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21: NORTHWEST
<br />The likelihood of increased disturbance (fire, insects, diseases,
<br />and other sources of mortality) and altered forest distribution
<br />are very high in areas dominated by natural vegetation, and
<br />the resultant changes in habitat would affect native species
<br />and ecosystems. Subaipine forests and alpine ecosystems are
<br />especially at risk and may undergo almost complete conversion
<br />to other vegetation types by the 2080s (A2 and B1;104 A2;199
<br />Ensemble A2, B1, B2;106). While increased area burned can
<br />be statistically estimated from climate projections, changes
<br />in the risk of very large, high -intensity, stand -replacing fires
<br />Description of Observed and Projected Changes
<br />Climate change will alter Northwest forests by increasing
<br />wildfire risk and insect and tree disease outbreaks, and
<br />by forcing longer -term shifts in forest types and species
<br />(see Ch 7: Forests). Many impacts will be driven by
<br />water deficits, which increase tree stress and mortality,
<br />tree vulnerability to insects, and fuel flammability.
<br />The cumulative effects of disturbance — and possibly
<br />interactions between insects and fires — will cause the
<br />greatest changes in Northwest forests.86,87 A similar
<br />outlook is expected for the Southwest region (see Ch.
<br />20: Southwest, Key Message 3).
<br />Although wildfires are a natural part of most Northwest
<br />forest ecosystems, warmer and drier conditions have
<br />helped increase the number and extent of wildfires in
<br />western U.S. forests since the 1970s.14,87,88,89 This trend
<br />is expected to continue under future climate conditions.
<br />By the 2080s, the median annual area burned in the
<br />Northwest would quadruple relative to the 1916 to
<br />2007 period to 2 million acres (range of 0.2 to 9.8 million
<br />acres) under the A1B scenario. Averaged over the region,
<br />this would increase the probability that 2.2 million acres
<br />would burn in a year from 5% to nearly 50%.14 Within
<br />the region, this probability will vary substantially with
<br />sensitivity of fuels to climatic conditions and local variability
<br />in fuel type and amount, which are in turn a product of forest
<br />type, effectiveness of fire suppression, and land use. For
<br />example, in the Western Cascades, the year-to-year variability
<br />in area burned is difficult to attribute to climate conditions,
<br />while fire in the eastern Cascades and other specific vegetation
<br />zones is responsive to climate.14 How individual fires behave in
<br />the future and what impacts they have will depend on factors
<br />we cannot yet project, such as extreme daily weather and
<br />forest fuel conditions.
<br />Higher temperatures and drought stress are contributing to
<br />outbreaks of mountain pine beetles90,91 that are increasing pine
<br />mortality in drier Northwest forests. This trend is projected
<br />to continue with ongoing warming.14,92,93,94Between now and
<br />the end of this century, the elevation of suitable beetle habitat
<br />Forest Mortality
<br />Figure 21.6. Forest mortality due to fire and insect activity is already
<br />evident in the Northwest. Continued changes in climate in coming
<br />decades are expected to increase these effects. Trees killed by a
<br />fire (left side of watershed) and trees killed by mountain pine beetle
<br />and spruce beetle infestations (orange and gray patches, right
<br />side of watershed) in subalpine forest in the Pasayten Wilderness,
<br />Okanogan Wenatchee National Forest, Washington, illustrates how
<br />cumulative disturbances can affect forests. (Photo credit: Jeremy
<br />Littell, USGS).
<br />is projected to increase as temperature increases, exposing
<br />higher -elevation forests to the pine beetle, but ultimately
<br />limiting available area as temperatures exceed the beetles'
<br />optimal temperatures.14,92,93 As a result, the proportion of
<br />Northwest pine forests where mountain pine beetles are most
<br />likely to survive is projected to first increase (27% higher in
<br />2001 to 2030 compared to 1961 to 1990) and then decrease
<br />(about 49% to 58% lower by 2071 to 2100).9z For many tree
<br />species, the most climatically suited areas will shift from their
<br />current locations, increasing vulnerability to insects, disease,
<br />and fire in areas that become unsuitable. Eighty-five percent of
<br />the current range of three species that are host to pine beetles
<br />is projected to be climatically unsuitable for one or more of
<br />those species by the 2060s,14,96 while 21 to 38 currently existing
<br />plant species may no longer find climatically appropriate
<br />habitat in the Northwest by late this century.
<br />Consequences and Likelihoods of Changes
<br />cannot yet be predicted, but such events could have enormous
<br />impacts for forest -dependent species.88 Increased wildfire
<br />could exacerbate respiratory and cardiovascular illnesses in
<br />nearby populations due to smoke and particulate pollution
<br />(Ch. 9: Human Health).107,108
<br />These projected forest changes will have moderate economic
<br />impacts for the region as a whole, but could significantly affect
<br />local timber revenues and bioenergy markets.109
<br />U.S. GLOBAL CHANGE RESEARCH PROGRAM 495 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
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