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Different forests, different fires

Virtually all forests in western North America burn at some point. Some types burn infrequently in high-intensity, stand-replacement fires, while still others experience a range of burning intensities and frequencies. The ponderosa pine forests of the American Southwest, growing in a semi-arid climate and subject to frequent lightning strikes, have a natural fire regime of frequent, low-intensity fires.

The Inner Basin of the San Francisco Peaks
The Inner Basin of the San Francisco Peaks. Photo courtesy of USGS.

The San Francisco Peaks outside Flagstaff contain a variety of forest types. Their base lies at an elevation of about 7,000 feet, in a forest composed mainly of ponderosa pine trees. Numerous fire history studies in this forest have shown that fire occurred often in these forests in the centuries preceding Euro-American settlement in the 1870s. Whether set by lightning or by Native Americans, these fires burned through the forest as frequently as every few years. Because fire was so common, fuels did not accumulate much. The forest remained open, with a grassy understory that fed frequent fires.

A thousand feet higher, ponderosa pine intermixes with Douglas-fir, aspen, limber pine, and other tree species. Research by Tom Heinlein, formerly of NAU’s Ecological Restoration Institute, has shown that fire was fairly common in this forest zone too. Some of these fires may have burned less frequently but more intensely than those at lower elevations, since there is typically more moisture, more growth, and hence more fuel here. Where fires did kill conifers, aspen groves sprang up.

Higher still ponderosa pines and Douglas-firs give way to Engelmann spruce and subalpine fir. This dense, shady forest receives much snowfall and remains too moist to burn in most years. But occasionally it does become flammable during dry periods. Early settlers wrote that intense, large fires did range in such places on occasion. These fires killed large swaths of mature trees and created room for new aspen groves, but they also generally burned in a patchwork pattern, sparing some conifer groves in drainages or on cool north-facing slopes.

At the highest elevations—above 10,500 feet or so—scattered bristlecone pines and subalpine firs present targets for lightning strikes, but the forest at these elevations is so open that widespread fires were very rare.

These different forest types and fire regimes make it difficult to generalize about forest fire in the Southwest. Frequent low-intensity fires in ponderosa pine forest are “natural,” but so are infrequent stand-replacing fires in spruce-fir forests—the sort of fires that observers often label “catastrophic.” Clearly forest managers, and anyone interested in forest issues, need to treat these different forest types differently.

P.F.

References

Arno, S. F., and S. Allison-Bunnell. 2002. Flames in our Forest: Disaster or Renewal? Washington, D.C.: Island Press.

Heinlein, T. A. 1996. Fire regimes and forest structure in lower mixed conifer forests: San Francisco Peaks, Arizona. M.S. thesis, Northern Arizona University, Flagstaff.

Swetnam, T. W., and C. H. Baisan. 1994. Historical fire regime patterns in the Southwestern United States since AD 1700. Pages 11-32 in C. Allen, ed. Fire effects in Southwestern Forests. Proceedings of the Second La Mesa Fire Symposium. USDA Forest Service General Technical Report RM-GTR-286, Los Alamos, New Mexico.

Last edited June 17, 2003

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