Page 6 - Climate Change Impacts in the United States

7: FORESTS

Forests can be a Source – or a Sink – for Carbon

Figure 7.4. Relative vulnerability of different forest regions to
climate change is illustrated in this conceptual risk analysis
diagram. Forest carbon exchange is the difference between
carbon captured in photosynthesis and carbon released by
respiration of vegetation and soils. Both photosynthesis and
respiration are generally accelerated by higher temperatures,
and slowed by water deficits, but the relative strengths
of these controls are highly variable. Western forests are
inherently limited by evaporation that exceeds precipitation
during much of the growing season. Xeric (drier) eastern
forests grow on shallow, coarse textured soils and experience
water deficits during long periods without rain. Mesic (wetter)
eastern forests experience severe water deficits only for
relatively brief periods in abnormally dry years so the carbon
exchanges are more controlled by temperature fluctuations.
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(Figure source: adapted from Vose et al. 2012 ).

Forest Carbon Sequestration and Carbon Management
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From the onset of European settlement to the start of the million tons of carbon per year. The amount of carbon taken
last century, changes in U.S. forest cover due to expansion up by U.S. land is dominated by forests (Figure 7.5), which have
of agriculture, tree harvests, and settlements resulted in annually absorbed 7% to 24% of fossil fuel carbon dioxide (CO 2)
net emissions of carbon. 37,38 More recently, with forests emissions in the U.S. over the past two decades. The best
reoccupying land previously used for agriculture, technological estimate is that forests and wood products stored about 16%
advances in harvesting, and changes in forest management, (833 teragrams, or 918.2 million short tons, of CO 2 equivalent
U.S. forests and associated wood products now serve as a in 2011) of all the CO 2 emitted annually by fossil fuel burning in
substantial carbon sink, capturing and storing more than 227.6 the United States (see also “Estimating the U.S. Carbon Sink” in
Ch. 15: Biogeochemical Cycles). 3
Forest Growth Provides an Important Carbon Sink
The future role of U.S. forests in the carbon cycle
will be affected by climate change through changes
in disturbances (see Figures 7.3 and 7.4), as well
as shifts in tree species, ranges, and productivity
(Figure 7.6). 19,38 Economic factors will affect any
future carbon cycle of forests, as the age class
and condition of forests are affected by the
acceleration of harvesting, 39,40 land-use changes
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such as urbanization, changes in forest types, and
bioenergy development. 41,43,44,45
Efforts in forestry to reduce atmospheric CO 2
levels have focused on forest management and
forest product use. Forest management strategies
include land-use change to increase forest area
(afforestation) and/or to avoid deforestation and
optimizing carbon management in existing forests.
Figure 7.5. Forests are the largest component of the U.S. carbon sink, but Forest product-use strategies include the use of
growth rates of forests vary widely across the country. Well-watered forests wood wherever possible as a structural substitute
of the Pacific Coast and Southeast absorb considerably more than the arid
southwestern forests or the colder northeastern forests. Climate change for steel and concrete, which require more carbon
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and disturbance rates, combined with current societal trends regarding emissions to produce. The carbon emissions offset
land use and forest management, are projected to reduce forest CO2 from using wood rather than alternate materials for
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uptake in the coming decades. Figure shows average forest growth as a range of applications can be two or more times the
measured by net primary production from 2000 to 2006. (Figure source: carbon content of the product. 47
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adapted from Running et al. 2004 ).

180 CLIMATE CHANGE IMPACTS IN THE UNITED STATES