Vegetation Transitions and Environmental Controls on Alpine Hydrology

Abstract Climate change is driving substantial vegetation shifts in mountain ecosystems, including colonization by pioneer species, grassland development, shrubification, and tree line advancement. These processes, jointly referred to as mountain greening, influence hydrological dynamics by affecting infiltration, water retention, and evapotranspiration. As snow and ice decline under global warming, understanding how greening impacts hydrology is crucial for predicting future water supply. This study investigated the ecohydrological consequences of greening in the Meretschi catchment in the Swiss Alps, using a plot-based sampling design across 42 plots representing five vegetation classes (bare ground, pioneer vegetation, grassland, dwarf shrub, and larger shrub/forest). High-frequency soil moisture and temperature data were collected, alongside detailed assessments of vegetation, soil properties, and topographic variables. We showed that vegetation mediated topographic influences on soil moisture and temperature, with distinct responses among vegetation classes. Pioneer vegetation exhibited lower soil moisture and temperature than more developed vegetation classes with deeper soils. Vegetation classes with taller aboveground biomass, such as dwarf shrubs and larger shrubs/forest, were associated with earlier snow melt and shorter snow-covered periods compared to grassland stages. Dwarf shrubs introduced additional seasonal effects, including pronounced fluctuations in soil moisture and temperature, as well as freezing during winter. Using a space-for-time substitution framework, we expect that continued transitions from pioneer to established and from grass to dwarf shrub communities will substantially modify hydrological processes in mountain catchments, altering both the timing and volume of water availability locally and downstream.