Floristic patterns of alluvial forests in Atlantic Forest and Pampa: Climate and geographic insertion as determining factors.

The aims of this study were to identify floristic assemblages for alluvial forests in the Atlantic Forest and Pampa regions in Brazil, assess the level of floristic similarity between assemblages, and determine environmental gradients and indicator species for these assemblages. Surveys carried out in alluvial forests in the Atlantic Forest and Pampa regions were selected, based on which a presence/absence matrix was built for tree species. A cluster analysis was performed to verify the existence of species assemblages. Floristic similarity was determined by means of the Sorensen Distance measure, from which a dendrogram was developed. The floristic matrix was ordinated by means of NMDS. A PCA was performed with climatic data from areas to determine environmental gradients. An assessment of indicator species was carried out afterwards. Two groups of areas not related to the separation of the regions became visible from the dendrogram and were corroborated by NMDS. Temperature, rainfall and altitude gradients were synthesized by the PCA. Gymnanthes klotzschiana and Andira fraxinifolia were the most relevant species, respectively, in the Paraná-Uruguay and Atlantic assemblages. Alluvial forests were gradually separated in two floristic assemblages associated with river basins and migration routes, while especially influenced by tropicality and altitude gradients.