Directional selection shifts trait distributions of planted species in dryland restoration

1. The match between species trait values and local abiotic filters can
restrict community membership. An often-implicit assumption of this
relationship is that abiotic filters select for a single locally optimal
strategy, though difficulty in isolating effects of the abiotic
environment from those of dispersal limitation and biotic interactions has
resulted in few empirical tests of this assumption. Similar constraints
have made it difficult to assess whether the type and intensity of abiotic
filters shift along gradients of environmental harshness, as predicted by
the stress dominance hypothesis. 2. We planted 9,216 plants of perennial
grass and forb species that had a range of functional trait values and
were assigned to a warm, intermediate, or cool temperature tolerance pools
across eight sites on the Colorado Plateau. We compared the distributions
of traits of surviving individuals to null distributions to evaluate
whether there were shifts in trait means and variation. Borrowing from
phenotypic selection concepts in evolutionary biology, we assessed support
for stabilizing, directional, and disruptive abiotic filtering of trait
distributions and whether these types of filtering varied with initial
species pool. 3. Functional composition was significantly different from
null distributions for nearly all traits at all sites, with trait
variation more restricted in harsher abiotic conditions, supporting the
stress-dominance hypothesis. Contrary to expectations, we primarily found
evidence for directional selection, which increased in frequency in warm
species pools while disruptive selection was found more often in cool and
intermediate species pools. 4. Synthesis: This study provides a controlled
experimental approach to test the effect of the abiotic environment on
plant trait filtering. We found that opportunistic strategies allowing for
rapid water acquisition during favorable periods improved survival at
warmer sites. Species with these strategies may be expected to benefit
from increasing aridity and may be selected for active management efforts.
More generally, the prevalence of directional selection may have important
implications for dynamic vegetation models that rely on trait
distributions for translating environmental variation into ecosystem
processes.