Climate Change Effects on Treeline Dynamics in the White Mountains, CA
Brian V. Smithers, UC Davis; Constance I. Millar, USDA Forest Service PSW; Malcolm P. North, USDA Forest Service PSW
Introduction
Climatic treeline is a function of temperature so treeline
advance is an expected sensitive indicator of the effects of
climate change on species distributions. Mean minimum
temperatures have increased in the White Mountains of
California, which have seen minimal human impact and have
been free of major glaciation, making these mountains an
ideal location to test for climate change effects on
treeline. White Mountain treelines are mostly dominated by
Great Basin bristlecone pine (Pinus longaeva) but
observations suggest that usually downslope-growing limber
pine (Pinus flexilis) is establishing above historical treeline. It
may be the current treeline advance is largely a function of
limber pine “leap-frogging” over bristlecone pine treeline.
Methods
We surveyed 87 treeline plots at 9 sites in the three soil
types. A series of 30m x 10m plots were placed as a
modified belt transect across upper treeline at each
site, moving up and down slope to follow the treeline
contour. Plots were placed above treeline at the
elevation where juveniles were in sufficient density that
they are likely represent future treeline. An additional
72 plots were surveyed in the mid-stand. In the plot, we
identified each sub-adult (<100 years old) tree to species, aged them using whorl
counts, and measured vertical distance to historical treeline using a rangefinder.
To assess potential seed sources for the plots, we used a wedge prism to estimate
basal area in each plot. Due to the low density of these forest, we used a 1 Basal Area
Factor (metric) prism, so a count of included stems is also the basal area in m2/hectare.
Results
Bristlecone pine, Pinus longaeva
Limber pine, Pinus flexilis
Due to low adult mortality and long life spans of both species,
establishing limber pine could exclude bristlecone pine from
upslope habitat limiting bristlecone pine’s ability to expand its
range in response to climate change. Local extirpations of
bristlecone pine are possible in such a scenario. This study
examines whether limber pine is overrepresented above
historical treeline in the White Mountains relative to locally
available seed sources.
Study Site
The White Mountains are a
Great Basin range east of the
Sierra Nevada in CA and reach
up to 4,344m (14,252 ft). Due to
the Sierra Nevada rain shadow,
they receive very little
precipitation and the dry
(<20in/yr.), cold climate makes
for stressful conditions which
few species can tolerate. This
creates a simple study system of
only two species of tree
inhabiting the sub-alpine forest.
Plots were surveyed in three main locations: Boundary Peak,
Patriarch Grove, and Campito Mountain, on the three main
treeline soil types in the region: granite, dolomite, and
quartzite.
The average vertical increase in
treeline is 20.1m. There are soil
specifics effects with treeline on
granite soils increasing 31.1m
(ANOVA, p<0.001). Across all soil
types above treeline, limber pine
density is significantly higher than
bristlecone pine (p<0.001). Basal
area estimates show that seed
sources are significantly higher for
bristlecone pine (p<0.001).
At mid-stand, there is a large
overall increase in basal area with
limber pine making up a greater
fraction of adults. Bristlecone pine
is the dominant seed source. Stem
densities show no significant
difference between species.
Granitic soil plots were dominated
by limber pine, and removing these
plots from analysis shows an even
stronger disconnect of seed source
(basal area) and establishment. On
dolomite soils, limber pine seed
source is almost negligible and the
mid-stand establishment is
dominated by bristlecone pine.
Above treeline, limber pine density
is still significantly higher than
bristlecone pine density.
Figure 1: Overall stem densities (#/m2) and basal areas (m2/ha) at
upper treeline and in mid-stand. Error bars represent the standard
error of the mean. Note that scales vary in each plot.
Discussion
In the White Mountains of California, treeline appears to be in
the early stages of advance. Due to extremely low mortality in
both limber and bristlecone pine, establishment of young
trees above treeline is likely to result in upslope expansion of
treeline. Bristlecone pine adults dominate at treeline, but
above treeline, young limber pine dominate, “leap-frogging”
over bristlecone pine to establish on newly available habitat.
This is especially true on dolomitic soils, a soil type that has
been historically dominated by bristlecone pine. Limber pine
seed sources are nearly absent on dolomite while they
dominate establishment. The reasons for this are unknown.
Ongoing research aims to target mechanisms for this
differential range shift between these species. A major factor
is likely to be the effect of dispersal. Limber pine relies on
Clark’s nutcracker for dispersal, a corvid bird that caches
seeds in the soil, some of which survive to germinate.
Bristlecone pine is largely dispersed by wind so its ability to
disperse is inherently different than that of limber pine. How
dispersal differences affect a pine species’ ability to respond
to climate change remains unknown.
Some individual Great Basin bristlecone pines are more than
5,000 years old, making them the oldest individual organisms
on earth. The tree ring record in these trees, alive and dead,
have been a major contribution to diverse disciplines such as
climatology, archaeology, astronomy, physiology, and
ecology. Adult bristlecone pines will largely be able to
withstand moderate climate change, but its ability to quickly
disperse into its future climatic range is unknown. Limber pine
appears much more able to take advantage of rapidly
changing climate. Its establishment above treeline is likely to
exclude bristlecone pine. Whether this an early-successional
response in which bristlecone pine will ultimately dominate
remains to be seen, but with such long-lived trees, this state,
even if it is “short-term,” is likely to last for thousands of
years. Local extirpations of bristlecone pine are certainly
possible in those time scales.
Acknowledgements
This work is being generously funded through grants from the
UC Davis Graduate Group in Ecology, UC Davis College of
Agriculture and Environmental Science, USDA Forest Service,
California Native Plant Society, and the White Mountains
Research Center.
Figure 2: Overall stem densities (#/m2) and basal areas (m2/ha) at
upper treeline and in mid-stand on dolomite soils. Error bars
represent the standard error of the mean. Note that scales vary in
each plot.
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