A QUANTITATIVE PHYTOGEOGRAPHIC ANALYSIS OF SPECIES RICHNESS IN FORESTCOMMUNITIES OF THE PARANA RIVER DELTA, ARGENTINA

A biogeographic analysis of 26 species of trees and shrubs occurring i n the Parana River Delta (32 degrees to 34 degrees 30' South latitude, 58 degrees to 61 degrees West longitude, Argentina) was performed to study the floristic and structural differences among deltaic woodland communities. Traditionally these variations have been interpreted as a consequence of the differential influence of local island geomorpholo gy and flood dynamics on floristic successions. We performed a cluster analysis of collection site, obtained maps showing the joint occurren ce of the species of each community, and related species ranges with t he presence of possible migration corridors. Results suggest that, in addition to local variations in topography and flood persistence, chan ges in floristic richness can be understood as a consequence of the di fferential efficiency of the Parana and Uruguay Rivers as migration co rridors for tropical species into temperate latitudes. Geomorphical st ability of riverbeds and fluvial dynamic differences between rivers ar e discussed in relation to how they may explain the more efficient mig ration of species along the flood plain of the Uruguay River compared to the Parana River. Results allow us to re-interpret the relative imp ortance of both rivers in the establishment of tropical and subtropica l species in temperate latitudes, and to add a regional explanation fo r the observed floristic heterogeneity.