Hydrological Regime and Climate Interactively Shape Riparian Vegetation Composition along the Colorado River, Grand Canyon

Questions: How closely do riparian plant communities track hydrological and climatic variation in space, and how do interactions among hydrological and climatic filters influence success of flow management strategies? Location: Grand Canyon, Arizona, USA. Methods: Multi-year vegetation surveys were conducted across three hydrological zones-active channel, active flood-plain and inactive flood-plain-within each of 42 sandbars that vary geographically in temperature and precipitation along a 400-km river segment. Ecological niche models were used to estimate locally optimal conditions of maximum inundation duration, elevation above daily peak flow, mean annual precipitation and mean maximum and minimum temperature for 16 of the most abundant woody and 58 of the most abundant herbaceous plant species. These estimates were used to calculate CWM environmental preferences, which were used to determine how closely vegetation preferences tracked local variation in environmental factors, and to assess interactive responses of species and communities to variation in hydrology and climate. Results: Communities closely tracked hydrological variation across zones, but less so within zones. Communities tracked variation in minimum temperature more closely than maximum temperature or precipitation. At the species level, woody plants that were more abundant in wetter hydrological conditions were also more abundant in wetter climatic conditions, and vice versa. This relationship was even stronger at the community level, where there were significant negative relationships between CWM preferences of inundation duration and temperature for both woody and herbaceous vegetation. Conclusions: The climate-hydrology linkages found in this system suggest that increasing temperatures and drought are likely to reduce the inundation tolerance of riparian vegetation within the Grand Canyon. Increasing the duration of high-flow events would likely reduce the abundance of encroaching woody vegetation but could also reduce the resilience of remaining vegetation to heatwaves and drought. The reinforcing effects of climatic and hydrological filters are likely to generally result in greater sensitivity of species composition to environmental change than if those environmental filters acted independently. These results have implications for predicting resource responses to environmental change, as well as prescriptions for direct vegetation management to enhance resilience.