Tree diversity and composition in Amazonia are known to be strongly determined by the water supplied by precipitation. Nevertheless, within the same climatic regime, water availability is modulated by local topography and soil characteristics (hereafter referred to as local hydrological conditions), varying from saturated and poorly drained to well-drained and potentially dry areas. While these conditions may be expected to influence species distribution, the impacts of local hydrological conditions on tree diversity and composition remain poorly understood at the whole Amazon basin scale. Using a dataset of 443 1-ha non-flooded forest plots distributed across the basin, we investigate how local hydrological conditions influence 1) tree alpha diversity, 2) the community-weighted wood density mean (CWM-wd) – a proxy for hydraulic resistance, and 3) tree species composition. We find that the effect of local hydrological conditions on tree diversity depends on climate, being more evident in wetter forests, where diversity increases towards locations with well-drained soils. CWM-wd increased toward better-drained soils in Southern and Western Amazonia. Tree species composition changed along local soil hydrological gradients in Central-Eastern, Western and Southern Amazonia, and those changes were correlated with changes in the mean wood density of plots. Our results suggest that local hydrological gradients filter species, influencing the diversity and composition of Amazonian forests. Overall, this study shows that the effect of local hydrological conditions is pervasive, extending overwide Amazonian regions, and reinforces the importance of accounting for local topography and hydrology to better understand the likely response and resilience of forests to increased frequency of extreme climate events and rising temperatures.

We used the Amazon Tree Diversity Network (ATDN) dataset, with plots distributed throughout the Amazon basin. Our analyses were restricted to 1-ha lowland terra-firme forest plots below 500 a.s.l. (excluding plots on white sand and inundated forests) and to individuals with a diameter ≥ 10 cm, excluding all lianas. Plots varied in dimensions and shapes, with most being square or rectangular 1 ha, while 11.7% were 250 × 40 m and following altitudinal contours . In addition, we considered only plots with at least 80% of individuals identified to species level. As species identification was done by different taxonomists, we excluded 18 634 individuals (8.45% of the total number of individuals; mean = 42, min = 0, max = 173 individuals per plot) that were not identified to the species level to avoid confusion with morphospecies synonymy. This introduced no bias in the analyses, as there was no association between the proportion of morphospecies per plot with the main variables of interest (i.e. local hydrological conditions). By including only those individuals identified at the species level, more robust patterns of alpha diversity and composition are expected. We also excluded plots with georeferencing problems, such as those with coordinates displaced from terra-firme towards rivers or lakes. Finally, we excluded 18 plots from areas without height above nearest drainage (HAND) data. Thus, we carried out the analyses using 443 plots, which total 210 801 individuals of 3527 species, distributed in 619 genera and 104 families.