Page 340 - Environment: The Science Behind the Stories

carefully controlled conditions (see Figure 1.8b, p. 28). These
prescribed burns, or controlled burns, clear away fuel loads, 100
nourish the soil with ash, and encourage the vigorous growth Temperature 15
of new vegetation. Because they are time-intensive and some- Number of large fires in West 14 Mean summer temperature (°C)
times are misunderstood by politicians and the public, pre- 0
scribed burns are conducted on only a small proportion of Wildfires 13
land (about 2 million acres per year). As a result, vast areas 1970 1975 1980 1985 1990 1995 2000
of American forests remain vulnerable to catastrophic fires. Year
In the wake of major fires in California in 2003, the
U.S. Congress passed the Healthy Forests Restoration Act. (a) Wildfires correlate with hotter summer temperatures
Although this legislation encouraged some prescribed burn-
ing, it primarily promoted the physical removal of small
trees, underbrush, and dead trees by timber companies. The
removal of dead trees, or snags, following a natural distur-
bance (such as a fire, windstorm, insect damage, or dis-
ease) is called salvage logging. From a short-term economic
standpoint, salvage logging may seem to make good sense.
However, ecologically, snags have immense value; the
insects that decay them provide food for wildlife, and many
animals depend on holes in snags for nesting and roosting.
Removing timber from recently burned land can also cause
soil erosion, impede forest regeneration, and promote fur-
ther wildfire (see THE SCIENCE BEHIND THE STORY, pp. 342–343).

WEIGHING THE ISSUES

HOW TO HANDLE FIRE? A century of fire suppression has
left vast areas of North American forests vulnerable to cata-
strophic wildfires. Prescribed burning helps to alleviate this (b) Mountain pine beetles kill more trees in a warmer climate
risk, yet we will never have adequate resources to conduct
careful prescribed burning over all these lands. Can you sug- FIGURE 12.18 Forests face increasing threats as temperatures
rise. The number of large wildfires in the western United States
gest solutions to help protect people’s homes in the wildland- (a) has risen along with the region’s summer temperatures. Warm
urban interface while improving the ecological condition of summers and mild winters have favored bark beetles (b), which are
forests? Do you think people should be allowed to develop killing vast areas of trees throughout the West. Source (a): Westerling,
homes in fire-prone areas? A.L., et al., 2006. Warming and earlier spring increase western U.S. forest wildfire
activity. Science 313: 940–943, Fig 1A. Reprinted with permission from AAAS.

Climate change and pest Scientists studying beetles and their impacts say there
outbreaks are altering forests are two primary reasons for today’s extraordinary outbreaks.
One is that past forest management has resulted in even-
Global climate change (Chapter 18) is now worsening wildfire aged forests across large regions, and many trees in these
risk by bringing warmer weather to most of North America forests are now at a prime age for beetle infestation. Plan- CHAPTER 12 • FOREST S, FOREST MAN A GEMENT, AND PR O TECTED AREAS
and drier weather to much of the American West. Scientific tation forests dominated by single species that the beetles
climate models predict further warming and drying (pp. 514, prefer are most at risk. The second reason is climate change.
504). Research suggests that the recent increase in fires in Milder winters allow beetles to overwinter further north, and
North America has been driven in part by warmer, drier cli- warmer summers speed up their consumption and reproduc-
mate (FIGURE 12.18a). tion. In Alaska, beetles have switched from a two-year life
Climate change also is promoting certain pest insects cycle to a one-year cycle. In parts of the Rocky Mountains,
that destroy forest trees. Bark beetles are small beetles that they now produce two broods per year instead of one. Mean-
feed within the bark of conifer trees. They attract one another while, droughts like those that have plagued the western
to weakened trees and attack en masse, eating tissue, laying and southern United States in recent years have stressed and
eggs, and bringing with them a small army of fungi, bacte- weakened trees, making them vulnerable to attack. On top
ria, and other pathogens (FIGURE 12.18b). Bark beetle infesta- of all this, beetle outbreaks create a positive feedback loop
tions can wipe out vast areas of trees with amazing speed, and (pp. 124–125); by killing trees, they reduce the amount of
today’s outbreaks are unprecedented. Since the 1990s, beetle carbon dioxide pulled from the air, and thereby intensify cli-
outbreaks have devastated more than 11 million ha (27 million mate change.
acres) of forest in western North America, killing an estimated Climate change benefits some species while harming oth-
30 billion conifer trees. ers, and as it interacts with pests, diseases, and management 339

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