Phenotypic Plasticity and Genetic Diversity Elucidate Rarity and Vulnerability of an Endangered Riparian Plant

AbstractEnvironmental change, accelerated by anthropogenic activities, threatens many species and can be especially challenging for rare species given their potentially limited capacity for migration and adaptation relative to more common species. The ability to acclimate via phenotypic plasticity could provide an important path to species persistence in the face of such change. We investigated the responses of an endangered plant species endemic to a highly dynamic riparian habitat in southeastern Tennessee, USA, and its most widespread congener to environmental change to elucidate their current statuses and future vulnerability. Specifically, we compared the population‐ and species‐level plasticity of rarePityopsis ruthiiand commonP. graminifoliato contrasting light, temperature, and water conditions in a growth chamber experiment to evaluate their potential to acclimate to environmental change. Contrary to our expectations,P. ruthiihad greater phenotypic plasticity than its common congener in response to both altered light and water availability. But this plasticity was not associated with increased fitness, suggesting that it was not adaptive. Concurrently, we genotyped these individuals at nine putatively neutral microsatellite loci to contrast genetic diversity across the range of each species. As expected,P. ruthiiexhibited reduced genetic diversity relative to its more common congener. Overall, our findings accord with the narrow range and current habitat specificity ofP. ruthii, especially its tolerance of highly variable water, and highlight its potential vulnerability to future environmental change.