We carried out an exploratory analysis of structural and diversity patterns and related them to environmental variables in a riverine forest. The sample, divided into 100 subplots, was placed on the banks of the Rio Uruguai, Turvo State Park, Southern Brazil (27º09' S; 53º53' W). All trees with the perimeter at breast height >15 cm were recorded. Vegetation description was based on phytosociological parameters and Shannon's diversity index. We measured edaphic and topographic environmental variables as well as canopy coverage. The relationship between species abundance in the subplots and environmental variables was assessed by canonical correspondence analysis - CCA and "partial" CCA. We found 82 species belonging to 30 families. The CCA showed a clear gradient, related hierarchically to mean elevation and thus to flooding susceptibility at different sites. Three groups of subplots were identified by cluster analysis based on mean elevation and flooding susceptibility. There were structural differences among these groups, with the highest basal area and maximum height medians at the middle elevation sites. Selectivity caused by frequent flooding on the species pool affects structure at the lower sites, with the most surprising difference being the lack of a typical forest canopy at least 10 m tall. On the other hand, the upper sites are structurally similar to interfluvial forest, thus constituting a transitional section. Diversity measured by species richness and by Shannon's index was higher for middle elevation sites. This pattern fits proposed models that relate moderate disturbance regime to the generation of environmental heterogeneity, and thus, opportunities of diversification for species.
partial canonical correspondence analysis; spatial autocorrelation; Seasonal Forest; Turvo State Park; topography
