Spatial patterns of Mexican pine-oak forests under different recent fire regimes

Patterns of spatial arrangement, tree density, and species composition were compared in three unharvested pine-oak forests under different recent fire regimes: (1) an uninterrupted frequent fire regime, (2) fire exclusion, and (3) life exclusion followed by the return of fire. Regeneration was dense and highly aggregated at all sites but the frequent-fire overstory was random to uniform in spatial distribution and relatively open while the fire-excluded sites had clumped overstory trees with a high density of smaller trees. Dominance by sprouting species was greatest at the fire-excluded sites. Mortality was spatially aggregated at all sites, consistent both with thinning by fire and density-dependent mortality, but competitive self-thinning appeared insufficient to counteract the increased tree density without fire. The return of fire after 29 years of exclusion reduced tree density but left overstory trees aggregated and led to vigorous oak anti alder sprouting. Frequent fire disturbance is considered essential to maintain open pine forests; fire exclusion with or without subsequent fire appears to lead to denser forests dominated by smaller trees of sprouting species.