Effects of slope aspect and topographic position on environmental variables, disturbance regime and tree community attributes in a seasonal tropical dry forest

Questions: What is the magnitude of the differences in environment and chronic human disturbance between contrasting slope aspects and topographic positions in a seasonally dry tropical forest? What is the effect of such topography-related differences on composition, structure and diversity attributes of the tree community of this forest? Location: Tziritzicuaro, Michoacan State, southern Mexico. Methods: Vegetation was sampled in 36 100-m(2) plots evenly distributed among three topographic positions (lower, middle and upper parts of a slope) and two slope aspects (N- and S-facing). Environment at these sites was described through modelling incoming solar radiation and in situ recording of temperature during 1 yr. Disturbance was visually assessed in the field to calculate a Chronic Disturbance Index. Vegetation structure and diversity were compared among the resulting combinations of slope and topographic position. PERMANOVA and CCA were used to examine the multivariate relationship among vegetation, topography and disturbance. Results: Slope aspects and topographic positions differed in terms of annual mean temperature, potential energy income and evapotranspiration. Conversely, disturbance was not so clearly related to topography. Regarding vegetation structure, significant differences were only found for individual sizes and abundance; these values increased towards the upper portion of S-facing slopes, but decreased with elevation in N-facing slopes. Species diversity (S, Jacknife 1 and Fisher's alpha) was higher in S-facing slopes and increased from lower to upper topographic positions. PERMANOVA showed that vegetation structure and diversity were influenced by topographic position (12.9%) and the interaction between soil moisture and chronic disturbance with slope aspect (8.2% and 8.3%, respectively). CCA showed that a modest proportion (21%) of variation in species composition is explained by the combination of environmental and disturbance variables. Conclusions: Slope aspect and topographic position represent axes of environmental and disturbance differentiation. Although vegetation attributes respond to these ecological factors, they do not show homogeneous responses. Floristic composition is clearly linked to environmental heterogeneity, while structural attributes and alpha-diversity appear to be more closely related to human disturbance and soil moisture, particularly on S-facing slopes. Integrating environmental heterogeneity and human disturbance with topographic variability enhances our understanding of large variation in tree community attributes in seasonal dry tropical forests.