Diversity and distribution of tree species in subtropical Andean forests (Argentina, Bolivia)

This study describes and predicts diversity and distribution of tree species in subtropical Andean forests of northwestern Argentina and southern Bolivia. Results are based on 30 1-ha plots distributed across elevational and latitudinal gradients over 13,000 km2. First, I considered tree-species composition and turnover in relation to plot size, spatial extent and distance. Species turnover was inversely related to plot size. Turnover was dramatic across elevations; within elevations, turnover was lowest but increased sharply at short distances (<500 m), to remain high and unchanged at larger distances (1--220 km). Half of regional turnover was detected across elevations, a third within elevations, and less than a fourth within plots. These findings suggest that tree distributions were constrained by limited dispersal within localities and by environmental factors within and across elevations. Second, I quantified the effects of spatial and environmental factors, both at local and regional extents, on species abundance and composition. Results indicated (1) a strong role of regional climatic factors on species distributions, with a secondary role for local factors (topography and forest structure); (2) that distribution of trees was associated with gradients in minimum temperature, total annual rainfall and moisture stress, and that these gradients were associated with leaf phenology and dispersal syndrome; and (3) that species abundance of juvenile and adult trees were highly correlated, except for few species affected by disturbance events and climatic change. Third, I used ecological niche models (GARP) to predict the distribution of 36 tree species, based on biotic, topographic and climatic factors. I obtained significant models for 29 species; model performance was unrelated to sample size, species abundance, and species habitat breadth. Species previously thought to be restricted to particular mountain sectors were found in new areas based on modeled maps, questioning the role of isolation in the distribution of tree species. I also used modeled distributions to identify overlapping occurrences of timber species. They were found concentrated along the eastern edge of Andean forests, adjacent to croplands. This finding suggests that agriculture has advanced over areas of high timber value, undermining potential economic benefits and conservation strategies based on sustainable logging.