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September 2007

Phenomenon: The Life Cycle of Wildfires

Global warming creates conditions that invite larger and more frequent forest fires. Here's how.

Forests and wildfires form an unlikely ecological yin and yang: In a healthy climate, the apparently destructive blazes actually keep an entire ecosystem in balance. Fire accelerates the decomposition of organic material, returning nutrients to the soil; it also helps evergreens’ cones to open and lodgepole and jack pine seeds to germinate. But if flames come too often, or burn too intensely, they can sterilize the landscape, harming wildlife and opening the door to invasive species. Unfortunately, numerous studies have documented that global warming creates conditions that invite larger and more frequent conflagrations–which lead in turn to higher carbon emissions. Here’s a look at the vicious cycle.

[1] Mercury rises
Average spring and summer temperatures throughout the West have increased 1.6°F in just the last two decades. As a result, mountain snowpacks, which keep the soil moist and serve as the main water source for Western forests and rivers, are melting one to four weeks earlier.

[2] Forests dry out
Within a month of snowmelt, trees weaken and vegetation dries, making forests more vulnerable to parasites and fires. Dry seasons now last 78 days longer, on average, than they did 20 years ago.

[3] Beetles thrive
The combination of warmer year-round temperatures and longer summers has spurred the proliferation of forest-slaying mountain pine beetles by speeding up the bugs’ maturation and metabolism. “What’s normally a two-year life cycle has shortened to one,” says Don McKenzie, research ecologist with the Pacific Wildland Fire Sciences Lab in Seattle and lead author of a 2004 study examining the link between climate change and wildfires. The beetles lay their eggs in pines, eventually killing the trees (see page 56) and adding more fuel to the ecological tinderbox. Spruce beetles have killed more than 90 percent of spruce trees on Alaska’s Kenai Peninsula.

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