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Frequent, low-intensity fire is a key disturbance agent in the long-needled pine forests of western North America, but little is known about the fire ecology of the Mexican forests which have been least affected by fire exclusion. We compared fire disturbance history and forest structure at four unharvested or lightly-harvested study sites differing in recent fire history. Frequent, low-intensity fires, recurring between 4 to 5 years for all fires and 6 to 9 years for widespread fires, characterized all the sites until the initiation of fire exclusion in the mid-twentieth century at three of the four sites. Although most fires in the study area are ascribed to human ignitions, evidence of both lightning and human-caused burning was observed on the study sites. A possible connection between fire occurrence and climate was indicated by a correspondence between regional fire years and positive extremes of the Southern Oscillation index, which is associated with cold/dry weather conditions. Forest ecosystem structures differed in ways consistent with the thinning and fuel consuming effects of fire. Two sites with extended fire exclusion were characterized by relatively dense stands of smaller and younger trees, high dead woody biomass loading, and deeper forest floors. In contrast, a site which had burned following a 29-year fire exclusion period, and the final site where frequent fires had continued up to the present, were both relatively open forests dominated by larger trees. The recently burned sites had lower dead woody biomass loading, especially of rotten woody fuels, and more shallow duff layers. The high regeneration density but low overstory density at the recently burned sites is also consistent with the thinning effect of low-intensity fire. Long-term management and conservation strategies for these forests should recognize the historic role of fire disturbance as well as the potential for changes in fire intensity and ecological effects following extended fire exclusion.

Fire regimes and forest structure in the Sierra Madre Occidental, Durango, Mexico