Riparian response guilds shift in response to flow alteration in montane streams of the southern Rocky Mountains

Abstract The southern Rocky Mountains serve as the headwaters of the West, providing water to agriculture, industry, recreation, and millions of people in 17 western states. Eighty percent of the water draining land managed by the United States Department of Agriculture Forest Service (Forest Service) is from snowmelt. The demand for this water continues to grow with increasing population throughout the arid West. The Forest Service is obliged to manage these watersheds for a diverse set of interests including extraction of forest products and other natural resources, providing recreational opportunities, and delivering fresh water. The Forest Service is simultaneously required to sustain the ecological condition of the managed lands for future generations. Satisfying these demands requires quantitative approaches to balance the ecological impacts against the benefits of water development. In this study, we (1) measured riparian vegetation at nine sites in the Medicine Bow‐Routt National Forests, (2) identified twelve trait‐based riparian vegetation‐flow response guilds (functionally similar groups of plants), (3) constructed reach‐scale hydraulic models for each site, (4) classified sites by hydro‐geomorphic characteristics, and (5) modeled observed guild distributions as a function of duration of stream inundation and site characteristics. To illustrate the application of these models in quantifying ecological impacts of water development and management decisions, flow scenarios were developed to simulate the effects of upstream diversions and reservoirs. Guild distribution models were then used to forecast alterations to vegetation patterns under the flow simulations. Even in these relatively moist montane riparian areas, stream inundation duration explained much of the variability in guild distributions. Controlling for groundwater contributions to the moisture regime improved the predictive ability of the models. Plot‐level guild diversity was found to be most correlated to the maximum height of species in the guild, shade tolerance, and seed mass. Plots were identified with relatively low species diversity and high guild diversity, suggesting niche partitioning and a target guild composition for restoring resilient riparian vegetation assemblages. We demonstrate the transferability of guild models across rivers to assist in the decision‐making process for evaluating water development and forecasting riparian vegetation response.