DOI: 10.5061/dryad.ngf1vhhqm
title: Atmospheric and soil drought risks combined shape community assembly of trees in a Tropical Dry Forest

description: 1. Predicting plant community assembly is challenging in part because the
 influence of environmental conditions via plant functional strategies and
 the relevance of mechanisms of community assembly change across habitats
 and these changes remain poorly studied. 2. To assess how environmental
 conditions drive species sorting in a tropical dry forest, we used the
 combined RLQ and Fourth-Corner methods to analyze changes in tree species
 assemblages among sites with distinct atmospheric and soil drought risks.
 We asked how variation in potential radiation, soil water availability and
 evapotranspiration interact with functional traits to explain the
 cross-site sorting of species lying along a continuum of drought coping
 strategies ranging from acquisitive to conservative resource-use. 3.
 Environment and traits were tightly related. Opposing common expectations
 on the success of strategies in resource limited environments,
 drought-tolerant conservative species with dense tissues and tardily
 deciduous leaves were favored on sites with higher resource (soil water)
 availability. Drought-avoiding acquisitive species with water storing
 tissues and thin, light leaves with short retention time periods were
 favored in sites with drier soils. A decoupling of stressing conditions
 caused by soil and atmospheric aridity combined with differences in
 species adaptations to each of these factors can explain the apparent
 discrepancies. 4. Synthesis. Drought stress gradients entailed shifts in
 community functional composition. We show that atmospheric and soil
 drought risks can be decoupled and jointly determine species distribution
 in relation to their functional strategies. By considering atmospheric
 drought stress, a key yet often overlooked factor in studies of community
 assembly, we offer a novel explanation to a seemingly contrasting pattern
 in tropical dry forests where species with acquisitive, rather than
 conservative strategies, predominate in the most resource-limited yet less
 water demanding environment. More generally, our results emphasize the
 need for detailed studies of the multiple habitat-dependent relationships
 between traits and environment to advance our predictive understanding of
 species distributions and community assembly.
description: Data on woody vegetation (free standing individuals with a
 diameter at breast height, DBH, ≥ 2.5 cm) were obtained from 36 100
 m<sup>2</sup> plots (10 × 10 m) located at low, middle and top
 parts of slopes in the North (N) and South (S) faces of three hills with
 elevations between 380 and 780 m a.s.l. (site description details on:
 Méndez-Toribio, Meave, Zermeño-Hernández, & Ibarra-Manríquez,
 2016). Such sampling encompasses the range of floristic and environmental
 variability in the area and represents water availability gradients that
 go from sites with high soil moisture and evapotranspiration (e.g., lower
 N-facing slopes) to sites with low soil moisture and evapotranspiration
 (e.g., higher S-facing slopes).             Air
 temperature (AT) above soil level, potential incoming solar radiation
 (Rad, including direct and diffuse radiation), potential
 evapotranspiration in the rainy season
 (<i>ET</i><sub>0</sub>, months with &gt; 100
 mm), and Topographic Wetness Index (TWI) were calculated for each sampling
 plot. Air temperature was recorded hourly from September 2012 to December
 2013, with Hobo® Pro v2, U23-001 data loggers placed on a branch of the
 tree closest to the plot center at 3 m above the ground, where most of the
 tree foliage develops. Rad was calculated with the “area solar radiation”
 module of the Spatial Analyst Tools in ArcGis 9.3 ESRI (2009) using a 20 m
 digital elevation model (DEM) as input. The DEM was built from topographic
 vector maps (1:50,000 scale) of the studied landscape (~ 4.02 × 109
 km<sup>2</sup>). Rad was defined as the quantity of solar
 energy (MJ m<sup>-2</sup>) on each pixel (20 × 20 m) of the
 image and estimated monthly for the year 2014 by assuming a clearly
 uniform sky. Air temperature and Rad were used to estimate
 <i>ET</i><sub>0</sub>, a crucial indicator of the
 potential atmospheric drought stress suffered by a plant (van der Maarel,
 2005). Potential evapotranspiration was estimated as:
 <i>ET</i><sub>0</sub> = 0.0135 (KT) (Rad) × 0.408
 (TD) (TC + 17.8), where TD = T<sub>max</sub>
 -T<sub>min</sub> (°C), TC = average daily air temperature (°C)
 and KT is an empirical coefficient for interior (i.e. non-coastal) regions
 equal to 0.162 (Zohrab, 2000). The TWI (Beven, 1997) was calculated from
 an hydrological model (TOPMODEL) via the Raster Calculator module of
 ArcGis 9.3. The TOPMODEL calculates the wetness index k = ln (A/tanβ). To
 this end, the DEM of the area is used to create a slope raster (β) and a
 flow direction raster. This latter raster is used to create a flow
 accumulation raster, which identifies the upslope contributing area (A)
 for each pixel. Further methodological details can be
 found in the source publication.  
description: The complement dataset pertaining to the species functional
 traits has been archived in the TRY database and is publicly available
 from https://www.try-db.org