DOI: 10.5061/dryad.qnk98sfkg
title: Dataset: Local hydrological conditions influence tree diversity and composition across the Amazon basin

description: 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.
description: 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.