Page 3 - Climate Change Impacts in the United States

7: FORESTS

Key Message 1: Increasing Forest Disturbances

Climate change is increasing the vulnerability of many forests to ecosystem changes
and tree mortality through fire, insect infestations, drought, and disease outbreaks.

Insect and pathogen outbreaks, invasive species, wildfires, increased forest productivity, but extreme climate events can
and extreme events such as droughts, high winds, ice potentially overturn such patterns. 10
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storms, hurricanes, and landslides induced by storms are all
disturbances that affect U.S. forests and their management Factors affecting tree death – such as drought, physiological
(Figure 7.1). These disturbances are part of forest dynamics, water stress, higher temperatures, and/or pests and pathogens
are often interrelated, and can be amplified by underlying – are often interrelated, which means that isolating a single
trends – for example, decades of rising average temperatures cause of mortality is rare. 11,12,13 However, in western forests
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can increase damage to forests when a drought occurs. there have been recent large-scale die-off events due to one
Disturbances that affect large portions of forest ecosystems or more of these factors, 14,15,16 and rates of tree mortality are
occur relatively infrequently and in response to climate well correlated with both rising temperatures and associated
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extremes. Changes in climate in the absence of extreme climate increases in evaporative water demand. In eastern forests,
events (and the forest disturbances they trigger) may result in tree mortality at large spatial scales was more sensitive

Forest Ecosystem Disturbances

© Melanie Stetson Freeman/Getty Images

A Montana saw mill owner inspects a lodgepole
pine covered in pitch tubes that show the tree
trying, unsuccessfully, to defend itself against
the bark beetle. The bark beetle is killing
lodgepole pines throughout the western U.S.

© Pete McBride/National Geographic Creative

Figure 7.1. An example of the variability and distribution of major ecosystem
disturbance types in North America, compiled from 2005 to 2009. Forest disturbance
varies by topography, vegetation, weather patterns, climate gradients, and proximity
to human settlement. Severity is mapped as a percent change in a satellite-derived
Disturbance Index. White areas represent natural annual variability, orange Warmer winters allow more insects to survive
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represents moderate severity, and red represents high severity. Fire dominates the cold season, and a longer summer allows
much of the western forest ecosystems, and storms affect the Gulf Coast. Insect some insects to complete two life cycles in a
damage is widespread but currently concentrated in western regions, and timber year instead of one. Drought stress reduces
harvest is predominant in the Southeast. (Figure source: modified from Goetz et trees’ ability to defend against boring insects.
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al. 2012; Copyright 2012 American Geophysical Union). Above, beetle-killed trees in Rocky Mountain
National Park in Colorado.

177 CLIMATE CHANGE IMPACTS IN THE UNITED STATES