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4. HOW DOES CLIMATE AFFECT GROWTH?
We find that while Engelmann
Spruce display relatively low
sensitivity to climate, Subalpine Fir
have much greater sensitivity,
particularly negative responses to
warm summer temperatures, and
trends of decreasing growth on
specific landscape positions.
2. STUDY SITE: SAN JUAN NATIONAL FOREST
0.6
0.4
0.4
0.2
0.2
0
0
−0.2
−0.2
−0.6
−0.6
9 8 7 6 5 4 3 2 1 12p11
11 10 9 8p7
7 6 p5
5 4 p3
3 2 1p1
99 8 7
1 12p11
p1
7 6 55 4 33 2 1
11 10 9 8p7
7 6 p5
5 4 p3
321
Month
Month
Year of growth Year preceding growth
> High sensitivity to climate
Subalpine fir show a negative growth response to warm summer temperatures.
Monthly Precipitation
Minimum Monthly Temperature
0.6
0.6
0.4
0.4
a)
a) The region of San Juan National
Forest has experienced a trend of
warming in recent years, observed
in NOAA Divisional Data (all of
Western Colorado) and SNOTEL
data within the San Juan Mountains.
b) Predictions of four regional
climate models forecast increasing
seasonal minimum and maximum
temperatures, with increases of
several degrees C by 2070.
b)
Projected
increase in
summer
T min
Projected
increase in
summer
T max
Adapted from: Rangwala el al., Mid-21st century projections in temperature extremes in the southern Colorado Rocky
Mountains from regional climate models, Climate Dynamics, (2012) 39:1823–1840
Model Diameter Elevation Aspect Eastness Site Temperature
1
x
x
2
x
x
x
3
x
4
x
x
x
x
5
x
x
AIC
214.5
214.7
228.9
215.2
230.2
BIC
237.5
241.0
242.1
244.8
246.6
R2
0.36
0.36
0.37
0.36
0.36
>  Growth declining over past 40 years
>  Trees with declining growth predicted by aspect and elevation
0.2
0.2
0
0
−0.2
−0.2
−0.4
−0.4
Engelmann Spruce
−0.6
99 8 77 6 55 4 33 2 11 12p11
11 10 9 8p77 6p5
5 4 p3
3 2 p1
1
99 8 77 6 5
5 4 33 2 11 12p11
11 10 9 8p77 6 p5
5 4 p3
3 2 p1
1
> Low sensitivity to climate
5. INTERACTIONS: CLIMATE AND PHYSIOGRAPHY
How do climate effects on growth vary across the landscape? Climatic and
physiographic drivers of tree growth were investigated using a Generalized Linear
Mixed Modeling approach:
fixed effects
random effects
BAI
~ climate + physiography + site + tree
BA
Tables: top 3 models for each species, ranked by model fit and physiography.
Subalpine Fir
Model Tmin Tmax Precip Elevation Aspect Eastness* AWC
1
x
x
2
x
3
x
x
x
Interaction
AIC
BIC
R2
Elev*Tmin
39260
39316
0.55
Eastness*Tmin
39398
39453
0.54
Aspect*Tmin
39378
39465
0.54
> growth explained by climate and physiography and their interactions;
specifically temperature, elevation and aspect. * Eastness = sine(azimuth)
Interaction:
Temperature
> No declines in growth
6. CONCLUSIONS
1.  Subalpine Fir show much greater sensitivity to climate than
Engelmann Spruce.
2.  Subalpine Fir growth is negatively correlated with warm
summer temperatures.
3.  Both Engelmann Spruce and Subalpine Fir growth are affected
by interactions between climate and local physiography.
4.  Growth of Subalpine Fir is declining, specifically on east facing
landscape positions.
0.08
Growth
(BAI/BA)
3. CLIMATE TRENDS IN SAN JUAN NATIONAL FOREST
Expectation of growth in healthy trees
−0.4
−0.4
−0.6
Methods of tree ring analysis:
•  detrended ring width
•  basal area (BA): the crosssectional area of tree
•  basal area increment (BAI),
cross-sectional area
added annually.
Subalpine Fir
Monthly Precipitation
Minimum Monthly Temperature
0.6
Engelmann Spruce
22 sampling sites were
chosen within Spruce-fir
forest to include varying:
•  elevation
•  aspect
•  soil type
To assess growth trends, we analyze BAI over time by aspect. In a healthy
tree, BAI should increase throughout the tree’s life; declines in BAI over time
indicate declining growth.
Subalpine Fir
Correlation
We use dendroclimatological techniques to explore climate-mediated changes in radial
growth over time in the high elevation forests of SW Colorado by sampling 450 Engelmann
Spruce and 210 Subalpine Fir from 22 sites across varying aspect, elevation, and soil type.
5. FACTORS AFFECTING GROWTH DECLINES
Investigating growth-climate relationships: Ring width data were detrended to
remove the effects of tree age and disturbance. The resulting detrended ring widths
were compared to monthly climate data from the NOAA Divisional Dataset
1895-2012 to calculate the correlation between growth and climate for each tree.
Correlation
1. CLIMATE CHANGE AND FOREST GROWTH
Forests of the western United States are in a period of rapid change, due in part to climate
driven changes in growth, establishment, and mortality. These changes are of particular
concern in the American Southwest where climate conditions are projected to become
increasingly hot and dry throughout the next century. While lower elevation trees of the this
region are already experiencing decreased radial growth in response to warmer temperatures,
the response of high elevation trees to future climate is highly uncertain.
Correlation
Contact: K. Kelsey;
kelseykc@colorado.edu
Correlation
GC23E
0691
Warm years
0.05
ACKNOWLEDGEMENTS
0.03
0.02
0.01
Cool years
Elevation (m)
2700 3000 3300
Elevation (m)
2700 3000 3300
Funding from US Department of Agriculture; NIFA Grant Award Number
2011-67003-03755 and Niwot Ridge LTER #DEB-1027341
Elevation (m)
In cool years, growth is greater at low elevation; in warm years, growth is greater at high elevation
Engelmann Spruce
Model Tmin Tmax Precip Elevation Aspect Eastness AWC
1
x
2
x
3
x
x
x
x
Interaction
AIC
BIC
R2
Aspect*Tmin
149571
149684
0.23
Eastness*Tmin 149630
Elev*Tmin
149634
> growth explained poorly by climate and physiography
149691
0.23
149695
0.23
Field and analytical help from D. Doak, M. Redmond,
L. Young, D. Fernandez, H. Smith, N. Volin
San Juan National Forest