Divergent physiological responses of hydric and mesic riparian plant species to a Colorado River experimental flow

Riparian plant species can differ in their responses to streamflow variation in ways that strongly influence the composition and functioning of riparian plant communities. Quantifying these differences and the potential asymmetry of responses to low- versus high-flow phases of stream fluctuations is important for predicting and managing vegetation responses to variation in flow regimes. We measured the physiological response of two riparian plant species with different habitat preferences to an experimental flow that progressed from a low-flow to high-flow phase. Schedonorous arundinaceus, a hydric grass growing in near-channel habitat, exhibited significant and biologically substantial declines in stem water potential (SWP) during the low-flow phase of the experiment and a saturating increase in leaf relative water content (LRWC) during the high-flow phase. These patterns are consistent with cavitation risk in response to low-flow anomalies and saturating responses to high-flow anomalies in this hydric species. Pluchea sericea, a mesic shrub growing at intermediate elevations above the channel, exhibited a decrease in LRWC during the low-flow phase and an increase in SWP during the high-flow phase. These patterns are consistent with protection from stem cavitation risk during drought through leaf dehydration and opportunistic increases in water status in response to high-flow anomalies in this mesic species. The asymmetrical responses of both species to low- versus high-flow phases demonstrate unique physiological responses to flow anomalies of contrasting directions attributable to species habitat preferences and functional strategies that can be used by managers to predict non-linear vegetation responses to flow variation.