GC41A-0111
Alpine Treeline Changes in the Central Rocky Mountains:
A Progress Report
University of Colorado Boulder
Connie A. Woodhouse, Department of Geography and Regional Development, University of Arizona
(conniew1@email.arizona.edu), and Jeffrey J. Lukas, INSTAAR, University of Colorado (lukas@colorado.edu)
INTRODUCTION
MONARCH PASS SITE
COTTONWOOD PASS
Mountain environments are expected to be extremely sensitive to human-induced changes in
climate, and the impacts of climate change on mountain regions are already welldocumented in western North America. Changes in the elevation, composition, and structure
of alpine treeline can reflect climate variability and change, as well as patterns of disturbance
which themselves may be mediated by climate variability. Information about past treelines
may provide insight into the likely character of future changes.
SITE DESCRIPTION
SITE DESCRIPTION
This site is directly east and above Monarch Pass on the
Continental Divide in Colorado. The current treeline forest
is composed of primarily Englemann spruce, with a few
subalpine fir. The actual treeline is variable in elevation,
but generally about 3500 meters. Remnants were scattered
in a line roughly 10-50 meters above the current treeline,
roughly tracking the topographic undulations in the current
treeline. Several old burns are embedded in the treeline. In
addition, there are saplings (Englemann spruce, lodgepole,
limber, and bristlecone pine) above the forest line and the
spruce tree islands.
This site is located about 1 km west
of Cottonwood Pass, at the
Continental Divide. There is no clear
evidence for a previously higher
treeline, but there is evidence of fire
and a denser forest prior to burning.
The forest below the burn is sparse,
with some living tree islands above.
The burn area is being colonized by a
few “krummholz” lodgepole, and by
limber and bristlecone pine trees.
The burned trees appear to be
Englemann spruce. Two standing
snags with bark were burned when
living will help determine the year of
the fire.
Studies of past treelines are possible in areas where wood remnants above the current
treeline are well preserved due to cold, dry climate conditions. Alpine treeline changes
during the late Holocene in the Sierra Nevada have been investigated by a number of
researchers (e.g., LaMarche and Mooney 1972, Scuderi 1987, Lloyd and Graumlich 1997,
Millar 2007), but to date, no similar studies of late Holocene treeline have been undertaken in
the central Rocky Mountains.
SAMPLING STRATEGY
In this study, we have begun an
investigation of characteristics of the
current and past alpine treeline in the
central Rocky Mountains. The goal of
the study is to document the past and
present treeline, and treeline
relationships to climate and fire, using
dendrochronological techniques. In
the summer of 2007, we sampled
remnants within and above treeline at
three sites, as well as living trees at
the current treeline (locations are
shown in the map, right). We also
recorded the presence of seedlings
and saplings establishing above the
current treeline and disturbances at
two of the sites in the form of burns.
The preliminary results for this work
are reported here.
41 N
COLORADO
Continental
Divide
Denver
B. CURRENT TREELINE
3 transects were run from the highest tree 100 meters
down into the forest. At 20 m intervals, the six closest trees
were measured (DBH and height to coring) and cored. All
other trees, remnants, sapling, and seedlings in a 10 m
radius were noted.
Sheep Mountain
Cottonwood Pass
Monarch Pass
0
50
100
150
200 km
37 N
102 W
109 W
Locations of 3 sites in this study
A. REMNANTS
43 remnants of bristlecone pine or limber pine above the
current treeline were photographed, GPSed, and cut with a
chainsaw.
C. SAPLINGS ABOVE THE CURRENT TREELINE
All saplings above the highest tree island were GPSed,
measured and species were noted. Two young trees were
cored.
D. BURNED AREAS
Remnants at two burns within the treeline were sampled
(12 and 20 samples) and GPSed.
SHEEP MOUNTAIN SITE
SITE DESCRIPTION
This site is located on the northern side of Sheep
Mountain, approximately 10 km west of Leadville, CO.
The forests going up to treeline are bands of 5-needle
(limber and bristlecone pine), and Engleman spruce,
grading into spruce krummholz. The sampling site was
in a high steep, southfacing drainage with a treeline of 5needle pines and many large, full tree remnants above
the living trees. Pine and spruce tree islands extended
the elevation of the remnants, higher along the side of
the drainage.
SAMPLING STRATEGY
Monarch Pass Site Questions
From the lower end of the burn, to the
elevation of the highest remnant
(3610-3680m), we defined a
rectangular area (100m across and
200m up/down slope). Within this
area, 34 remnants were sampled,
photographed and GPSed. Each
living tree > 10 cm in diameter was
cored, and the locations and
diameters of smaller living trees was
recorded.
Ÿ
What is the age of the remnant treeline? To what does it correspond,
climatically?
Ÿ
What is the age structure/species composition of the current forest to
treeline?
Ÿ
What are the dates of the fires in the two burned areas? Are they concurrent
with each other, and concurrent with the Cottonwood Pass burn?
SAMPLING STRATEGY
24 remnants were sampled, starting at the highest
(3545m) to the lowest (3520m) which graded into the
current treeline forest. Each remnant was photographed
and GPSed. In addition, we cored 22 old living trees (a
mix of bristlecone and limber) growing in a stand below
the remnants and around to a cliff above Limber Grove
(a previously sampled stand of old limber pine).
Ÿ
What are the ages of the oldest saplings growing above the current treeline?
Will the saplings survive, eventually raising the current treeline or die in harsh
conditions?
Preliminary Results
Cottonwood Pass Site
Questions
Ÿ
What is the date of the fire? From the
Remnant Treeline
To date, 21 remnants have been dated. The inside and outside dates are shown (below), while
tree abundance (binned by century after Lloyd and Graumlich 1997), are shown (below, right ). No
adjustments have been made yet for the loss of sapwood and lack of pith in most samples, but
results give a rough first look at approximate dates these trees were alive and growing above the
current treeline.
Sheep Mountain Pass Questions
Ÿ
What are dates of remnants and we determine the date of trees establishment?
era of miners or earlier? Concurrent
drought and/or the Monarch burn?
Ÿ
When did the living trees become established?
Ÿ
What was the species and age
Ÿ
Are the remnants a distinctly different population
composition of the burned forest?
(time-wise) from the living trees?
Ÿ
Are there trees currently in this burn area
Ÿ
If not, can an extended tree-ring chronology be
that are survivors of the fire or have all of
them established since the fire?
compiled from both remnant and living tree collections?
Ÿ
What climate variable limits tree-growth at this site?
Monarch Pass samples
Ÿ
Is the climate signal different for the limber and
Transect Plot
bristlecone pine trees?
Remnants - inside dates
300 BC - AD 1
A
AD 400 - 800
Undated
Current treeline
Summit
3626m
C
Monarch
Pass
3448m
200m
B
Locations of remnant samples and transects
(A, B, C) for sampling the current treeline
forest are shown above. Undated remnants
either do not overlap in age with dated
remnants or have too few rings to accurately
crossdate.
A comparison with
Salzer and
Kipfmueller’s
.
(1997) temperature
reconstruction (left)
suggests
trees at this site are
sensitive to
summer
temperatures
The current treeline forest is composed almost entirely of Engelmann spruce trees, saplings and
seedlings. Estimated pith dates of the trees, grouped into 20-yr age classes, are spread from the
1630s to the 1960s, with a peak in the late 19th century (above). A preliminary chronologies (12
samples) of standard ring widths shows a rapid shift from high growth to low growth just prior to an
increase in pith dates, but whether this is related or just a coincidence is unknown (above. top).
Within the forest, a large number of Englemann spruce
seedlings and saplings has established in the last two
decades (right). In contrast, above the current treeline,
small trees, saplings and seedlings are mostly a mix of
spruce (11) and 5-needle pines (19, limber and
bristlecone).
References
LaMarche, V. C., and H. A. Mooney. 1967. Altithermal timberline advance in western United States. Nature 213, 980982.
Lloyd, A.H. and L.J. Graumlich. 1997. Holocene dynamics of treeline forests in the Sierra Nevada. Ecology 78, 11991210.
Millar, C.I. 2007. Treeline Response to Climate: Up, Down, All Around. Presentation for CNPS at White Mountain
Research Station, CA.
Salzer, M.W. and K.F. Kipfmueller. 2005. Reconstructed temperature and precipitation on a millennial timescale from
tree rings in the southern Colorado Plateau, U.S.A. Climatic Change 70, 465 - 487
Scuderi, L. A. 1987. Late-Holocene upper timberline variation in the southern Sierra Nevada. Nature 325, 242-244.
Acknowledgements
This work was made possible through the interest and support of Robert S. Thompson and the Earth Surface Processes
- Central Region of the U.S. Geological Survey. We thank Mark Losleben, Kurt Chowanski, and Cody Routson for field
assistance.