Direct and biodiversity-mediated effects of climate on grassland productivity across the Alps

Understanding how climate shapes ecosystem productivity through both energetic constraints and biodiversity‑mediated pathways remains a major challenge in global change ecology, particularly in mountain grasslands where rapid warming and strong environmental gradients interact. Here, we disentangle and map direct climatic controls on productivity from indirect effects mediated by canopy functional structure across the European Alps. Using Sentinel‑2 time series (2017–2024), we quantified canopy functional structure from spectral proxies of pigment investment and water status, functional richness, and vegetation productivity using near‑infrared reflectance of vegetation (NIRv). We combined causal inference with piecewise structural equation modelling to partition total climate effects into direct and trait‑mediated components and used varying‑coefficient models to evaluate how these pathways vary along environmental gradients. We identified two concurrent pathways linking climate to productivity. Warmer growing seasons directly increased productivity but simultaneously reduced canopy pigment investment and water status, generating negative indirect effects that partly offset direct gains. Greater winter snow accumulation reduced productivity both directly, by shortening the effective growing season, and indirectly, through increases in canopy water content associated with lower productivity. Climatic water deficit produced opposing effects, with weakly positive direct impacts counteracted by negative indirect effects mediated by pigment‑related traits, resulting in minimal net productivity change. Trait diversity showed relatively weak climate responses and modest contributions to indirect effects. Environmental context strongly modulated climate–vegetation relationships. Baseline temperature and moisture availability most strongly shaped climate effects on canopy traits and productivity, whereas biodiversity–functioning relationships varied little across space. Strongest positive productivity responses to warming occurred in moist, mid‑elevation regions, while responses weakened or became neutral at high elevations, snow‑rich areas, and dry sites.