Page 5 - Climate Change Impacts in the United States

7: FORESTS

Forest Vulnerability to Changing Climate

Figure 7.3. The figure shows a conceptual
climate envelope analysis of forest vulner-
ability under current and projected future
ranges of variability in climate parameters
(temperature and precipitation, or alter-
natively drought duration and intensity).
Climate models project increasing temper-
atures across the U.S. in coming decades,
but a range of increasing or decreasing
precipitation depending on region. Episodic
droughts (where evaporation far exceeds
precipitation) are also expected to increase
in duration and/or intensity (see Ch. 2:
Our Changing Climate). The overall result
will be increased vulnerability of forests
to periodic widespread regional mortality
events resulting from trees exceeding their
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physiological stress thresholds. (Figure
11
source: Allen et al. 2010 ).

Large-scale die-off and wildfire disturbance events could have investments more risky (Figure 7.4). Western forests could
potential impacts occurring at local and regional scales for also lose substantial amounts of carbon storage capacity.
timber production, flooding and erosion risks, other changes For example, an increase in wildfires, insect outbreaks, and
in water budgets, biogeochemical changes including carbon droughts that are severe enough to alter soil moisture and
storage, and aesthetics. 29,30,31 Rising disturbance rates can nutrient contents can result in changes in tree density or
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increase harvested wood output and potentially lower prices; species composition.
however, higher disturbance rates could make future forest
Key Message 2: Changing Carbon Uptake

U.S. forests and associated wood products currently absorb and store the equivalent of
about 16% of all carbon dioxide (CO 2) emitted by fossil fuel burning in the U.S. each year.
Climate change, combined with current societal trends in land use and forest
management, is projected to reduce this rate of forest CO 2 uptake.

Climate-related Effects on Trees and Forest Productivity
Forests within the United States grow across a wide range of season in the last thirty years, yet earlier spring growth may be
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latitudes and altitudes and occupy all but the driest regions. negated by mid-summer drought.
Current forest cover has been shaped by climate, soils,
topography, disturbance frequency, and human activity. By the end of the century, snowmelt may occur a month
Forest growth appears to be slowly accelerating (less than 1% earlier, but forest drought stress could increase by two
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per decade) in regions where tree growth is limited by low months in the Rocky Mountain forests. In the eastern United
temperatures and short growing seasons that are gradually States, elevated CO 2 and temperature may increase forest
being altered by climate change (for species shifts, see Ch. 8: growth and potentially carbon storage if sufficient water
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Ecosystems). Forest carbon storage appears to be increasing is available. 1,31,36 Despite recent increases in forest growth,
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both globally and within the United States. Continental-scale future net forest carbon storage is expected to decline due to
satellite measurements document a lengthening growing accelerating mortality and disturbance.

179 CLIMATE CHANGE IMPACTS IN THE UNITED STATES