COS 167-2 - Riparian plant occurrence and cover are differentially influenced by complex stressors and temperature interactions

Abstract: 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, such as the Colorado River, with flow patterns altered by hydropower production. We used a large dataset from the Colorado River downstream of Glen Canyon Dam, Arizona, USA to address three hypotheses: 1) within a species, plant occurrence and cover will exhibit similar responses to topography and temperature-related variables, but dissimilar responses to disturbance or competition-related environmental variables; 2) flow regime patterns unique to hydropower production will have independent effects on plant occurrence and cover relative to other hydrological variables (e.g., elevation above baseflow); and 3) gradients in mean annual temperature (MAT) will alter species response to hydrological and topographic variables. Plant species occurrence and cover class were recorded from 2016-2020 at approximately 10,500 plots spanning a 5°C 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) occurrence and cover exhibited differences in their controlling environmental variables for all species and in their direction of responses for 9 of 25 species. Contrary to hypothesis 1, the direction of responses did not reliably correspond or differ by variable type. 2) Daily flooding from hydropower plays a significant role in riparian plant occurrence (2 species) and cover (5 species), with indications that one species, Salix exigua, is particularly sensitive to hydropower flows by exhibiting significant reductions in presence (mean effect size = -51.5) and cover (-0.34). Finally, 3) for all species, higher MAT mediates responses to other environmental variables, particularly overhanging plants and distance to baseflow. Maintaining diverse riparian plant communities will require a range of flow patterns that account for sediment regimes and changing temperatures. Differential impacts of MAT on plant responses to other environmental variables could lead to compositional shifts that would not be predicted by simply considering temperature or flow changes in isolation. Increasing MAT may make some species more sensitive to flows associated with hydropower. Hence, efforts to manage single important aspects of river flows can be expected to have more variable outcomes and potentially be less successful than using multivariate, whole ecosystem approaches.