PARENT SESSION
Oral Session #54: Fire Ecology -- Trees, forests, woodlands.
Presiding: E. Menges
Wednesday, August 7. 8:00 AM to 11:30 AM. Apache Meeting Room, TCC.
Historical fire in pine-oak forests, Sierra Madre Occidental, Mexico: Influence of climate and land use.
Heyerdahl, Emily*,1, Alvarado, Ernesto2, 1 USDA Forest Service, Rocky Mtn. Res. Stn., Missoula, MT2 University of Washington, Seattle, WA
ABSTRACT- Our objective was to infer the drivers of temporal variation in fire regimes in pine-oak forests of the Sierra Madre Occidental in north central Mexico. We reconstructed a multicentury history (1772-1994) of the occurrence of surface fires from 1,469 fire scars on 180 trees sampled at 8 sites over about 700 km in the states of Durango and Chihuahua. We compared our fire histories to existing tree-ring reconstructions of winter and early summer precipitation and the Southern Oscillation Index. Fire intervals were similar among our sites, with Weibull median fire intervals of 3 to 6 years. Most fires probably burned in the warm, dry spring, based on the intra-ring position of fire scars (98% formed during the season of radial dormancy or early in the growing season) and the seasonality of precipitation, lightning and modern fires in this region. However, some fall or winter fires may have occurred. Annual variation in precipitation and El Niño/Southern Oscillation were strong drivers of current year's fire, probably through their effects on fuel moisture. Extensive fires generally burned during dry years but not during wet ones. Extensive fires also typically burned during La Niña years, which tend to have dry winters in this region. Climate in prior years was also a strong driver of fire, through its effect on fuel amount. Widespread fires often burned following one to two wet years and also following El Niño years, which tend to have wet winters in this region. Likewise, fires were not widespread following dry years and following La Niña years. Prior year's climate probably affected the growth of grass and herbaceous fuel. Changes in land use, rather than climate, probably caused the near cessation of fire that we reconstructed at some sites because these shifts did not occur synchronously (some ca. 1900, some ca. 1950). Frequent surface fires continued to burn until the time of sampling at some sites.
KEY WORDS: surface fire history, Mexico, tree rings, climate