Vegetation Transitions and Environmental Controls on Alpine Hydrology

Climate warming is rapidly transforming mountain ecosystems through processes such as colonization by pioneer species, grassland development, shrub expansion, and tree line advancement. These vegetation transitions, collectively driving mountain greening, have important consequences for hydrological dynamics. Yet, their ecohydrological interactions remain poorly understood. We investigated how different vegetation types, used as a space-for-time proxy for vegetation transitions, modify soil moisture, soil temperature, and snow dynamics in the Meretschi catchment (Swiss Alps) using a plot-based sampling design spanning five vegetation classes, from bare ground to shrub and forest communities. High-frequency soil moisture and temperature measurements (TOMST TMS-4) were combined with detailed vegetation, soil, and topographic data across 42 plots. Our results show that vegetation mediates topographic effects on soil moisture (R² = 0.65; standardized effect = 0.58) and soil temperature (R² = 0.74; standardized effect = 0.34), with pioneer vegetation maintaining lower soil moisture and temperature than more developed communities. Taller vegetation, including dwarf shrubs and larger shrubs/forest, was associated with snowmelt starting ~22 days earlier, ending ~44 days earlier and snow-covered periods being ~68 days shorter.  Dwarf shrub communities further introduced strong seasonal variability in soil moisture and temperature. Using a space-for-time approach, we anticipate that continued vegetation transitions from pioneer to established and from grassland stages toward shrub-dominated communities will alter both the timing and volume of water availability in mountain catchments. These findings highlight the need to integrate vegetation change into predictions of future alpine water resources.