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, there are still outstanding questions about how they interact and vary across closely related species. 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 characterization, and a controlled dry-down experiment to understand how geographic distribution and evolutionary background among three closely related Phlox wild flower species and their F1 hybrids explain their responses to water availability. We infer that the species occupy distinct niches that diverge along a primary axis of water availability and soil moisture. Each species has a distinct 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.