Atmospheric and soil drought risks combined shape community assembly of trees in a Tropical Dry Forest

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.