene, as
evidenced by the maintenance of forest composition and the ability to rebound after such
events (Minckley et al., 2012). However, it is certain that future warming will exceed


62
what we have experienced in the late Holocene, likely pushing forests beyond the natural
range of variability. We are headed into uncharted territory, and how forests and
disturbances will respond depends on the conditions that result from continued warming.
Nonetheless, lessons from the past are invaluable and can provide suggestions for
management of central Colorados subalpine forests. The US Forest Service is expected
to be the primary agency that can utilize this information for management purposes;
however, private landowners, conservation groups, science agencies, etc. may also find
the information pertinent. While central Colorados subalpine forest composition
remained unchanged, the abundances of plant species in the forests did, influencing forest
structure and the amount of biomass available to burn. Since the availability of fuels in
the forest dictates fire severity, it would prove useful for managers to monitor the fuel
structure of their forest in the future. With an expected decrease in fuels with a more
open-canopy forest, the controls for fire may differ in the future as compared to
conditions were currently seeing. However, this change may not occur until the legacy of
the dense, spruce-fir forests with high-severity fires of the LIA are phased out. The
large-scale, drought-driven, high-severity fires seen in the recent past will likely continue
until the new forest structure is established, and are balanced with climate. While plant
species in subalpine forests are resilient to change, they have responded in the past by
moving along elevational gradients. Since treeline is temperature dependent, an upslope
migration in treeline is expected, and managers need to be cognizant that lower elevation
pine trees will likely encroach into the subalpine zone, while spruce and fir trees will
continue to climb in elevation. If, and when this movement is constrained by natural


63
barriers (e.g., elevational limit) or human-caused habitat fragmentation, mitigation may
be required to ensure the climatic niche of ecosystem dominant conifers is satisfied.


64
CHAPTER VII
CONCLUSION
In conclusion, the Mirror Lake paleo-record of the late Holocene offers new,
high-resolution data for the subalpine zone of central Colorado. This data allows us to
better understand the forest structure and disturbance history of central Colorados forests
as they responded to climate changes during the last 4000 years. The research was
structured around the following two questions:
1. What was the late Holocene vegetation and disturbance history of subalpine forests in
central Colorado, and how does climate change influence this history?
The subalpine forests of central Colorado exhibited changes in forest structure in
relation to the seasonal timing of precipitation. As summer insolation decreased through
the late Holocene, precipitation shifted from being more summer-to-winter dominated at
-2300 cal yr BP. The vegetation shifted from being a closed-canopy spruce-fir forest to a
more open-canopy forest with greater pine abundance. This forest structure remained
until the LIA, when cool and wet conditions resulted in the return to a closed-canopy
spruce-fir forest. However, forest composition remained relatively constant through the
late Holocene, suggesting plant species are inherently resilient to climate changes and
disturbances, with only fluctuation in abundance.
High-severity fires characteristic of the subalpine zone were prevalent when
spruce-fir forests were closed, evidenced by substantial levels of charcoal in the record
and numerous fire events. A period (-2300 to 1700 cal yr BP) of reduced fire activity


65
was attributed to a more open-canopy forest canopy with greater abundance of non-
arboreal taxa and pine trees. The subsequent period of low fire activity (-850 to 450 cal
yr BP) was attributed to climate influences, likely due to few convective thunderstorms
resulting from enhanced droughts possibly due to high-pressure systems over the western
US. Overall, fire activity was highly dependent on a combination of forest fuel loads
(amount of biomass available to bum) and climatic conditions conducive for fire (likely
wet periods followed by extensive drought). This is most evident from the period of
megadroughts during the MCA and LIA, which correlated to fire events in the Mirror
Lake record. There was no evidence of historical beetle outbreaks in the Mirror Lake
record suggesting they did not occur within the watershed over the last 4000 years.
Further studies examining modem outbreaks and how they are recorded in the sediment
record are needed to determine the nature and extent of insect outbreaks in Colorado
forests. Avalanches were evident in the record during the most recent part of the record
(1600 cal yr BP to present). This suggests that increased winter-precipitation led to
higher frequency of avalanches. Further studies refining the methodology for detecting
avalanches in sediment records would be useful.
2. How does the late Holocene disturbance and forest reconstruction inform
current and future management?
Understanding the historical conditions that have given rise to modern forest
structure is imperative for proper management. There is inherent resiliency of conifer
forests to climate changes; therefore, if, and when a shift in ecosystem dynamics is
evident in the paleo record, it is important to take note. There were no compositional
changes in the forest, however structural changes and abundances of individual species


66
did occur in response to the timing of precipitation and changes in temperature. It is
likely that forests are currently experiencing conditions seen during the MCA and
subalpine forests may transition to a more open forest structure with greater pine
abundance and an upslope shift in treeline. The sediment record from Mirror Lake
suggest that fire frequency and severity in central Colorado subalpine forests have
historically varied in response to forest structure and climate variability. However, other
factors such as fire suppression and the legacy from the dense tree growth due to
conditions of the LIA have likely led to the widespread, severe disturbance events of the
20th and 21st centuries. Subalpine forests will undoubtedly change under future warming,
and depending on the degree and nature of the warming, climate may take these forests
into uncharted territory.
Further research is needed to provide additional, high-resolution records focusing
on the history of disturbance events, such as the prevalence of fire, beetle outbreaks, or
avalanches that would greatly enhance our understanding of the historical range of
natural variability. Limited studies exist that evaluate and document insect outbreaks or
avalanches as major forms of disturbance in the past in subalpine ecosystems. Further
research will improve methodology and provide better baseline data on insect
infestations. Additionally, because the spatial extent of each study is limited, combining
regional records utilizing the same proxies can provide a greater geographical range and
provide more useful tools for forest managers.


67
REFERENCES
Anderson, L. (2012). Rocky Mountain hydroclimate: Holocene variability and the role of
insolation, ENSO, and the North American Monsoon. Global and Planetary
Change, 92-93, 198-208.
Anderson, R.S., Allen, C.D., Toney, J.L., Jass, R.B., and Bair, A.N. (2008a). Holocene
vegetation and fire regimes in subalpine and mixed conifer forests, southern
Rocky Mountains, USA. International Journal of Wildland Fire 17, 96-114.
Anderson, R.S., Jass, R.B., Toney, J.L., Allen, C.D., Cisneros-Dozal, L.M., Hess, M.,
Heikoop, J., and Fessenden, J. (2008b). Development of the mixed conifer forest
in northern New Mexico and its relationship to Holocene environmental change.
Quaternary Research 69, 263-275.
Anderson, R.S., Smith, S.J., Lynch, A.M., and Geils, B.W. (2010). The pollen record of a
20th century spruce beetle (Dendroctonus rufipennis) outbreak in a Colorado
subalpine forest, USA. Forest Ecology and Management 260, 448-455.
Asmerom, Y., Polyak, V.J., and Burns, S.J. (2007). Solar forcing of Holocene climate:
new insights from a speleothem record, southwestern United States. Geology 35,
1-4.
Bartlein, P.J., Anderson, K.H, Anderson, P.M., Edwards, M.E., Mock, C.J., Thompson,
R.A., Webb, R.S., Webb III, T., and Whitlock, C. (1998). Paleoclimate
simulations for North America over the past 21,000 years: features of the
simulated climate and comparisons with paleoenvironmental data. Quaternary
Science Reviews 17, 549-585.
Berger, A. and Loutre, M. (1991). Insolation values for the climate of the last 10 million
of years. Quaternary Science Reviews 10, 297-317.
Berwyn, B. (2014). Colorado wildfire activity near historic low this year. The Colorado
Independent, http://www.coloradoindependent.com/150405/colorado-wildfire-
activity-near-historic-low-this-year.
Birks, H.J. and Gordon, A.D. (1985). Numerical methods in Quaternary pollen
analysis. Academic Press Inc. London Ltd.
Blikra, L.H. andNemec, W. (1998). Postglacial colluvium in western Norway:
depositional processes, facies and palaeoclimatic record. Sedimentology 45, 909-
959.


68
Briles, C.E., Whitlock, C., and Meltzer, D.J. (2012). Last glacial-interglacial
environments in the southern Rocky Mountains, USA and implications for
Younger Dryas-age human occupation. Quaternary Research 77, 96-103.
Brugger, K. A. (2006). Late Pleistocene climate inferred from the reconstruction of the
Taylor River glacier complex, southern Sawatch Range, Colorado.
Geomorphology 75, 318-329.
Caffrey, M.A. and Doerner, J.P. (2012). A 7000-year record of environmental change,
Bear Lake, Rocky Mountain National Park, USA. Physical Geography 33, 438-
456.
Calder, J.W., Stopka, C.J., and Shuman, B.N. (2014). High-elevation fire regimes in
subalpine ribbon forests during the Little Ice Age and Medieval Period along the
Continental Divide, Colorado, U.S.A. Rocky Mountain Geology 49, 75-90.
Colorado Geological Survey. (2015). Geologic Hazards: Avalanches (Snow).
http://coloradogeologicalsurvey.org/geologic-hazards/avalanches-snow/defmition/
Cook, E.R., Woodhouse, C.A., Eakin, M.C., Meko, D.M., and Stahle, D.W. (2004).
Long-term aridity changes in the western United States. Science 306, 1015-1018.
Dean, W.E. (1974). Determination of carbonate and organic matter in calcareous
sediments by loss on ignition comparison to other methods. Journal of
Sedimentary Petrology 44, 242-248.
Doesken, N.J., Pielke, R.A., and Bliss, O.A.P. (2003). Climate of Colorado.
Climatography of the United States 60
http://climate.atmos.colostate.edu/climateofcolorado.php.
Emslie, S.D., Stiger, M., and Wambach, E. (2005). Packrat middens and late Holocene
environmental change in southwestern Colorado. The Southwestern Naturalist 50,
209-215.
Fall, P.L. (1997a). Fire history and composition of the subalpine forest of western
Colorado during the Holocene. Journal of Biogeography 24, 309-325.
Fall, P.L. (1992a). Pollen accumulation in a montane region of Colorado, USA: a
comparison of moss polsters, atmospheric traps, and natural basins. Review of
Palaeobotany andPalynology 72, 169-197.
Fall, P.L. (1992b). Spatial patterns of atmospheric pollen dispersal in the Colorado Rocky
Mountains, USA. Review of Palaeobotany and Palynology 74, 293-313.


69
Fall, P.L. (1997b). Timberline fluctuations and late Quaternary paleoclimates in the
southern Rocky Mountains, Colorado. GSA Bulletin 109, 1306-1320.
Falk, D.A., Heyerdahl, E.K., Brown, P.M., Farris, C., Fule, P.Z., McKenzie, D.,
Swetnam, T.W., Taylor, A.H., and Van Horne, M.L. (2011). Multi-scale controls
of historical forest-fire regimes: new insights from fire-scar networks. Frontiers
in Ecology and the Environment 9, 446-454.
Feiler, E.J., Anderson, S.R., and Koehler, P.A. (1997). Late Quaternary
paleoenvironments of the White River plateau, Colorado, U.S.A. Arctic and
Alpine Research 29, 53-62.
Froyd, C. A. and Willis, K. J. (2008). Emerging issues in biodiversity & conservation
management: the need for a palaeoecological perspective. Quaternary Science
Reviews 27, 1723-1732.
Gedye, S.J., Jones, R.T., Tinner, W., Ammann, B., and Oldfield, F. (2000). The use of
mineral magnetism in the reconstruction of fire history: a case study from Lago di
Origlio, Swiss Alps. Palaeogeography, Palaeoclimatology, Palaeoecology 164,
101-110.
Gleason, H.A. (1939). The individualistic concept of the plant association. American
Midland Naturalist 21, 92-110.
Grove, J.M. (1988). The Little Ice Age. Methuen, London, UK.
Higuera, P.E., Briles, C.E., and Whitlock, C. (2014). Fire-regime complacency and
sensitivity to centennial-through millennial-scale climate change in Rocky
Mountain subalpine forests, Colorado, USA. Journal of Ecology 102, 1429-1441.
Higuera, P.E., Gavin, D.G., Barlein, P.J., and Halett, D.J. (2010). Peak detection in
sediment charcoal records: impacts of alternative data analysis methods on fire
history interpretations. International Journal of Wildland Fire 19, 996-1014.
Jackson, S.T. and Overpeck, J.T. (2000). Responses of plant populations and
communities to environmental changes of the late Quaternary. Paleobiology 26,
194-220.
Jimenez-Moreno, G. and Anderson, S.R. (2012). Pollen and macrofossil evidence of late
Pleistocene and Holocene treeline fluctuations from an alpine lake in Colorado,
USA. The Holocene 23, 68-77.
Jimenez-Moreno, G., Anderson, S.R., Atudorei, V., and Toney, J.L. (2011). A high-
resolution record of climate, vegetation, and fire in the mixed conifer forest of
northern Colorado, USA. Geological Society of America Bulletin 123, 240-254.


70
Jimenez-Moreno, G., Fawcett, P.J., and Anderson, S.R. (2008). Millennial- and
centennial-scale vegetation and climate changes during the late Pleistocene and
Holocene from northern New Mexico (USA). Quaternary Science Reviews 27,
1442-1452.
Johnson, B.G., Jimenez-Moreno, G., Eppes, M.C., Diemer, J.A., and Stone, J.R. (2013).
A multiproxy record of postglacial climate variability from a shallowing, 12-m
deep sub-alpine bog in the southeastern San Juan Mountains of Colorado, USA.
The Holocene 0, 1-11.
Jones, G.D., Bryant, V.M., Lieux, M.H., Jones, S.D., and Lingren, P.D. (1995). Pollen
of the southeastern United States: with emphasis on melissopalynology and
entomopalynology. American Association of Stratigraphic Palynologists
Foundation, College Station, TX.
Kapp, R.O., Davis, O.K., and King, J.E. (2000). Ronald O. Kapps pollen and spores.
American Association of Stratigraphic Palynologists Foundation, College Station,
TX.
Keane, R.E., Hessburg, P.F., Landres, P.B., and Swanson, F.J. (2009). The use of
historical range and variability (HRV) in landscape management. Forest Ecology
and Management 258, 1025-1037.
Kent, B., Gebert, K., McCaffrey, S., Martin, W., Calkin, D., Schuster, E., Martin, I.,
Bender, H.W., Alward, G., Kumagai, Y., Cohn, P.J., Carroll, M., Williams, D.,
and Ekarius, C. (2003). Social and economic issues of the Hayman Fire.
http://www.fs.fed.us/rm/pubs/rmrs_gtrl 14/rmrs_gtrl 14 315 395
Kitzberger, T., Brown, P.M., Heyerdahl, E.K., Swetnam, T.W., and Veblen, T.T. (2007).
Contingent Pacific-Atlantic Ocean influence on multicentury wildfire synchrony
over western North America. Proceedings of the National Academy of Sciences
104, 543-548.
Kulakowski, D. and Veblen, T.T. (2007). Effect of prior disturbance on the extent and
severity of wildfire in Colorado subalpine forests. Ecology 88, 759-769.
Kulakowski D., Veblen, T.T., and Bebi, P. (2003). Effects of fire and spruce beetle
outbreak legacies on the disturbance regime of a subalpine forest in Colorado.
Journal of Biogeography 30, 1445-1456.
Long, C.J., Whitlock, C., Bartlein, P.J. and Millspaugh, S.H. (1998). A 9000-year fire
history from the Oregon Coast Range, based on a high-resolution charcoal study.
Canadian Journal of Forestry 28, 774-787.
Lynch, E.A. (1996). The ability of pollen from small lakes and ponds to sense fine-scale
vegetation patterns in the Central Rocky Mountains, USA. Review of
Palaeobotany andPalynology 94, 197-210.


71
Mann, M. (2002a). Little Ice Age. The Earth system: physical and chemical
dimensions of global environmental change: Volume 1 Encyclopedia of Global
Environmental Change, John Wiley & Sons, Ltd, Chichester, pp. 504-509.
Mann, M. (2002b). Medieval Climatic Optimum. The Earth system: physical and
chemical dimensions of global environmental change: Volume 1 Encyclopedia of
Global Environmental Change, John Wiley & Sons, Ltd, Chichester, pp. 514-516.
Mann, M. (2002c). The value of multiple proxies. Science 297, 1481-1482.
Mann, M., Zhang, A., Hughes, M.K., Bradley, R.S., Miller, S.K., and Rutherford, S.
(2008). Proxy-based reconstructions of hemispheric and global surface
temperature variations over the past two millennia. Proceedings of the National
Academy of Sciences 105, 13252-13257.
Meko, D.M., Woodhouse, C.A., Baisan, C.A., Knight, T., Lukas, J.J., Hughes, M.K., and
Salzer, M.W. (2007). Medieval drought in the upper Colorado River Basin.
Geophysical Research Letters 34, L10705.
Minckley, T.A., Shriver, R.K., and Shuman, B. (2012). Resilience and regime change in a
southern Rocky Mountain ecosystem during the past 17000 years. Ecological
Monographs 82, 49-68.
Moore, M.M., Covington, W.W., and Fule, P.Z. (1999). Reference conditions and
ecological restoration: a southwestern ponderosa pine perspective. Ecological
Applications 9, 1266-1277.
Morecroft, M.D., Crick, H.Q.P., Duffield, S.J., and Macgregor, N.A. (2012). Resilience
to climate change: translating principles into practice. Journal of Applied Ecology
49, 547-551.
Morris, J.L. (2013). Using lake sediment records to reconstruct bark beetle disturbances
in western North America. Frontiers of Biogeography 5, 219-226.
Morris, J.L., Mustaphi, C.J.C., Carter, V.A., Watt, J., Derr, K., Pisaric, M.F.J., Anderson,
R.S., Brunelle, A.R. (2014). Do bark beetle remains in lake sediments correspond
to severe outbreaks? A review of published and ongoing research. Quaternary
International http ://dx. doi. org/10.1016/j. quaint.2014.03.022
National Oceanic and Atmospheric Administration (NOAA). (2014). ENSO Impacts on
United States Winter Precipitation and Temperature. Climate Prediction Center.
http://www.cpc.ncep.noaa.gov.
Polyak, V.J. and Asmerom, Y. (2001). Late Holocene climate and cultural changes in the
southwestern United States. Science 294, 148-151.


72
Ray, A.J., Barsugli, J.J., Averty, K.B., Wolter, K., Hoerling, M., Doesken, N., Udall, B.,
and Webb, R.S. (2008). Climate change in Colorado: a synthesis to support water
resources management and adaptation, (ed. W.W.A. CU-NOAA, Colorado Water
Conservation Board). CU-Boulder University Communications, Marketing &
Creative Services.
Reimer, P.J., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk, R.C., Buck, C.E.,
Cheng, EL, Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P.,
Haflidason, EL, Hajdas, I., Hatt, A.C., Heaton, T.J., Hogg, A.G., Hughen, K.A.,
Kaiser, K.F., Kromer, B., Manning, S.W., Niu, M., Reimer, R.W., Richards, D.A.,
Scott, E.M., Southon, J.R., Turney, C.S.M., and van der Plicht, J. (2013). IntCall3
and Marinel3 radiocarbon age calibration curves 0-50000 years cal BP.
Radiocarbon 55, 1869-1887.
Schoennagel, T., Veblen, T.T., Romme, W.H., Sibold, J.S., and Cook, E.R. (2005).
ENSO and PDO variability affect drought-induced fire occurrence in Rocky
Mountain subalpine forests.
http://spot.colorado.edu/~schoenna/images/Schoennagel2005EcoApps.pdf
Sherriff, R.L, Veblen, T.T., and Sibold, J.S. (2001). Fire history in high elevation
subalpine forests in the Colorado Front Range. Ecoscience 8, 369-380.
Shulmeister, J., Rodbell, D.T., Gagan, M.K., and Seltzer, G.O. (2006). Inter-hemispheric
linkages in climate change: paleo-perspectives for future climate change. Climate
of the Past 2,167-185.
Shuman, B., Henderson, A.K., Colman, S.M., Stone, J.R., Fritz, S.C., Stevens, L.R.,
Power, M.J., and Whitlock, C. (2009). Holocene lake-level trends in the Rocky
Mountains, U.S.A. Quaternary Science Reviews 28, 1861-1879.
Sibold, J.S. and Velen, T.T. (2006). Relationships of subalpine forest fires in the
Colorado Front Range with interannual and multidecadal-scale climatic variation.
Journal of Biogeography 33, 833-842.
Sibold, J.S., Veblen, T.T., Chipko, K., Lawson, L., Mathis, E., and Scott, J. (2007).
Influences of secondary disturbances on lodgepole pine stand development in
Rocky Mountain National Park. Ecological Applications 17, 1638-1655.
Smol, J.P. and Cumming, B.F. (2000). Tracking long-term changes in climate using algal
indicators in lake sediments. Journal of Phycology 36, 986-1011.
Stuiver, M. and Polach, H. A. (1977). Discussion: reporting of 14C data. Radiocarbon
19, 355-363.


73
Swetnam, T.W., Allen, C.D., and Betancourt, J.L. (1999). Applied historical ecology:
using the past to manage for the future. Ecological Applications 9, 1189-1206.
Toney, J.L. and Anderson, S.R. (2006). A postglacial palaeoecological record from the
San Juan Mountains of Colorado USA: fire, climate and vegetation history. The
Holocene 16, 505-517.
Trouet, V., Diaz, H.F., Wahl, E.R., Viau, A.E., Graham, R., Grahan, N., and Cook, E.R.
(2013). A 1500-year reconstruction of annual mean temperature for temperate
North America on decadal-to-multidecadal time scales. Environmental Research
Letters 8, 1-10.
United States Forest Service (USFS). (2013). Aerial Forest Health Survey Results.
http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprd3791328.pdf.
United States Geological Survey (USFS). (2014). Geologic Provinces of the United
States: Rocky Mountains.
http://geomaps.wr.usgs.gov/parks/province/rockymtn.html.
Vasskog, K., Nesje, A., Storen, E.N., Waldmann, N., Chapron, E., and Ariztegui, D.
(2011). A Holocene records of snow-avalanche and flood activity reconstructed
from a lacustrine sedimentary sequence in Oldevatnet, western Norway. The
Holocene 21, 597-614.
Veblen, T.T., Hadley, K.S., Reid, M.S., and Rebertus, A.J. (1991). The response of
subalpine forests to spruce beetle outbreak in Colorado. Ecology 72, 213-231.
Veblen, T.T., Hadley, K.S., Nel, E.M., Kitzberger, T., Reid, M., and Villalba, R. (1994).
Disturbance regime and disturbance interactions in a Rocky Mountain subalpine
forest. Journal of Ecology 82, 125-135.
Veblen, T.T., Kitzberger, T., and Donnegan, J. (2000). Climatic and human influences on
fire regimes in ponderosa pine forests in the Colorado Front Range. Ecological
Applications 10, 1178-1195.
Vierling, L.A. (1998). Palynological evidence for late- and postglacial environmental
change in central Colorado. Quaternary Research 49, 222-232.
Weber, W.A. and Wittmann, R.C. (2012). Colorado Flora Eastern Slope: A field guide
to the vascular plants. University Press of Colorado, Boulder, CO.
Weber, W.A. and Wittmann, R.C. (2011). Colorado Flora Western Slope: A field guide
to the vascular plants. University Press of Colorado, Boulder, CO.
White, P.S. and Pickett, S.T. (1985). The Ecology of natural disturbance and patch
dynamics. Academic Press, Orlando, FL.


74
Whitlock, C. and Larsen, C.P.S. (2001). Charcoal as a Fire Proxy. In: Smol, J.P., Birks,
H.J.B., Last, W.M. (Eds.), Tracking Environmental Change Using Lake
Sediments: Volume 3 Terrestrial, Algal, and Siliceous indicators, Kluwer
Academic Publishers, Dordrecht, pp. 75-97.
Whitlock, C., Briles, C.E., Fernandez, M.C., and Gage, J. (2012). Holocene vegetation,
fire and climate history of the Sawtooth Range, central Idaho, USA. Quaternary
Research 15, 114-124.
Whitlock, C., Higuera, P.E., McWethy, D.B., and Briles, C.E. (2010). Paleoecological
perspectives on fire ecology: revisiting the fire-regimes concept. The Open
Ecology Journal 3, 6-23.
Whitlock, C., Shafer, S.L., and Marlon, J. (2003). The role of climate and vegetation
change in shaping past and future fire regimes in the northwestern US and the
implications for ecosystem management. Forest Ecology and Management 178,
5-21.
Willis, K.J. and Bhagwat, S.A. (2010). Questions of importance to the conservation of
biological diversity: answers from the past. Climate of the Past 6, 759-769.
Willis, K.J., Bailer, R.M., Bhagwat, S.A., and Birks, H.J.B. (2010). Biodiversity
baselines, thresholds and resilience: testing predictions and assumptions using
palaeoecological data. Trends in Ecology and Evolution 25, 583-591.
Wright Jr., H.E., Mann, D.H., and Glaser, P.H. (1983). Piston cores for peat and lake
sediments. Ecology 65, 657-659.


75
APPENDIX A
KEY TO ABBREVIATIONS USED IN
APPENDICES B, C, D, AND E


Age (cal yr BP)
Depth (cm)
MS (cgs)
%0
%C
Volume (cm3)
Charcoal count
Count/Volume
Deposition (yr/cm)
Age in calendar years BP
Depth in centimeters
Magnetic susceptibility in
Percent organic matter
Percent carbonates
Charcoal sample volume
# of pieces per cm3
Charcoal concentration
Sediment deposition rate


77
Pollen Count Abbreviations
Name Abbreviation
Total Pinus spp. PN
Total Picea spp. PI
Pseudotsuga spp. PS
Abies spp. AB
Cupressaceae CU
Ephedra spp. EP
Poaceae PO
Quercus spp. QU
Artemisia spp. AR
Asteraceae AS
Ambrosia spp. AM
Amaranthaceae sd. AN
Sarcobatus spp. SA
Salsola spp. SS
Chenopodium spp. CH
Rosaceae RO
Spiraea spp. SP
Eriogonum spp. ER
Polygonum spp. PY
Alnus spp. AL
Be tula spp. BE
Umbelliferae UM
Fabaceae FA
Liliaceae LI
Malvaceae MA
Plantago spp. PL
Thalictrum spp. TH
Salix spp. SX
Myriophyllum spp. MY
Selaginella spp. SE
Equisetum spp. EQ
Botrychium spp. BO
Dryopteris spp. DR
Degraded DE
Tricolpate grain w/clave TR
Unknown UN
Lycopodium spike LY
Terrestrial pollen ZP
Terrestrial & aquatic pollen and spores IT


78
APPENDIX B
MAGNECTIC SUSCEPTIBILITY
FOR MIRROR LAKE


79
Depth Age MS
(cm) (cal yr BP) (cgs)
6.5 11 1.82E+00
7 17 2.86E+00
7.5 23 1.19E+00
8 29 1.06E+00
8.5 35 3.18E+00
9 41 3.41E+00
9.5 47 1.48E+00
10 53 6.14E-01
10.5 59 1.63E+00
11 65 2.52E+00
11.5 71 2.02E+00
12 78 2.10E+00
12.5 84 2.62E+00
13 90 2.48E+00
13.5 96 2.36E+00
14 102 2.18E+00
14.5 108 2.51E+00
15 115 1.54E+00
15.5 121 1.83E+00
16 127 3.44E+00
16.5 134 4.13E+00
17 140 4.57E+00
17.5 146 2.63E+00
18 153 1.97E+00
18.5 159 1.82E+00
19 166 3.00E+00
19.5 172 2.59E+00
20 178 2.05E+00
20.5 185 2.95E+00
21 192 1.08E+00
21.5 198 1.18E+00
22 205 1.66E+00
22.5 211 2.15E+00
23 218 1.74E+00
23.5 224 1.14E+00
24 231 1.27E+00
24.5 238 3.45E+00
25 245 4.59E+00
25.5 251 3.83E+00
26 258 1.92E+00
Depth Age MS
(cm) (cal yr BP) (cgs)
26.5 265 1.75E+00
27 272 3.09E+00
27.5 278 1.25E+00
28 285 1.08E+00
28.5 292 1.99E+00
29 299 2.51E+00
29.5 306 2.10E+00
30 313 2.05E+00
30.5 320 2.03E+00
31 327 8.06E-01
31.5 334 1.09E+00
32 341 1.24E+00
32.5 348 2.02E+00
33 355 2.31E+00
33.5 362 1.66E+00
34 369 3.13E+00
34.5 376 1.33E+00
35 384 2.19E+00
35.5 391 2.90E+00
36 398 3.72E+00
36.5 405 2.24E+00
37 412 1.66E+00
37.5 420 2.49E+00
38 427 2.87E+00
38.5 434 4.26E+00
39 442 4.08E+00
39.5 449 3.62E+00
40 456 2.88E+00
40.5 464 4.60E+00
41 471 4.95E+00
41.5 479 3.21E+00
42 486 2.31E+00
42.5 494 2.58E+00
43 501 2.56E+00
43.5 509 1.40E+00
44 516 9.82E-01
44.5 524 2.09E+00
45 532 1.82E+00
45.5 539 3.63E+00
46 547 1.47E+00


80
Depth Age MS
(cm) (cal yr BP) (cgs)
46.5 555 2.04E+00
47 562 3.39E+00
47.5 570 3.43E+00
48 578 4.95E+00
48.5 586 5.10E+00
49 593 3.28E+00
49.5 601 2.18E+00
50 609 2.52E+00
50.5 617 3.38E+00
51 625 3.73E+00
51.5 633 1.08E+00
52 641 8.42E+00
52.5 649 3.83E+00
53 657 4.54E+00
53.5 665 3.36E+00
54 673 1.79E+00
54.5 681 3.02E+00
55 689 3.28E+00
55.5 697 3.84E+00
56 705 4.30E+00
56.5 713 3.27E+00
57 721 3.34E+00
57.5 730 3.07E+00
58 738 2.87E+00
58.5 746 3.14E+00
59 754 3.89E+00
59.5 763 2.04E+00
60 771 9.31E-01
60.5 779 3.38E+00
61 788 5.00E+00
61.5 796 3.20E+00
62 804 1.73E+00
62.5 813 1.63E+00
63 821 2.32E+00
63.5 830 9.11E-01
64 838 2.68E+00
64.5 847 4.30E+00
65 855 1.91E+00
65.5 864 7.00E-01
66 872 6.93E-01
Depth Age MS
(cm) (cal yr BP) (cgs)
66.5 881 1.23E+00
67 890 1.76E+00
67.5 898 1.31E+00
68 907 1.40E+00
68.5 916 2.40E+00
69 924 2.03E+00
69.5 933 2.10E+00
70 942 1.48E+00
70.5 951 1.37E+00
71 959 1.31E+00
71.5 968 1.51E+00
72 977 1.15E+00
72.5 986 1.82E+00
73 995 1.24E+00
73.5 1004 1.19E+00
74 1013 2.18E+00
74.5 1022 2.72E+00
75 1031 4.10E+00
75.5 1040 4.09E+00
76 1049 2.79E+00
76.5 1058 2.26E+00
77 1067 1.42E+00
77.5 1076 1.68E+00
78 1085 1.47E+00
78.5 1094 1.38E+00
79 1103 2.37E+00
79.5 1113 1.37E+00
80 1122 1.55E+00
80.5 1131 1.30E+00
81 1140 9.22E-01
81.5 1150 5.70E-01
82 1159 5.44E-01
82.5 1168 5.75E-01
83 1178 7.65E-01
83.5 1187 2.13E+00
84 1196 2.22E+00
84.5 1206 2.86E+00
85 1215 2.58E+00
85.5 1225 2.26E+00
86 1234 1.26E+00


81
Depth Age MS
(cm) (cal yr BP) (cgs)
86.5 1244 1.61E+00
87 1253 2.49E+00
87.5 1263 2.46E+00
88 1272 2.38E+00
88.5 1282 2.71E+00
89 1292 1.99E+00
89.5 1301 2.10E+00
90 1311 1.66E+00
90.5 1321 2.75E+00
91 1330 1.91E+00
91.5 1340 1.15E+00
92 1350 2.21E+00
92.5 1360 2.03E+00
93 1369 3.63E+00
93.5 1379 3.32E+00
94 1389 2.46E+00
94.5 1399 2.21E+00
95 1409 2.74E+00
95.5 1419 2.50E+00
96 1429 2.10E+00
96.5 1439 2.27E+00
97 1449 1.40E+00
97.5 1459 2.28E+00
98 1469 1.95E+00
98.5 1479 2.06E+00
99 1489 1.01E+00
99.5 1499 1.05E+00
100 1509 2.09E+00
100.5 1519 3.14E+00
101 1530 1.98E+00
101.5 1540 1.81E+00
102 1550 1.61E+00
102.5 1560 1.80E+00
103 1570 1.56E+00
103.5 1581 2.05E+00
104 1591 1.85E+00
104.5 1601 2.88E+00
105 1612 3.53E+00
105.5 1622 3.56E+00
106 1633 3.34E+00
Depth Age MS
(cm) (cal yr BP) (cgs)
106.5 1643 1.71E+00
107 1653 2.08E+00
107.5 1664 2.55E+00
108 1674 2.63E+00
108.5 1685 1.96E+00
109 1695 9.43E-01
109.5 1706 1.31E+00
110 1717 1.01E+00
110.5 1727 1.79E+00
111 1738 1.49E+00
111.5 1748 2.51E+00
112 1759 1.92E+00
112.5 1770 1.67E+00
113 1781 2.33E+00
113.5 1791 1.91E+00
114 1802 1.91E+00
114.5 1813 1.86E+00
115 1824 1.88E+00
115.5 1835 1.98E+00
116 1845 2.95E+00
116.5 1856 2.25E+00
117 1867 7.77E-01
117.5 1878 1.03E+00
118 1889 2.11E+00
118.5 1900 1.78E+00
119 1911 1.81E+00
119.5 1922 1.72E+00
120 1933 1.21E+00
120.5 1944 6.71E-01
121 1955 1.99E+00
121.5 1966 2.08E+00
122 1977 2.94E+00
122.5 1989 2.50E+00
123 2000 2.64E+00
123.5 2011 2.12E+00
124 2022 2.41E+00
124.5 2033 1.18E+00
125 2045 1.17E+00
125.5 2056 1.81E+00
126 2067 1.95E+00


82
Depth Age MS
(cm) (cal yr BP) (cgs)
126.5 2079 1.54E+00
127 2090 1.87E+00
127.5 2101 1.25E+00
128 2113 1.62E+00
128.5 2124 1.42E+00
129 2136 8.81E-01
129.5 2147 1.21E+00
130 2159 1.38E+00
130.5 2170 7.81E-01
131 2182 1.15E+00
131.5 2193 1.22E+00
132 2205 7.34E-01
132.5 2217 1.12E+00
133 2228 1.19E+00
133.5 2240 1.62E+00
134 2252 1.74E+00
134.5 2263 1.59E+00
135 2275 1.27E+00
135.5 2287 1.31E+00
136 2298 9.80E-01
136.5 2310 1.01E+00
137 2322 1.06E+00
137.5 2334 1.03E+00
138 2346 1.63E+00
138.5 2358 1.80E+00
139 2370 2.27E+00
139.5 2381 2.93E+00
140 2393 1.99E+00
140.5 2405 1.24E+00
141 2417 1.93E+00
141.5 2429 1.06E+00
142 2441 1.90E+00
142.5 2454 1.16E+00
143 2466 2.14E+00
143.5 2478 2.11E+00
144 2490 2.08E+00
144.5 2502 1.73E+00
145 2514 2.11E+00
145.5 2526 2.02E+00
146 2539 2.90E+00
Depth Age MS
(cm) (cal yr BP) (cgs)
146.5 2551 2.47E+00
147 2563 2.30E+00
147.5 2575 3.14E+00
148 2588 2.42E+00
148.5 2600 1.67E+00
149 2613 3.97E+00
149.5 2625 1.80E+00
150 2637 9.43E-01
150.5 2650 1.31E+00
151 2662 1.58E+00
151.5 2675 3.04E+00
152 2687 2.75E+00
152.5 2700 2.95E+00
153 2712 2.63E+00
153.5 2725 3.09E+00
154 2737 2.76E+00
154.5 2750 2.94E+00
155 2763 3.13E+00
155.5 2775 2.90E+00
156 2788 3.14E+00
156.5 2801 4.02E+00
157 2813 3.81E+00
157.5 2826 2.33E+00
158 2839 2.20E+00
158.5 2852 2.38E+00
159 2865 2.14E+00
159.5 2877 8.52E-01
160 2890 7.37E-01
160.5 2903 8.04E-01
161 2916 1.18E+00
161.5 2929 2.39E+00
162 2942 1.99E+00
162.5 2955 2.12E+00
163 2968 2.10E+00
163.5 2981 2.85E+00
164 2994 2.29E+00
164.5 3007 1.56E+00
165 3020 1.84E+00
165.5 3033 2.79E+00
166 3046 2.23E+00


83
Depth Age MS
(cm) (cal yr BP) (cgs)
166.5 3060 1.62E+00
167 3073 1.30E+00
167.5 3086 2.23E+00
168 3099 2.00E+00
168.5 3112 2.43E+00
169 3126 1.74E+00
169.5 3139 1.39E+00
170 3152 1.92E+00
170.5 3166 2.44E+00
171 3179 1.77E+00
171.5 3192 2.09E+00
172 3206 2.21E+00
172.5 3219 2.23E+00
173 3233 2.39E+00
173.5 3246 2.27E+00
174 3260 2.56E+00
174.5 3273 1.86E+00
175 3287 1.78E+00
175.5 3300 2.69E+00
176 3314 2.53E+00
176.5 3328 1.91E+00
177 3341 1.70E+00
177.5 3355 2.14E+00
178 3369 1.72E+00
178.5 3382 8.99E-01
179 3396 9.13E-01
179.5 3410 6.52E-01
180 3424 8.04E-01
180.5 3437 2.14E+00
181 3451 2.09E+00
181.5 3465 1.94E+00
182 3479 7.71E-01
182.5 3493 1.35E+00
183 3507 2.36E+00
183.5 3521 1.09E+00
184 3535 1.14E+00
184.5 3549 1.69E+00
185 3563 1.82E+00
185.5 3577 2.52E+00
186 3591 1.88E+00
Depth Age MS
(cm) (cal yr BP) (cgs)
186.5 3605 1.58E+00
187 3619 2.23E+00
187.5 3633 1.95E+00
188 3647 2.52E+00
188.5 3661 2.69E+00
189 3675 2.55E+00
189.5 3690 2.98E+00
190 3704 3.17E+00
190.5 3718 3.92E+00
191 3732 3.13E+00
191.5 3747 3.00E+00
192 3761 3.51E+00
192.5 3775 5.02E+00
193 3790 3.38E+00
193.5 3804 1.59E+00
194 3819 2.14E+00
194.5 3833 2.62E+00
195 3848 2.01E+00
195.5 3862 2.10E+00
196 3877 1.28E+00
196.5 3891 2.01E+00
197 3906 2.37E+00
197.5 3920 1.50E+00
198 3935 1.35E+00
198.5 3949 2.73E+00
199 3964 2.61E+00
199.5 3979 2.12E+00
200 3993 2.25E+00
200.5 4008 1.59E+00


84
APPENDIX C
LOSS-ON-IGNITION
FOR MIRROR LAKE


Depth (cm) Age (cal yr BP) %0 %C
7 17 9.8 2.2
9 41 8.3 2.0
11 65 7.7 1.6
13 90 8.9 1.9
15 115 8.3 1.5
17 140 7.3 1.1
19 166 10.5 0.9
21 192 10.6 1.0
23 218 17.0 1.3
25 245 7.7 1.6
27 272 7.7 1.4
29 299 8.4 0.8
31 327 7.8 1.2
33 355 8.6 2.1
35 384 8.1 1.4
37 412 8.5 1.6
39 442 5.9 2.8
41 471 6.1 0.9
43 501 4.0 1.1
45 532 8.2 1.7
47 562 5.9 1.4
49 593 4.1 1.1
51 625 4.4 1.0
53 657 3.1 1.0
55 689 4.7 1.2
57 721 5.4 1.4
59 754 7.4 1.8
61 788 9.9 1.7
63 821 13.0 2.1
65 855 5.4 1.2
67 890 10.3 3.0
70 942 8.1 1.3
72 977 8.0 1.1
74 1013 7.5 1.1
76 1049 3.6 0.7
78 1085 15.1 1.7
80 1122 2.2 0.4
82 1159 21.9 1.5
84 1196 6.9 1.2


Depth (cm) Age (cal yr BP) %0 %C
86 1234 8.4 0.9
88 1272 6.9 1.1
90 1311 6.9 0.9
92 1350 7.3 0.9
94 1389 6.5 0.9
96 1429 6.2 0.9
98 1469 7.9 1.0
100 1509 9.5 1.3
102 1550 8.4 1.0
104 1591 12.5 1.1
105 1612 9.1 0.9
109 1695 10.3 0.7
111 1738 17.9 1.0
113 1781 9.4 0.3
115 1824 7.6 0.9
117 1867 15.2 0.9
119 1911 15.1 1.3
121 1955 17.0 1.0
123 2000 10.6 1.1
125 2045 17.6 1.5
127 2090 17.7 1.6
129 2136 18.6 1.8
131 2182 21.5 1.5
133 2228 12.6 0.9
135 2275 14.8 1.5
137 2322 11.4 0.9
139 2370 11.3 1.4
141 2417 12.0 1.1
143 2466 23.5 1.8
145 2514 12.6 2.0
147 2563 7.8 1.3
149 2613 9.5 1.4
151 2662 9.9 1.4
153 2712 10.0 1.5
155 2763 11.5 1.8
157 2813 10.8 1.8
159 2865 13.4 1.9
161 2916 12.8 1.8
163 2968 20.2 2.6


87
Depth (cm) Age (cal yr BP) %0 %C
165 3020 18.7 2.2
167 3073 15.0 2.7
169 3126 13.4 2.3
171 3179 12.8 2.4
173 3233 13.6 2.7
175 3287 15.8 2.7
111 3341 9.6 1.6
179 3396 20.4 2.7
181 3451 12.0 1.6
183 3507 20.1 2.3
184.5 3549 14.2 1.6
186.5 3605 12.3 1.4
188.5 3661 11.7 1.7
190.5 3718 12.6 2.1
192.5 3775 8.2 1.6
194.5 3833 15.8 2.6
196.5 3891 15.9 2.4
198.5 3949 15.4 2.1
200.5 4008 17.0 2.8


88
APPENDIX D
POLLEN COUNTS AND SUMS
FOR MIRROR LAKE


89
Depth (cm) Age (cal yr BP) PN PI PS AB CU EP PO QU AR
6 6 171.5 28.5 1 3 1 2 8 6 30
10 53 124.5 19.5 0 4.5 2 0 9 0 21
14 102 173 39 0 2 2 0 3 4 30
17.5 146 124 13.5 0 3 0 0 12 2 18
20 178 153 29 1 2 5 2 7 3 32
24 231 188.5 21.5 2 4 0 0 7 7 25
28 285 139 13 1 1 1 1 3 2 25
32 341 190 21 1 3 0 0 11 3 12
34 369 183 20 1 7 0 0 9 6 21
38 427 121 8.5 1 0 2 1 5 0 27
42 486 186.5 15 0 6.5 1 0 5 3 22
44 516 213 16 0 3 0 0 5 3 23
46 547 144.5 12 1 1.5 3 0 2 2 13
48 578 206 12 1 6.5 1 3 5 0 23
52 641 164.5 12.5 1 2.5 0 0 10 2 22
58 738 201 13 0 1.5 0 0 14 3 24
62 804 103 20 2 2 0 1 6 1 19
64 838 176 12.5 1 4 0 0 9 1 20
69 924 172.5 11 0 0 1 0 8 2 23
74 1013 192 12.5 1 2.5 0 0 10 1 19
80.5 1131 178 21 1 2.5 0 0 15 2 18
87 1253 165.5 18 0.5 5 0 0 9 3 26
93 1369 156 23 1 4 1 0 10 3 19
99 1489 171 8.5 0 0 0 1 9 1 17
105 1612 176.5 22.5 1 4 0 0 13 1 21
110 1717 171.5 16.5 1 1.5 0 0 10 0 21
117 1867 166.5 10.5 0 1 0 2 6 1 50
121 1955 174.5 13 1 0.5 0 0 7 3 19
125 2045 197.5 9.5 1 0 1 0 2 1 28
129 2136 163 10 0 0 0 1 3 5 25
133 2228 141 8.5 1 0 0 0 4 2 62
135 2275 175 16.5 0 2 0 0 18 3 20
141 2417 108.5 11.5 2 3 0 0 10 0 27
149 2613 94 23 0 6 1 0 10 2 25
157 2813 141.5 34 5 5 1 1 6 1 33
159 2865 203.5 19.5 0 4.5 0 0 8 0 23
161 2916 202.5 20.5 2 4 1 0 6 2 17
163 2968 171.5 16 1 2.5 0 0 11 2 34


90
Depth (cm) Age (cal yr BP) PN PI PS AB cu EP PO QU AR
165 3020 124.5 28.5 2 5 1 0 7 3 39
173 3233 160.5 38.5 1 4 4 2 8 1 27
181 3451 146.5 25.5 4 2 0 1 2 0 40
187.5 3633 201.5 23 0 2.5 0 0 6 0 16
195.5 3862 164.5 25 0 2.5 0 0 13 2 17
201.5 4038 182 21.5 0 0.5 0 2 8 0 21


Full Text

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ABST RACT Picea Abies Pinus Pinus

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DEDICATION

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ACKNOWLEDGEMENTS

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TABLE OF CONTENTS CHAPTER I. INTRODUCTION 1 II. BACKGROUND INFORMATION 5 Physical environment Geology 5 Climate 6 Vegetation 7 Disturbance 8 Proxy data and natural archives 11 Regional environmental history 12 Ecosystem resilience and range of natural variability 16 III. SITE DESCRIPTION 19 IV. METHODS AND DATA ANALYSIS 24 Field methods 24 Laboratory methods 24 Core preparation, lithology, and chronology 24 Magnetic susceptibility & loss-on-ignition 25 Pollen 25 Charcoal 27 Data analysis 27 Polle n 27 C harcoal V. RESULTS 31 Chronology 31 Lithology 33 Magnetic susceptibility & loss-on-ignition 35 Pollen 35 Charcoal 39 VI. DI SCUSSION 43 The climate of the late Holocene (4000 yr BP to present) 43 29 5

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Ecosystem response to late Holocene climatic conditions 49 Summer-dominant precipitation regime 49 Winter-dominant precipitation regime 50 Ins ect outbreaks 56 Avalanches 57 Management impli cations 59 VII. CONCLUSION 64 REFERENCES 67 APPENDIX A. KEY TO A BBREVIATIONS USED IN A PPENDICES B, C, D, AND E BY ORDER OF APPEARANCE 75 B. MAGNECTIC SUSCEPTIBILITY FOR MI RROR LAKE 78 C. LOSS-ON-IGNITION FOR MI RROR LAKE 84 D. PO LLEN COUNTS AND SUMS FOR MI RROR LAKE 88 E. CHA RCOAL SAMPLE VOLUME, CHA RCOAL COUNT, CHA RCOAL CONCENTRATION, AND DEPOSITION RATE F OR M I RROR LAKE 97 75

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LIST OF TABLES TABLE 1. Plant species list within Mirror Lake watershed and Taylor Park 23 2. Uncalibrated radiocarbon dates and calibrated 14C ages for Mirror Lake 32 3. Regional site descriptions 45

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LIST OF FIGURES FIGURE 1. Site map 20 2. Age-versus-depth curve based on 14C dates 32 3. Core lithology, loss-on-ignition, and magnetic sus ceptibility 34 4. Pollen percentages, pollen accumulation rates (PAR), pollen zones, (green line) and constrained cluster analysis output (CONISS) 36 5. Mean fire return interval, peak magnitude, charcoal accumulation rates (CHAR) (gray line is background charcoal accumulation rates BCHAR), and pollen zones (gr een line) 40 6. Climate history for the southern Rocky Mountains and Mirror Lake environmental history 44

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CHAPTER I INTRODUCTION

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CHAPTER II B ACKGROUND INFORMATION Physical environment Ge ology

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Climate

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Vege tation

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Disturbance

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Dendroctonus rufipennis Dendroc tonus ponderosae

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. Picea Abies Pinus

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Proxy data from natural archives

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Regional environmental history

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Pinus Picea Abies Querc us Pinus Quercus Picea Abies Pinus

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Picea Abies Pinus Picea Abies

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Artem isia

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Ecosystem resilience and range of natural variability

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Pinus

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CHAPTER III S ITE DESCRIPTION

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# TincupTaylor Park ReservoirMirror Lake Subalpine-spruce/fir Mixed conifer lodgepole/douglasfir Woodland pinyon / ponderosa/juniper Steppe shrub/grassland Tundra, rock ice Continental Divide Roads Mt. Princeton Mt. Yale Cottonwood Pass Tincup pass (CR 267) 0481216 2 km 1 2 4 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19Figure 1. Site map. a. Location of Mirror Lake in Colorado. Regional study site locations (lakes): (1) Lily Pond (2) Cottonwood Pass Pond (3) Copley, Red Lady Fen, Red Well Fen, Splains, Splains Gulch Meadow, Keystone Ironbog, Alkali Basin (4) Hunters (5) Little Molas (6) De Herrera (7) Brazos Ridge Marsh (8) Stewart Bog (9) Chihuahueos Bog (10) Alamo Bog (11) Cumbres Bog (12) Lost Park (13) Kite (14) Bison (15) Tiago (16) Hidden (17) Seven Lakes (18) Summit (19) RMNP: Thunder, Sand Beach, Odessa, Lone Pine, Bear b. Photograph taken at Mirror Lake in July 2014 looking from the south towards the north. c. Taylor Park plant species distribution and location of Mirror Lake within the park. d. Contour lines depicting elevation (m) and topographic features within the Mirror Lake watershed. Red lines indicate avalanche paths and x indicates coring location.a. b. c. d. 3950 3750 3650 3550 3750 3200 3250 3300 3650 3550 3 4 5 0 3850 3540 3800 3950 4050 4100 3350 0 0.8 1.6 0.4 km Mirror Lake 0 0.8 1.6 0.4 km East Willow Creek

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Picea engelmannii Abies bifolia Picea pungens Pinus contorta latifolia Pseudotsuga me nziesii Artemisi a tridentata Seriphidium tridentatum

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Salix planifol ia Salix brachycarpa Salix glauca Juncus Carex Populus tre muloides

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Scientific Name Co mmon Name Abies bifolia* subalpine fir Achillea millefolium* co mmom yarrow Acomastylis alpine avens Al nus inca na subsp. tenuifolia* alder Amaranthaceae* amaranth family Ambrosia ragweed Antennaria spp .* pussytoes Apiaceae* parsley family Aquilegia coerulea* R.M. columbine Arceuthobium americ anum* dwarf mistletoe Arctost aphylos uva-ursi* kinnikinnick Arnica cordifolia* heart-leaved arnica Artemisia tridentata (or Ser phidium tridentatum ) big sagebrush Aster spp.* aster Asteraceae* aster family Betula gla ndulosa* bog birch Bistorta bistortoides* bistort Boraginaceae* borage family Brassicaceae* mustard family Bromus brome Calamagrostis pur purascens* purple reedgrass Carex aquatilis* sedge Carex utriculata* sedge Caryo phyllaceae* pink fam il y Castilleja spp.* paintbrush Chrysothamnus parryi rabbitbrush Chrysothamnus viscidiflorus yellow rabbitbrush Cirsium spp.* thistle Cyperaceae* sedge family Deschampsia cespitosa* tufted hairgrass Dryas mountain dryad Erio phorum angustifolium* cotton-sedge Erythronium gr andiflorum* avalanche lily Isoetes quillwort Juncus rush Oxytropis spp.* locoweed Penta phylloides florib unda* shrubby cin quefoil Phleum commutatum* alpine timothy Picea engelmannii* engelmann spruce Picea p ungens* Colorado blue spruce Pinus albic aulis whitbark pine Pinus aristata bristlecone pine Pinus contorta subsp. latifolia* lo dgepole pine Pinus flexilis limber pine Poa bluegrass Poaceae* grass family P op ulus tremuloides* quaking aspen Pseudotsuga menziesii* do uglas fir Psychrophila leptosepala* marsh-marigold Pterido phyta* ferns and fern allies Quercus gambelii gambel oak Quercus turbinella shrub live oak Ran unculus spp.* buttercup Ribes spp.* current Rosa woodsii* Woods' rose Rubacer parviflorum* thimbleberry Salix brachycar pa* short-fruit willow Salix glauca* grey-leaved willow Salix planifolia* plane-leaved willow Salix spp .* willow Sarco batus spp.* greasewood Saxifragaceae* rock breaker family S par ganium spp.* bur-reed Taraxacum officinale* dandelion Vaccinium caespitosum* dwarf blueberry Vaccinium myrtillus subsp. oreophilum* m yrtle blueberry Valeria na spp.* v aleri an Veratrum tenuipetalum* corn husk lily Vicia spp.* vetch Viola spp.* violet *Present within Mirror Lake watershed N omenclature follows Weber and Wittmann ( 2012).Table 1. Plant species list within Mirror Lake watershed and Taylor ParkScientific Name Co mmon Name spp .* spp.* spp.* spp.* spp.* spp.* spp .* Artemisia frigida fringed sage S elaginella spp.* spike moss Juniperus comm unis subsp. alpina* co mmon juniper Pinus ponderosa subsp. scopulorum ponderosa pine

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CHAPTER IV M ETHODS AND DATA ANALYSIS Field methods Laboratory methods Core preparation, lithology, and chronology

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Ma gnetic susceptibility & loss-on-ignition Poll en

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Ly copodium Pinus Pinus

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Charcoal Data analysis Pollen Pinus Pinus contorta latifolia Pinus ponderosa scopulorum Pinus Pinus Picea

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Picea engelmannii Picea pungens Ps eudotsuga Ps eudotsuga menziesii Abies Abies bifolia Juniperus c ommunis alpina Quercus Quercus gambelii Quercus turbinella Artemisia Artemisia tridentata Bromus Poa Phleum commutatum Deschampsia cespitosa Sarcobatus Artemisia frigida Ambrosia Aster Cirsium Antennaria Achillea millefolium Arnica cordifolia Taraxacum officinale Pentaphylloides floribunda Ribes Dryas Acomastylis Rubacer parviflorum Rosa woodsii

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Charcoal

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CHAPTER V R ESULTS Chronology

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(-63) 0 500 1000 1500 2000 2500 3000 3500 20 40 60 80 100 120 140 160 0 180Age (cal yr BP) Figure 2. Age-versus-depth curve based on 14C dates. Blue circles represent dated material based upon the medium probability, with the record beginning at -63 cal yr BP (2013 CE). The line of the curve is based upon a 2nd order polynomial. See Table 2 for age information. y = 0.0456x2 + 11.162x-63Depth below mud-water a 14C age determinations from Center for Accelerator Mass Spectrometry (CAMS) at the Lawerence Livermore National Laboratory (LLNL).b Depth below mud surface.c 14C age based upon Libby half life of 5,568 years following conventions of Stuiver and Polach (1977).d 95% confidence interval.e 14C calibrated ages derived from Calib 7.0.1 calibration curves (Reimer et al., 2013). Depth to age erroneous, dates not used in age-model.Macrofossil 380 30 Macrofossil 2995 30 Macrofossil5265 30 165297 165300 165298* 165299* D1-A D3-A D2-A D2-A 0 39.5 171 67 104Macrofossil5620 30 426 3071 505 3251 Material dated Radiocarbon date (14C yr BP)c Lab no.aCore Depth (cm)bUpper age range (cal yr BP)dLower age range (cal yr BP)dCalibrated age (cal yr BP; Medium Prob.)e-63 449 3178 Table 2. Uncalibrated radiocarbon dates and calibrated 14C for Mirror Lake

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Lithology Core description and analysis

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0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 0510152025 % Organic Content Age (cal yr BP)0.02.04.06.08.0 (cgs) Figure 3. Core lithology, loss-on-ignition, and magnetic susceptibility. RSE3RSE2RSE1 20 40 60 80 100 120 140 160 180 200 Unconsolidated sediment Dark fine detritus gyttja Fine detritus gyttja Coarse detritus gyttja Sand Rapid sedimentation Event (RSE) RSE1 : 11 cm removed RSE2 : 6 cm removed RSE3 : 6.5 cm removedDepth (cm) Loss-on-ignitionMagnetic susceptibility Lithology

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Magnetic susceptibility & loss-on-ignition Poll en Pinus Artemisia Picea, Sarcobatus Ambrosi a Abies Quercus Psuedotsu ga

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0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 Age (cal yr BP) Zone 1a Zone 1b Zone 2a Zone 2b0204060 Total Pinus 020 Total Picea 0 Psuedotsuga 0 Abies 0% Pollen (dashed line represents 4x exaggeration) Cupressaceae 0 Quercus 020 Artemisia 0 Poaceae 0 Amaranthaceae 0 Sarcobatus 0 Ambrosia 0 Asteraceae 0 Rosaceae 20 40 60 80 100 120 140 160 180 200 Depth (cm)Pollen accumulation rates (grains cm-2 yr-1)Figure 4. Pollen percentages, pollen accumulation rates (PAR), pollen ratios, pollen zones (green line), and constrained cluster analysis output (CONISS). Trees Shrubs & herbs 150065001150016500 Total terrestrial PAR 080016002400 Picea PAR 200400600800Total sum of squares CONISS Pollen zones 0 Picea : Abies ratio Picea : Pinus ratio Ratios LIA MCA .04.08.12.16.20 .6.7.8.91.0

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Pinus Pinus Pinus Pinus Pseudotsuga Abies Pinus PiceaAbies Pseudotsuga Artemisia Pinus Picea Abies, Pseudotsuga Artemisia. Pinus Pice a Abies Picea Pinus Artemisia

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Pinus Picea Psuedotsuga Abies Artemisi a Sarcobatus Ambrosia Quercus Picea Pinus Picea Pinus Pinus Picea Psue dotsuga Abies Sarcobatus Ambrosia Artemisia

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Pice a Pinus Pinus Pinus Pic eaArtemisia Picea Picea Pinus Picea Picea Charcoal

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010203040 peak magnitude (pieces cm -2 yr -1) 00.511.5 CHAR (pieces cm -2 yr -1) 0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 Age (cal yr BP) 20 40 60 80 100 120 140 160 180 200 Depth (cm)FRI (yr fire-1) 0200400 500-yr mean 95% CI Zone 1a Zone 1b Zone 2a Zone 2b Pollen zones Figure 5. Mean fire return interval, peak magnitude, charcoal accumulation rates (CHAR) (gray line is background charcoal accumulation rates BCHAR), and pollen zones (green line). MCA LIA the peaks represent individual fires. The summaries of fire-return intervals are reported on subcentennial timescales to compare with the pollen data. Throughout the record, BCHAR, defined by an 800-year running average of CHAR, averaged 0.3 particles cm-2 yr-1 and ranged between 0.52 and 0.002 particles cm-2 yr-1. There were 28 fire events recorded during the last 4000 years. A high signal-to-noise index (SNI) will indicate good separation betw een peaks and BCHAR values (Higuera et al., 2014). The mean SNI at Mirror Lake is 3.87, indi cating strong likelihood that the peaks registered are fires. F RI, or the time period between fires, using a 500-year m ean, ran g ed from 30 to 400 years.

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CHAPTER VI DIS CUSSION The climate of the late Holocene (4000 yr BP to present)

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-50 1000 2000 3000 4000 5000 -1 0 1 2 3 4 -5 -4 -3 -2 -1 0 %Winter Summer-10 -8 -6 -4 -2 (m)18O () -5 -4 -3 18O () -18 -17 -16 -15 -50 1000 2000 3000 4000 5000 -0.8 -0.5 -0.2 0.2 0.5 PDSI0 8 -8 Age (cal yr BP) C Frequent, high-severity fire, high biomass burned Frequent, high-severity fire, high biomass burned Infrequent fire, low biomass burnedInfrequent fire, low biomass burned Frequent, high-severity fire, moderate biomass burned Closed-canopy Palmer Drought Severity Index, Grid Point 131 Temperature anomaly, Northern Hemisphere, C Hidden Lake, CO water level (depth below modern) Precipitation, Pink Panther Cave, NM Seasonal precipitation balance, Bison Lake, CO Snow Wetter Drier Rain Precip. balance Warmer Cooler Less drought More drought Forest structure Fire history Lake level Insolation Insolation anomaly, solstice, 40 N. lat. Age (cal yr BP) -4 4 12 % AvalanchesRSE1 RSE2 RSE3 Mirror Lake environmental history a. b. c. d. e. f. Figure 6. Climate history for the southern Rocky Mountains and Mirror Lake environmental history. MCA: red box (1000 to 700 yr BP), LIA: blue box (650 to 80 yr BP), NAP: non-arboreal pollen. Data sources: (a) Cook et al., 2008 (b) Mann et al., 2009 (c) Shuman et al., 2009 (d) Asmerom et al., 2007 (e) Anderson, 2011 (f) Berger and Loutre, 1991 Fire events Winter Summer Forest change Picea-Abies forest Closedcanopy Open-canopy More Pinus & NAP than before More closed-canopy More Pinus than before Less Pinus

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Lake nameNearest localityLatitude (N) Longitude (W) Elevation (m) Vegetation type Type of dataReference Mirror Buena Vista, CO3847106553,347Subalpine Pollen, charcoal, MS, LOI This study Lily PondBuena Vista, CO3866106873,208 SubalpineMontane Pollen, charcoal, MS, LOI Briles et al., 2012 Cottonwood Pass Pond Buena Vista, CO3890106453,670TundraPollen, macrosFall, 1997b Copley Crested Butte, CO382810753,350SubalpinePollen, macrosFall, 1997b Red Lady FenCrested Butte, CO382010703,350 TundraSubalpine Pollen, macrosFall, 1997b Red Well FenCrested Butte, CO383010753,290 TundraSubalpine Pollen, macrosFall, 1997b Splains Crested Butte, CO38010703,160SubalpinePollen, macrosFall, 1997b Splains Gulch Meadow Crested Butte, CO38010703,150SubalpinePollen, macrosFall, 1997b Keystone IronbogCrested Butte, CO38210702,920 SubalpineMontane Pollen, macrosFall, 1997a,b Alkali BasinCrested Butte, CO38510602,750SteppePollen, macrosFall, 1997a,b HuntersAlamosa, CO 3760106003,516 TundraSubalpine Pollen, charcoal, MSAnderson et al., 2008a Little MolasSilverton, CO 3740107203,370 TundraSubalpine Pollen, charcoal, MS, TOC, macros Toney and Anderson, 2006 De HerreraAlamosa, CO 3744106563,343SubalpineCharcoal, MSAnderson et al., 2008a Brazos Ridge Marsh Alamosa, CO 3655106153,222SubalpineCharcoal, MSAnderson et al., 2008a Stewart BogSanta Fe, NM35010553,100 SubalpineMontane Pollen, MS Jimnez-Moreno et al., 2008 Chihuahueos BogSanta Fe, NM3620106002,925Montane Pollen, charcoal, MS, macros, C/N Anderson et al., 2008a,b Alamo BogSanta Fe, NM3540106502,630MontaneCharcoal, MSAnderson et al., 2008a Cumbres BogAlamosa, CO 37810673,050Subalpine Pollen, MS, LOI, diatoms Johnson et al., 2013 Lost ParkFairplay, CO 3970105203,079 SubalpineMontane Pollen, LOIVierling, 1998 Kite Fairplay, CO 3997106773,665TundraPollen, MS, macrosJimnez-Moreno and Anderson, 2012 Bison Glenwood Springs, CO3955107043,255Subalpine 18O Anderson, 2012 Tiago Steamboat Springs, CO4049106662,770Montane Pollen, charcoal, MS, TOC, 13C Jimnez-Moreno et al., 2011 HiddenSteamboat Springs, CO4008106662,710MontaneLake level Shuman et al., 2009 Seven LakesSteamboat Springs, CO4036106073,277 TundraSubalpine Charcoal Calder et al., 2014 SummitSteamboat Springs, CO4024106063,149 TundraSubalpine Charcoal Calder et al., 2014 ThunderRMNP, CO 4031105833,231Subalpine Charcoal, pollenHiguera et al., 2014 Sand BeachRMNP, CO 4032105603,140Subalpine Charcoal Higuera et al., 2014 OdessaRMNP, CO 4092105123,051Subalpine Charcoal, pollenHiguera et al., 2014 Lone PineRMNP, CO 403105393,016Subalpine Charcoal, pollenHiguera et al., 2014 Bear RMNP, CO 4087105832,888 SubalpineMontane Pollen, charcoal, MSCaffrey and Doerner, 2012 Abbreviations: MS: magnetic susceptibility, LOI: loss-on-ignition, macros: macrofossils, TOC: total organic carbon, C/N: carbon to nitrogen ratio, 18O: oxygen isotope ratios, 13C: carbon isotope ratios, RMNP: Rocky Mountain National Park Table 3. Regional site descriptions

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late Holocene resulted from shifts in insola tion influencing large-scale climate drivers that determined the timing and amount of pre cipitation and regional temperatures. The late Holocene climate of Colorado was characterized by a general cooling trend in temperature, with the exception of the MCA and the last few decades, and increased winter precipitation. These general trends were punctuate d by heightened periods of drought on yearlyto -decadal scales (

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Ecosystem response to late Holocene climatic conditions Summer-dominant precipitation regime Picea engelmannii Abies bifolia Pinus Pseudotsuga Pinus Artemisi a Picea

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Winte r-dominant precipitation regime Picea Abies Picea Pi nus Pinus Artemisia

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Pinus Artemisia Picea Abies Pinus Artemisia Artemisia Quercus Pinus Picea Abies Juniperus communis

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Pinus Pinus contorta latifolia Picea Abies Pinus Artemisia Sarcobatus Ambrosia Pinus Pice a Abies Pinus Pice a Abies Pinus

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Pinus Pinus Pinus Picea Abies

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Picea Abies Pinus Pinus Pinus

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Picea Picea Pinus Abies Pinus Picea Pinus Pinus

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Pinus Insect outbreaks Insect outbreaks such as Dendroctonus rufipennis Dendroctonus ponderosae Picea Abies Picea Abies

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Picea Abies Pinus Pinus Picea Picea Abies Picea Pinus Pinus Picea Avalanches

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Management implications

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Pinus Picea Abies

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CHAPTER VII CONCLUS ION

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REFERENCES Global and Planetary Change International Journal of Wildland Fire Quaternary Research Forest Ecology and Management Geology Quaternary Science Reviews Quaternary Science Reviews The Colorado Independent Sedimentology

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Quaternary Research Geomorphology Physical Geography Rocky Mountain Geology Science Journal of Sedimentary Petrology Climatography of the United States The Southwestern Naturalist Journal of Biogeography Review of Palaeobotany and Palynology Review of Palaeobotany and Palynology

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GSA Bulletin Frontiers in Ecology and the Environment Arctic and Alpine Research Quaternary Science Reviews Palaeogeography, Palaeoclimatology, Palaeoecology Am erican Midland Naturalist Journal of Ecology International Journal of Wildland Fire Paleobiology The Holocene Geological Society of America Bulletin

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Quaternary Science Reviews The Holocene Forest Ecology and Management Proceedings of the National Academy of Sciences Ecology Journal of Biogeography Canadian Journal of Forestry Review of Palaeobotany and Palynology

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Encyclopedia of Global Environmental Change Encyclopedia of Global Environmental Change Science Proceedings of the National Academy of Sciences Geophysical Research Letters Ecological Monographs Ecological Applications Journal of Applied Ecology Frontiers of Biogeography Quaternary International Science

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Radiocarbon coscience Climate of the Past Quaternary Science Reviews Journal of Biogeography Ecological Applications Journal of Phycology Radiocarbon

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Ecological Applications The Holocene Environmental Research Letters The Holocene Ecology Journal of Ecology Ecological Applications Quaternary Research

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Quaternary Research The Open Ecology Journal Forest Ecology and Management Climate of the Past Trends in Ecology and Evolution Ecology

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APPENDIX A KEY TO ABBREVIATIONS USED IN APPENDICES B, C, D, AND E

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Pollen Count Abbreviations Pinus Picea Pseudotsuga Abies Ephedra Quercus Artemisia Ambrosia s.l. Sarcobatus Salsola Chenopodium Spiraea Eriogonum Polygonum Alnus Betula Plantago Thalictrum Salix Myriophyllum Selaginella Equisetum Botrychium Dryopteris Lycopodium

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APPENDIX B MAGNECTIC SUSCEPTIBILITY FOR MIRROR LAKE

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APPENDIX C LOSSON -IGNITION FOR MIRROR LAKE

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APPENDIX D POLLEN COUNTS AND SUMS FOR M IRROR LAKE

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APPENDIX E CHARCOAL SAMPLE VOLUME, CHARCOAL COUNT, CHARCOAL CONCENTRATION, AND DEPOSITION RATE FOR MIRROR LAKE