Topography-driven energy redistribution triggers ecohydrological regime shifts and shrub–grass divergence in a high-altitude cold catchment

Terrain-induced radiation heterogeneity influences snow persistence and vegetation composition, but these interactions remain poorly represented in land surface models. Here, we integrated a three-dimensional topographic radiation scheme (3D-TopoRad) into the Community Terrestrial Systems Model coupled with the Functionally Assembled Terrestrial Ecosystem Simulator (CTSM-FATES). Using high-resolution terrain data and slope-parallel observations, we conducted and evaluated 30 m simulations in the Waerma alpine catchment on the eastern Tibetan Plateau. The correction reduced shortwave radiation root mean square errors by 65.6% on sunny slopes and 81.0% on shady slopes, with improvements extending to surface energy and carbon fluxes. The simulations suggest that snow connects winter radiation contrasts to growing-season conditions. Although radiation differences were greatest in winter, longer snow cover duration on shady slopes was associated with cooler, wetter soils into spring and summer. A breakpoint near 161.98 W m –2 in the spatial relationship between annual mean shortwave radiation and snow cover duration marked a transition between contrasting simulated ecohydrological regimes. The low-radiation regime occurred predominantly on shady and semi-shady slopes with longer-lasting snow and wetter growing-season soils, whereas the high-radiation regime occurred mainly on sunny and semi-sunny slopes with shorter-lasting snow and drier soils. These contrasting environments supported shrub–grass mixtures on shady slopes and grass dominance on sunny slopes. However, shrub abundance peaked at intermediate snow durations, consistent with a balance between snow-associated water availability and growing-season constraints. Shady slopes also supported greater carbon uptake and higher ecosystem water use efficiency despite similar total latent heat fluxes across aspects. These findings highlight the importance of seasonal radiation–snow–soil interactions in shaping alpine vegetation composition and carbon–water exchange.