Page 4 - Climate Change Impacts in the United States

7: FORESTS

to forest structure (age, tree size, and species composition) and Effectiveness of Forest Management
air pollutants than climate over recent decades. Nonetheless,
mortality of some eastern tree groups is related to rising in Reducing Wildfire Risk
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temperature and is expected to increase as climate warms.
Future disturbance rates in forests will depend on changes
in the frequency of extreme events as well as the underlying
changes in average climate conditions. 9,20 Of particular concern
is the potential for increased forest disturbance as the result
of drought accompanied with warmer temperatures, which
can cause both wildfire and tree death. Temperatures have
generally been increasing and are projected to increase in the
future (see Ch. 2: Our Changing Climate). Therefore, although
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it is difficult to predict trends in future extreme events,
there is a high degree of confidence that future droughts will
be accompanied by generally warmer conditions. Trees die
faster when drought is accompanied by higher temperatures,
so short droughts can trigger mortality if temperatures are Figure 7.2. Forest management that selectively removes trees
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higher. Short droughts occur more frequently than long to reduce fire risk, among other objectives (a practice referred
droughts. Consequently, a direct effect of rising temperatures to as “fuel treatments”), can maintain uneven-aged forest
may be substantially greater tree mortality even with no structure and create small openings in the forest. Under some
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change in drought frequency. conditions, this practice can help prevent large wildfires from
spreading. Photo shows the effectiveness of fuel treatments in
Arizona’s 2002 Rodeo-Chediski fire, which burned more than
Given strong relationships between climate and fire, even 400 square miles – at the time the worst fire in state history.
when modified by land use and management, such as fuel Unburned area (left) had been managed with a treatment that
treatments (Figure 7.2), projected climate changes suggest removed commercial timber, thinned non-commercial-sized
that western forests in the United States will be increasingly trees, and followed with prescribed fire in 1999. The right side
affected by large and intense fires that occur more of the photo shows burned area on the untreated slope below
frequently. 16,23,24,25 These impacts are compounded by a legacy Limestone Ridge. (Photo credit: Jim Youtz, U.S. Forest Service).
of fire suppression that has resulted in many U.S. forests
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becoming increasingly dense. Eastern forests are less likely
to experience immediate increases in wildfire, unless a point is
reached at which rising temperatures combine with seasonal
dry periods, more protracted drought, and/or insect outbreaks
to trigger wildfires – conditions that have been seen in Florida
(see Ch. 17: Southeast).

Rising temperatures and CO 2 levels can increase growth or
alter migration of some tree species; 1,27 however, the relation-
ship between rising temperature and mortality is complex. For
example, most functional groups show a decrease in mortal-
ity with higher summer temperatures (with the exception of ©Daryl Pederson/AlaskaStock/Corbis
northern groups), whereas warmer winters are correlated with
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higher mortality for some functional groups. Tree mortality
is often the result of a combination of many factors; thus in-
creases in pollutants, droughts, and wildfires will increase the
probability of a tree dying (Figure 7.3). Under projected climate
conditions, rising temperatures could work together with for- Climate change is contributing to increases in wildfires across
est stand characteristics and these other stressors to increase the western U.S. and Alaska.
mortality. Recent die-offs have been more severe than pro-
jected. 11,14 As temperatures increase to levels projected for dure only limited abnormal water stress, reinforcing the idea
mid-century and beyond, eastern forests may be at risk of die- that trees in wetter as well as semiarid forests are vulnerable
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off. New evidence indicates that most tree species can en- to drought-induced mortality under warming climates. 28

178 CLIMATE CHANGE IMPACTS IN THE UNITED STATES