Variation in response to water availability across Phlox species

Plants adapt to environmental variation both by evolving divergent trait
means and by plastically adjusting trait expression in response to local
conditions. While these dual strategies are essential for persistence in
diverse environments, how they interact and vary across closely related
species is understudied. For plants, water availability is a particularly
important selective force that shapes species distributions, selects for
growth habit and life history strategy, and can dictate individuals’
plastic expressions of trait values and reproductive success. Here, we use
ecological niche modeling, field soil collections, and a controlled
drought experiment to test how biogeography and evolutionary history
influence responses to limited water availability in three closely related
Texas annual Phlox species and their F1 hybrids. We
infer that the species occupy distinct niches that diverge along a primary
axis of water availably and soil moisture. Each species has a distinct
vegetative growth habit that does not match broad predictions of
divergence in response to water availability. Nevertheless, we find that
all the species show a significant morphological response to controlled
soil dry down with reduced biomass, smaller leaves, and fewer flowers, as
would be predicted in a response to drought. We find that Phlox
drummondii, which occupies intermediate habitats, exhibits the strongest
plastic response to water limitation, despite it not having the broadest
environmental niche. Additionally, most hybrids involving P.
drummondii display intermediate phenotypes in both wet and dry
treatments, while hybrids between P.
cuspidata and P. roemeriana show phenotypes
consistent with hybrid vigor. These results challenge the hypothesis that
species from broader environments evolve greater plasticity. Instead, the
most plastic species did not have the broadest niche, suggesting
plasticity and niche breadth may evolve independently.