As in the Mojave, the fire problem in the Sonoran Desert is worsening. The 1994 fire in Saguaro National Park was spread by red brome. And Esque says his team's surveys in remote, unburned areas of the park have revealed that penetration by exotic grasses -- including a perennial, drought-adapted species from Africa known as buffelgrass -- is far worse than was previously known. "There wasn't a fire problem in this area before the exotic species came in," says Schwalbe. "Now we're seeing a biome conversion, from palo verde and saguaro habitat to a mesquite-acacia savannah with a Mediterranean exotic grass understory. That's the future of the Sonoran Desert -- especially near roads."

Southwest Forests

Some of the most extensive and detailed records of past fire activity come from the southwestern United States. Over thousands of years, this region's widespread ponderosa pine forests have been shaped and structured by fire. Historically, frequent low-intensity ground fires maintained open, park-like forests with grassy understories. Although such fires are often very local in nature, a broad historical perspective reveals regional-scale patterns of fire incidence and intensity, driven by climatic variability.

Dr. Craig Allen has studied the history and effects of fire in the Jemez Mountains of northern New Mexico since 1986. He and his collaborators employ several different methods for reconstructing the fire history of the Jemez and neighboring Sangre de Cristo mountains. Fires that do not kill a tree often leave a scar, which is recorded in the tree's annual growth ring. By carefully examining the tree rings, researchers can determine the year and often even the season in which the fire occurred.

Allen's team has put together over 4,500 fire dates, from over 600 trees, logs and stumps. "The Jemez is one of the better-sampled landscapes of
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Contact: Catherine Haecker
catherine_haecker@usgs.gov
707-826-5645
United States Geological Survey
17-Sep-1999