Tuesday, August 07, 2007

Tree Deaths (and Wildfire Risk) in Sierra Nevada Increase With Climate Change

In light of last month's catastrophic Lake Tahoe wildfire, which destroyed dozens of homes, a new study's finding that tree deaths (and thus fuel for more wildfires) are increasing due to warming temperatures is particularly sobering. Here's the author's news release via the U.S. Geological Survey (USGS):

Apparent climatically induced increase of tree mortality rates in a temperate forest: After tracking the fates of more than 20,000 trees in a network of old—growth forest plots in the Sierra Nevada of California for over two decades (1983 — 2004), USGS scientists found that tree death rates have increased significantly over the past 20 years. Death rates increased not only for all trees combined, but also across most elevational zones and for the two dominant groups of conifers, firs and pines. The rising death rate for trees was paralleled by increasing summer drought due to warming temperatures. These findings suggest that Sierran forests, and potentially other forests of dry climates, may be sensitive to temperature—driven increases in drought, making them vulnerable to extensive die—back during otherwise normal periods of reduced precipitation.

With more dead trees equaling more fuel for fires, local officials will need to carefully consider how to tweek zoning laws to keep new homes out of vulnerable areas, as well as ensure that building codes and property maintenance requirements are adequate (and adequately enforced) to minimize risk to homes and businesses.

Possible building code solutions include mandating fireproof roofs, a layer of fireproofing around homes at ground level, and keeping piles of fireplace logs in a fire-proof location, rather than right on the side of the house. Brush and pine needles should also be cleared from the area immediately around homes.

What about forest thinning treatments to reduce fuels in the forests surrounding Tahoe homes and businesses, you may ask? These kinds of treatments are generally intended to slow the spread of crown fires by increasing the distance between trees. Interestingly, however, I've heard reports from Tahoe locals that it was actually both ground fires and, especially, airborne embers from burning trees and houses, not crown fires, that resulted in the destruction of many homes. Ground fires would not be significantly slowed by forest thinning. In fact, the fire spread right through areas that were thinned only months ago, jettisoning burning embers high into the air, many apparently landing on and igniting inadequately fireproofed homes. Some think that thinning may have even made the area more vulnerable the type of fire that occurred by increasing exposure to the drying influences of sunlight and wind. For the many homes that were not only located in highly fireprone habitats, but also inadequately fireproofed, the result proved disastrous.

Whatever the solutions -- better zoning, much better fireproofing of structures, or adjustments to forestry practices -- this new finding that warming temperatures are increasing tree death and wildfire danger provides yet another illustration of how climate change is very much a people issue.

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