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We inventoried 10 ha of late-successional and seasonally inundated black-water floodplain (igap) forest along four river sections of the Negro River, Central Amazonia, Brazil. The aim of the study was to test if tree species composition and diversity changes along the river course, and whether these changes reflect the different geological formations of the Negro River. On a continental-wide scale, we assessed alpha-diversity patterns of black-water flooded forests across the Amazon and Orinoco basins. Phytosociological analyses include family and species importance, species similarity, and Fisher's alpha-diversity, as well as Detrended Correspondence Analysis. A total of 6.126 individuals were recorded, belonging to 243 tree species. Only few tree species occurred in more than one river section, and floristic composition changed abruptly from one section to the other. Tree species richness ranged from 57 to 79 species ha(-1), and alpha-diversity was highest (27.24) in the lower river section upon sediments of Pliocene-Pleistocene origin. We found a gradual decrease in species diversity with increasing age of the geological formations. The igap forest is relatively species-poor, which we interpret to be the result of general low nutrient availability in alluvial substrates of the Negro River.
