Patterns of floristic diversity and composition in floodplain forests across four Southern Amazon river tributaries, Brazil

Floodplain forests cover extensive areas of the Amazon basin, but the number of tree inventories is low. Vast floodplain regions therefore remain floristically unknown. We present a quantitative inventory of floodplain forests from four Amazon river basins (Jutaí, Juruá, Tefé and Purus), investigate within- and between-basin floristic similarity, and examine patterns of tree species dominance and distribution. Twelve hectares with all trees ≥10 cm dbh were inventoried; three hectares in each river basin. Rarefaction curves were used to compare species richness across study areas. GNMDS was used to investigate within- and between-basin floristic similarity, combined with an analysis of similarity (ANOSIM) to test for significant differences. In total, 7722 stems representing 518 species were recorded. Inventory plots from the same river basin were clearly clustered in the GNMDS ordination and the ANOSIM showed that floristic composition differed significantly both between and within study areas. Fabaceae was the most abundant family and Eschweilera albiflora the most abundant species. Only nine species were highly abundant in more than one study area, whereas 220 species were recorded in only one sample plot. Our results demonstrate high levels of beta-diversity in Amazonian floodplain forests. The high number of uncommon species is consistent with other studies.