The response of vegetation zonation in Rocky Mountain ecotones to climate change
ID: GC23E-0685
1
Foster (acf7m@virginia.edu),
Adrianna C.
1
Shuman and
J.K.
H.H.
1
Shugart
1. Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904-4123, USA
Model Validation
Introduction
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Biomass (tonnes C/ha)
Forests in the Rocky Mountains are a crucial part of the North American carbon budget
(Schimel et al. 2002). However, disturbances like insect outbreaks, fire, and windthrow, along
with climate change, threaten the vitality of these important ecosystems (Joyce et al. 2014;
IPCC 2007; Bentz et al. 2010; Rehfeldt et al. 2006). It is difficult to predict how vegetation will
respond to climate change alone. Vegetation is able to respond to variability in climate over
moderate time and space scales (Holling 1992) with mediation in the form of stomatal closure
during drought conditions, or differential allocation to above- and belowground tissues (Katul
et al. 2012). Over longer time and space scales, climate change may cause shifts in the species
composition of a landscape, as species migrate into optimal climate zones (Shugart &
Woodward 2011). The Rocky Mountains have several vegetation zones (Figure 3), typically a
result of changing temperature and moisture up the mountain (Peet 1984). These zones may
shift with climate change, and tree species at the top of the mountain (the subalpine zone) may
not have an optimal climate zone. Climate change is also predicted to change the frequency and
severity of disturbances (Joyce et al. 2014; Bentz et al. 2010). Because of this, it is important to
analyze how climate change, both alone and with changes in disturbances, will affect the
vegetation of Rocky Mountain landscapes.
Gap models have been successful at investigating the response of trees to temperature and
disturbance and in predicting shifts in species dominance due to changes in climate (Lasch &
Linder 1995; Shuman et al. 2011; Bugmann 2001). The University of Virginia Forest Model
Enhanced (UVAFME) is an individual-based gap model that follows the annual growth,
establishment, and death of individual trees on independent patches of a landscape (Yan &
Shugart 2005). Each patch is equivalent to the size of a dominant tree crown (0.08 ha), and the
average of several hundred of these patches simulates the average biomass and species
composition of a forested landscape through time. In this study, UVAFME was calibrated to
field sites in the Colorado and southern Wyoming Rocky Mountains and validated against
expected species zonation. The model was then run under a climate change scenario to
determine what effect climate change might have on these forests.
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Pseudotsuga
menziesii
Populus tremuloides
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Pinus flexilis
Pinus ponderosa
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Pinus edulis
30
Pinus contorta
Picea engelmannii
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Juniperus scopulorum
10
Abies lasiocarpa
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a.
Biomass (tonnes C/ha)
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1900 m
JUNIcomm
PINUpond
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PSEUmenz
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PINUcont
POPUtrem
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ABIElasi
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PINUpond
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PINUcont
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PINUedul
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JUNIcomm
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PINUflex
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POPUtrem
PICEenge
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Biomass (tonnes C/ha)
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ABIElasi
Year
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c.
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Biomass (tonnes C/ha)
Subalpine zone
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PICEenge
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ABIElasi
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PINUflex
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PINUcont
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POPUtrem
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Subalpine fir
Lodgepole pine
Limber pine
Upper montane
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Engelmann
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spruce
PSEUmenz
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Limber pine
Douglas fir
JUNIcomm
Year
Quaking aspen
Lower montane
2400 m
Figure 3. Schematic of typical species
present in the Colorado/southern
Wyoming Rocky Mountains at different
elevational zones (Veblen & Lorenz 1991;
Burns & Honkala 1990; Peet 1984).
Ponderosa pine
b.
Juniper
1800 m
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PINUedul
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PINUpond
PINUcont
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PSEUmenz
ABIElasi
Year
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3200 m
climate change starts
4°C 15% precip
PICEenge
ABIElasi
PSEUmenz
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PINUcont
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POPUtrem
PINUpond
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PINUflex
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JUNIcomm
PINUedul
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Year
&!!
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&&!
&(!
'!!
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Conclusions
)(! "!!!
significant decrease in
biomass
significant increase in
biomass
no significant change
Table 1. Significance, based on
t-tests (p = 0.05), of changes in
biomass between non-climate
change runs and runs with
climate change included for
every 20 years of simulation.
Each 20 year step in the climate
change runs was compared to
the climax condition biomass
(year 500) from the non-climate
change run.. Species with
biomass lower than 1 tC ha-1
across all years and in both runs
were considered negligible/nonsignificant.
• The UVAFME model appears to accurately simulate the vegetation zonation that exists along an
elevational gradient in the Colorado and southern Wyoming Rocky Mountains. The changes in species
distributions in Figure 1 going from low to high elevation correspond well to what is typical of a
gradient in the Rocky Mountains in this region (Figure 3). Further validation will be conducted once
disturbance is added to the model.
• The climate sensitivity test conducted (increase in temperature of 4°C and decrease in precipitation of
15%) indicates an overall loss of biomass at both 1900 and 2200 m (Figure 4 a & b). The species
composition also changes at 2200 m in response to altered climate, with significantly lower
Pseudotsuga menziesii (Douglas fir), Pinus contorta (lodgepole pine), and P. ponderosa (ponderosa
pine) biomass, and higher P. edulis (pinyon pine) biomass (Figure 4b).
• As these climate change runs were conducted without disturbance, their results may change,
PSEUmenz
50
160
PINUpond
15
%(!
Future Work
4°C 15% precip
120
4°C 15% precip
JUNIcomm
60
%&!
particularly for the subalpine zone (3200 m), where disturbance frequency and intensity are predicted to
increase with climate change.
climate change starts
2200 m
80
20
PINUedul
40
Biomass (tonnes C/ha)
climate change starts
Biomass (tonnes C/ha)
1900 m
%#!
Pinyon pine
70
0
25
%$!
'$! '&! '(!
at 3200 m. This increase was also accompanied by a change in species composition: a decrease in Abies
lasiocarpa (subalpine fir) biomass, and the introduction of P. menziesii.
Pinyon pine
Climate Change Results
a.
• There was an overall increase in biomass with increases in temperature and decreases in precipitation
Quaking aspen
Juniper
0
•  Species biomass output (tC ha-1) for runs with altered climate change was compared
against that of a mature forest (year 500) for runs without climate change using t-tests
(Table 1).
PICEenge
70
0
•  Model was run for 200 independent plots in a Monte Carlo style simulation at elevations
from 1600 m to 3600 m at intervals of 100 m for a minimum of 500 years.
•  A climate sensitivity analysis was then conducted at three elevations by increasing
temperature linearly by 4°C and decreasing precipitation linearly by 15% from years 500
to 800, after which climate stabilized at these altered values (Figure 4).
Figure 2. Simulated mixed species biomass
dynamics (tC ha-1) for a field site in southern
Wyoming at three elevations of : 1900 m (a),
corresponding to a typical pinyon pine
(PINUedul) – juniper (JUNIcomm) system; 2200
m (b), corresponding to a Douglas fir
(PSEUmenz) – ponderosa pine (PINUpond)
forest; and 3200 m (c), corresponding to the
typical subalpine zone (Engelmann spruce
(PICEenge) and subalpine fir (ABIElasi)).
PINUedul
20
PINUflex
•  Nine species and corresponding site and soil information for four sites were added to the
model to represent forests of the Rockies from the lower montane to the subalpine zone.
•  Biomass of each species at year 500 for each elevation was compared to realistic curves of
biomass up an elevational gradient (Figures 1, 2, 3).
Figure 1. Simulated biomass (tC ha-1) at
year 500 of nine Rocky Mountain
species at different elevations for the
Glacier Lakes Site (southern Wyoming).
UVAFME was run every 100 m from
1600 m to 3600 m. Disturbance was not
included in these model runs.
Elevation (m)
Methods
•  UVAFME calibrated to field sites in Wyoming and Colorado using data on species
composition, climate, and soil parameters from the US Forest Service, Burns & Honkala
(1990), and local weather stations (NCDC 2014).
Statistical Tests
Figure 4. Simulated biomass output (tC ha-1)
from model runs with climate change for (a)
1900 m, (b) 2200 m, and (c) 3200 m. For each
climate change run temperature increased
linearly by 4°C and precipitation decreased by
15% from years 500 to 800 at which point
climate stabilized at these altered values.
The spruce beetle (Dendroctonus rufipennis (Kirby)) is an important mortality agent of
Engelmann spruce (Picea engelmannii Parry ex Enelm.) in the subalpine zone of the Rocky Mountains
(Bebi et al. 2003; Bentz et al. 2010). Spruce beetle outbreaks have caused widespread tree mortality
across the northwestern United States and Canada. With increasing temperatures due to climate
change, the frequency and severity of spruce beetle outbreaks are predicted to increase, further
endangering the future of western subalpine forests (DeRose & Long 2012; Bentz et al. 2010).
Future work on this project will involve developing a spruce beetle disturbance subroutine to add
to UVAFME. This subroutine will calculate the probability for spruce beetle induced mortality based
on several environmental and climate factors. Along with other disturbances already included in the
model (fire and windthrow) UVAFME will be run with the added beetle subroutine and climate change
to determine the impact of the spruce beetle – climate change interaction on western subalpine forests.
Acknowledgements
We are grateful to the NASA Virginia Space Grant Consortium and to the National Fish and Wildlife Foundation for their support. We are
also grateful for advice and much-needed in-the-field help from Jose Negron. References are included in hand-out version.