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 r ecent fire regimes: (I) an uninterrupted frequent fire regime, (2) fir e exclusion, and (3) fire exclusion followed by the return of fire. Re generation was dense and highly aggregated at all sites but the freque nt-fire overstory was random to uniform in spatial distribution and re latively open while the fire-excluded sites had clumped overstory tree s with a high density of smaller trees. Dominance by sprouting species was greatest at the fire-excluded sites. Mortality was spatially aggr egated at ail sites, consistent both with thinning by fire and density -dependent mortality, but competitive self-thinning appeared insuffici ent to counteract the increased tree density without fire. The return of fire after 29 years of exclusion reduced tree density but left over story trees aggregated and led to vigorous oak and alder sprouting. Fr equent fire disturbance is considered essential to maintain open pine forests; fire exclusion with or without subsequent fire appears to lea d to denser forests dominated by smaller trees of sprouting species.