Climate change is likely to dramatically modify stream hydrology across the United States. As global climate warms, seasonal precipitation and temperature regimes are likely to shift, altering the magnitude and timing of floods and minimum flows in many streams. Changes to historic flow regimes directly affect riparian forest composition and structure, because many riparian forests are comprised of species with life strategies that evolved around natural flow regimes. Here we paired riparian forest monitoring data with historic and projected climate and streamflow data to investigate how riparian forest communities may change as stream hydrology, temperature, and precipitation change in the interior Columbia River basin, USA. We compared historic streamflow (1915-2006) and climate (1970-2000) with streamflow and climate projected under the A1B climate scenario through 2040 and 2080. We identified changes in mean annual discharge, summer flow, annual flood discharge, and center of flow mass and a suite of bioclimatic variables over the study period. We simplified riparian forest composition to composite gradients using detrended correspondence analysis (DCA), correlating species abundance to the DCA. We modeled reaches’ multivariate forest composition in response to historic climate and streamflow metrics, projecting vegetation composition in the DCA for the projected streamflow and climate scenarios.
Our results show that streamflow will be significantly impacted by climate change. For example, under the A1B climate scenario, the center of flow mass was forecast to shift forward 27 days while mean summer flow would decrease by 7.9 cfs by 2080. Over the same timeframe mean annual temperature was projected to increase from 5 °C to 8.3 °C. Our vegetation composition models showed that many reaches with forests comprised of hydrophytic riparian and wetland species that rely on flooding for dispersal or fluvial disturbance would shift toward upland trees, generalist shrubs, and xeric forbs, associated with a less dynamic flow regime and warmer climate. Projected site scores showed that reaches were less likely to exhibit riparian understory species like Carex and Salix that are key elements of many small streams. Modeled forest composition within the DCA space was generally less diverse, indicating reductions in riparian specialist species. Our results provide evidence that keystone riparian plant species will be susceptible to future changes in climate and hydrology. Watershed managers should account for these changing hydroclimatic conditions when planning landscape management activities or floodplain habitat restoration that already consider anthropogenic stressors like logging and grazing.