Wildfire ecology in the western United States, part 2

September 17, 1999
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Is a burned forest a healthier forest? Certainly by some standards, but Stephenson prefers to say that fire restores stability and resiliency to forest ecosystems. "We're restoring a forest structure that's more stable, meaning if you give it a shove it's less likely to be bent out of shape. And it's more resilient, because if you do bend it out of shape it will bounce back quicker."

Fire management and restoration programs in the Sierra National Parks now reflect much of what researchers like van Wagtendonk and Stephenson have learned about the behavior and ecology of wildfires. The current prescribed burning program, says Stephenson, is highly successful. "It's an excellent example of how research has fed into management and changed management direction."

'The daunting thing is that it's going to take a long time to replace what has been destroyed in the last 20 years ... We're looking at centuries if we rely only on natural processes for recovery.'

Nevertheless, says van Wagtendonk, "so much needs to be done, it's hard to get ahead of the game." One major constraint is smoke, which limits the amount of prescribed burning that can be done. Fire managers must work to stay within the bounds of clear air standards, and limit the amount of smoke descending on local communities. Stephenson says that while only a few prescribed fires create a smoke problem, these can erode public support for fire restoration. Continuing education is vital, he says, for people to understand that without some fire, both forests and human communities face the ever-growing danger of a major conflagration.

Interior and coastal shrub lands
While the decline of old-growth forests has long been a high-profile issue in the West, the widespread loss of arid shrub lands has gone practically unnoticed.

But in the sagebrush ecosystems of the Great Basin and the Columbia River Basin, fire and a non-native plant species known as cheatgrass are together transforming ecological communities across a vast area. These changes may be irreversible, says USGS ecologist Dr. Steve Knick of the USGS Forest and Rangeland Ecosystem Science Center.

Knick studies these transformations at the Snake River Birds of Prey National Conservation Area in southwestern Idaho. Here, as in much of the Great Basin, the dominant vegetation -- sagebrush and other shrubs adapted to the harsh seasonal climate -- is disappearing. Of the roughly 100,000 hectares of shrub land present in the National Conservation Area in 1979, only 46,000 hectares remain.

To put it simply, Knick says, the shrub land is burning up. Wildfire incidence has increased by a factor of three since 1980, and fires are getting larger. But behind this increase -- and in turn capitalizing on it -- is the fast-spreading, exotic annual grass. "It's a synergistic thing," says Knick. "Cheatgrass promotes fire spread, and the larger fires eliminate more shrubs."

Knick says that fire has always been a factor in sagebrush ecosystems, creating openings in the shrub canopy and constraining the density of woody plants much the same as in forests. But an understory of native bunch grasses, which grow in isolated patches, tends to limit the intensity of blazes in these systems and prevent them from spreading over a wide area.

Cheatgrass, which has been advancing since the early 1900s, in part due to overgrazing and drought, creates a continuous carpet of fuel. And cheatgrass thrives on recently burned land, thus perpetuating the altered fire regime. If the shrubs in an area don't have time to recover before the next fire hits, they eventually disappear. "Fire has gone from maintaining a shrub land, to destroying a shrub land, to ultimately maintaining an exotic grassland," Knick says.

Since 1910, chaparral fire frequency has not changed and fire size has not increased. Intense fires were equally common in the years before widespread fire suppression as today, and do not appear to be the result of fuels build-up.

Using data from a number of sources including satellite imagery, historical records of fire frequency and behavior, and ground measurements of vegetation, Knick's team has developed a computer model for predicting long-term changes resulting from different scenarios of burning and regeneration of vegetation. The model shows that in shrub lands with a cheatgrass understory, fire can easily trigger a rapid transition to grassland. But once established, these grassland systems tend to be relatively stable, even when fire is suppressed.

"The daunting thing is that it's going to take a long time to replace what has been destroyed in the last 20 years," Knick says. "We're looking at centuries if we rely only on natural processes for recovery." Knick's work suggests that preserving intact shrub lands through more active fire suppression may be the only way to halt the losses. Restoration practices based on prescribed burning, as have been successfully carried out in forest ecosystems, may not work well in invaded shrub lands.

"In forests you can use prescribed burning to remove a lot of the fine fuels, with the expectation that they are going to take several years to grow back," Knick says. "In shrub lands dominated by cheatgrass, the cheatgrass will be back next year. It's using a disturbance to try to eliminate a species that likes disturbance."

While interior shrub land ecosystems have only a limited tolerance for fire, a very different kind of fire dynamic exists in the chaparral shrub lands of coastal California. Ecologists have long known that chaparral ecosystems burn extensively and often, and much of the dominant vegetation in these systems is highly adapted to a fire-prone environment. Many plants have seeds that require fire to germinate, or need the kind of disturbed habitat fires leave behind in order to grow.

Dr. Jon Keeley, a USGS research ecologist with the Western Ecological Research Center, has studied the physiological adaptations that link the life cycles of chaparral vegetation with the natural regime of frequent brushfires. Upon reproduction, many species drop seeds that remain dormant in the soil "seed bank" until fire creates favorable growth conditions. When the area burns, these seeds receive a number of cues that may cause them to germinate. While seed germination in some species is stimulated by heat, in many others the onset of plant growth requires chemical exposure to combustion products such as charred wood.

In the Mojave Desert of California and Nevada, and the Sonoran Desert of Arizona, researchers are grappling with a fire and invasive species problem similar to that affecting Great Basin shrub lands.

Recently Keeley and Dr. C. J. Fotheringham, of California State University, Los Angeles, published a study demonstrating that for many species, smoke can also trigger seed germination. In some species smoke alone is sufficient to induce growth, while in others a combination of factors is required. Because of the diverse cues through which vegetation may respond to fire, blazes of different intensities or degrees of smoke production may result in different plants dominating the post-fire recovery. Of particular interest is their discovery, detailed last year in the journal Science, that nitrogen oxides, which are also important components of air pollution, are the chemicals in smoke responsible for germination of some species.

Keeley and his collaborators have also examined historical patterns of California shrub land wildfires. Life and property losses from shrub land fires in California have been increasing in recent decades. It has long been thought that fire suppression has played the same role in chaparral shrub land as it has in forests, creating a build-up of fuels that eventually leads to more destructive fires.

Surprisingly however, a close analysis of state fire records revealed a different story. This June, in the journal Science, Keeley and his co-authors reported that since 1910, chaparral fire frequency has not changed and fire size has not increased. The researchers found that large, intense fires were equally common in the years before widespread fire suppression as today, and do not appear to be the result of fuels build-up. In this highly fire-prone ecosystem, suppression efforts appear not to have greatly altered normal patterns of fire incidence. Keeley says the greater financial cost of fires today is more likely the result of constant urban expansion into areas subject to frequent burning.

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