DOI: 10.5061/dryad.1023m1d
title: Data from: Growth responses to soil water potential indirectly shape local species distributions of tropical forest seedlings

description: 1. Local tree species distributions in tropical forests correlate strongly
 with soil water availability. However, it is unclear how species
 distributions are shaped by demographic responses to soil water
 availability. Specifically, it remains unknown how growth affects species
 distributions along water availability gradients relative to mortality. 2.
 We quantified spatial variation in dry season soil water potential (SWP)
 in the moist tropical forest on Barro Colorado Island, Panama, and used a
 hierarchical Bayesian approach to evaluate relationships between
 demographic responses of naturally regenerating seedlings to SWP (relative
 growth rates and first-year mortality) and species distributions along the
 SWP gradient for 62 species. We also tested whether species that were more
 abundant at the wet or dry end of the gradient performed better (1) at
 their ‘home end’ of the gradient (‘best at home’ hypothesis) and (2) ‘at
 home’ compared to co-occurring species (‘home advantage’ hypothesis). 3.
 Four and five species responded significantly to SWP in terms of growth or
 mortality, respectively. Growth (but not mortality) responses were
 positively related to species distributions along the SWP gradient;
 species with a more positive (negative) growth response to SWP were more
 abundant at higher (lower) SWP, i.e. at wetter (drier) sites. In addition,
 wet distributed species grew faster on the wet end of the SWP gradient
 than on the dry end (‘best at home’) and grew faster on the wet end than
 dry distributed species (‘home advantage’). Mortality rates declined with
 seedling size for all species. Thus, seedling growth responses to SWP
 indirectly shaped local species distributions by influencing seedling size
 and thereby mortality risk. 4. Synthesis. By demonstrating how growth
 responses to spatial variation in soil water availability affect species
 distributions, we identified a demographic process underlying niche
 differentiation on hydrological gradients in tropical forests. Recognizing
 the role of these growth responses in shaping species distributions should
 improve understanding of tropical forest composition and diversity along
 rainfall gradients and with climate change.
description: Soil moisture in the
 50-ha plot on BCI, PanamaBCI_SWP_SWC.txt holds
 soil water potential (SWP) and soil water content (SWC) at the 200
 seedling census sites in the 50-ha plot on BCI, Panama. We measured SWP
 and SWC during four sampling rounds in the 2015 dry season (February,
 March, April) and the 2016 dry season (March). We calculated the median
 SWP per site to characterize dry-season soil water
 status.BCI_SWP_SWC.txtSoil water retention curves in the 50-ha plot on BCI, PanamaBCI_retention_curves.txt holds soil water retention curves from 25 seedling census sites in the 50-ha plot on Barro Colorado Island, Panama. We constructed the curves using soil samples that we first used to measure SWP and SWC (see file BCI_SWP_SWC.txt). We used the curves to determine outliers in SWP measurements.BCI_retention_curves.txt