USGS studies wildfire ecology in the western United States

res of shrubland present in the National Conservation Area in 1979, only 46,000 hectares remain.

To put it simply, Knick says, the shrubland 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 shrubland, to destroying a shrubland, to ultimately maintaining an exotic grassland," Knick says.

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 shrublands 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 y

Contact: Catherine Haecker
United States Geological Survey

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