Environmental and soil variables affecting the structure and floristic woody composition of oak forests of northeastern Mexico

The objective of this study was to investigate the influence of environmental and soil factors on the structure and floristic woody composition of oak forests. Oak forests of the Sierra de Zapaliname in northeastern Mexico (25 degrees N) are distributed in canyons and northwest slopes with higher humidity, surrounded by montane chaparral. We carried out a vegetation inventory across an altitudinal gradient. All trees with diameter at breast height of >= 3 cm were identified and measured. In addition, the cover of understory species was measured in each plot. Using multivariate techniques, we detected two oak forest associations: Quercus greggii - Q. mexicana forest and Q. saltillensis - Q. laeta forest. The forests studied are rich in woody species, including 19 tree and 50 shrub species. The number of tree species decreased at lower elevations, but the shrub species increased. Environmental variables evaluated included elevation, precipitation, temperature, soil pH and electrical conductivity, and soil percentage of sand, clay, and silt. All these variables revealed significant differences for the two oak forest associations. With respect to biotic parameters, significant differences (P < 0.05) in basal area were observed. Multivariate analysis revealed the species distribution along a moisture gradient across elevation. The ecological study of oak forests provides the basis for future research on forest dynamics and can guide conservation efforts to maintain species diversity and endemism in the mountain studied.