Hydrologic and Ecological Consequences of Changing Snowmelt Patterns in the East River Catchment

Mountainous watersheds are increasingly experiencing snowpack decline and warmer winters. Several studies have shown that earlier and diminished snowpack alters streamflow and runoff dynamics while also affecting groundwater reserves. These changes in the hydrologic cycle —particularly early snowmelt and reduced snowpack—are hypothesized to disrupt coupled plant-microbial interactions, possibly due to the temporal discontinuity between microbial nutrient release and vegetation greening around the snowmelt period. This study aims to quantify how variations in snowmelt timing and snowpack depth influence nitrogen fluxes and plant phenology along a lower montane hillslope site in the East River catchment, Crested Butte, CO. Specifically, we compare nitrogen cycling and plant dynamics across three hydrologic conditions observed at the site: an average snow year (2016), a deep snowpack year (2017), and a sparse, early-melting snowpack year (2018). To achieve this, we developed a hillslope-to-floodplain transect model using ecosys — a comprehensive plant ecosystem model that integrates surface energy exchange, microbial metabolism, vegetation phenology/physiology, and lateral and vertical hydrologic and biogeochemical fluxes. Our results indicate that in low-snow years, surface soil water deficits suppress forb production while favoring deep-rooting shrubs, thereby altering vegetation nitrogen demand. In contrast, in high-snow years, soil and saprolite water availability extends into the monsoon season, sustaining forb growth. Furthermore, we find that monsoon timing and temperatures during the vernal window play a critical role in regulating plant productivity and the growing season length. Overall, these findings highlight the strong spatial and temporal linkages between snowmelt-driven nutrient release and plant phenology along the hillslope, underscoring the broader implications of shifting snow dynamics on watershed-scale ecosystem function.