Hotter Temperatures Alter Riparian Plant Response to River Regulation and Environmental Gradients

Hotter Temperatures Alter Riparian Plant Response to River Regulation and Environmental Gradients

Emily Palmquist^1*, Kiona Ogle², Bradley Butterfield³, Thomas Whitham⁴, Gerard Allan⁵, Patrick Shafroth⁶ 

¹U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, Flagstaff, AZ, USA

²School of Informatics, Computing and Cyber Systems, Northern Arizona University, Flagstaff, AZ, USA

³Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, USA

⁴Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, USA

⁵Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ, USA

⁶U.S. Geological Survey, Fort Collins Science Center, Fort Collins, CO, USA

River regulation and climate change are altering key variables that control riparian plant species occurrence and cover. Simultaneous changes to air temperature, river flows, and sediment regimes could result in novel environmental conditions and unanticipated riparian plant responses. These patterns may be pronounced in river systems with flow patterns altered by hydropower production, such as the Colorado River. We used a large dataset from the Colorado River downstream of Glen Canyon Dam, Arizona, USA to address three questions: 1) Does hotter mean annual temperature modify riparian plant response to hydrology, substrate composition, topography, and cover of other plant species? 2) Does the timing of hydropower tides affect plant responses, particularly by benefiting some species over others? 3) Are there species dependent upon fine sediments that are impacted by dam-induced erosion?  Plant species occurrence and cover class were recorded from 2016-2020 at approximately 10,500 plots spanning a 5°C mean annual temperature (MAT) gradient, along with environmental covariates. For a suite of species, an ordinal, zero-augmented Bayesian model was used to evaluate plant responses to MAT, hydrology, substrate, topography, and their interactions with MAT. We found that: 1) MAT modified species response to every variable for at least one species. 2) The timing of hydropower tides significantly impacts occurrence and cover. 3) Proportion of sand, a limited resource below dams, was as important as hydrology for both occurrence and cover. Hotter MAT, dam-controlled flow patterns, limited fine sediments, and their interactions are driving riparian plant occurrence and cover in a dryland river system. Not only will climate and river regulation independently alter plant distributions, but their interactions could also lead to unexpected and novel communities. Maintaining diverse riparian plant communities will require a range of flow patterns that support different life history stages, provide reliable groundwater, and mitigate fine sediment loss.