Habitat specifity, endemism and the neotropical distribution of Amazonian white-water floodplain trees

.The Amazon basin is covered by the most species-rich forests in the world and is considered to house many endemic tree species. Yet, most Amazonian ecosystems lack reliable estimates of their degree of endemism, and causes of tree diversity and endemism are intense matters of debate. We reviewed the spatial distribution of 658 of the most important flood-tolerant Amazonian white-water (varzea) tree species across the entire Neotropics by using data from herbaria, floras, inventories and checklists. Our results show that 90% of the varzea tree species are partially or widely distributed across neotropical macro-regions and biomes. Chi-square analyses indicated that varzea species richness in non-varzea macro-regions was dependent on the flooding gradient and the longitudinal position. Cluster analysis combined with association tests indicated four significant patterns of varzea species distributions depending on species flood-tolerance (low vs high) and spatial distribution (restricted vs widespread). We predict that the predominance of Andean substrates is the most important factor that determines the distribution of varzea tree species within and beyond the Amazon basin and explains the high floristic similarity to the Orinoco floodplains. Distribution patterns in other extra-Amazonian macro-regions are more likely linked to climatic factors, with rainforest climates housing more varzea species than savanna climates. 130 tree species were restricted to South-American freshwater floodplains, and 68 (> 10%) were endemic to Amazonian varzea. We detected two centers of endemism, one in the western Amazon characterized by low and brief floods, and one in the central Amazon, characterized by high and prolonged floods. Differences in taxonomic composition of endemic centers in the western and central Amazon are the result of different abiotic factors (i.e. flood regimes), as well as the regional species pools from where the species are recruited from. We hypothesize that numerous morphological, physiological and biochemical adaptations permit survival of trees in flooded environments. Furthermore, these adaptations are independently derived across many taxa and result in a highly specialized flora. We attribute higher than expected levels of endemism to the great spatial extent and age of floodplain ecosystems in the Amazon basin, and highlight the role of Amazonian varzea as an potential driver in speciation and diversification processes.