Direct and Trait-Mediated Effects of Climate on Grassland Productivity Across the Alps

Understanding how climate shapes ecosystem productivity through energetic constraints and changes in the functional properties expressed by vegetation remains challenging. This is central to trait-based biodiversity-ecosystem functioning research in mountain grasslands where warming and environmental gradients interact. Here, we distinguish direct climatic controls on productivity from indirect pathways operating through changes in canopy functional composition, physiological state, and within-plot spectral functional heterogeneity 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, within-plot spectral functional heterogeneity using the functional richness (FRic) metric, and vegetation productivity using near-infrared reflectance of vegetation (NIRv). We combined panel-data models with piecewise structural equation modeling to partition climate associations into direct and trait-mediated components operating through canopy functional properties and used varying-coefficient models to evaluate how these pathways vary along environmental gradients. We identified two pathways linking climate to productivity. Warmer growing seasons increased productivity directly but were associated with a lower chlorophyll-to-carotenoid pigment balance, consistent with greater relative carotenoid investment, generating an opposing indirect pathway. Greater winter snow accumulation reduced productivity primarily through a direct pathway, while its canopy-mediated pathways were uncertain. Climatic water deficit reduced canopy water status and within-plot spectral functional heterogeneity, but its direct and total associations with productivity were not distinguishable from zero. Environmental context modulated climate-vegetation relationships. Baseline temperature and moisture availability strongly shaped climate effects on canopy traits and productivity, whereas canopy-property-productivity relationships varied little across space. Positive productivity responses to warming were stronger in wetter and more seasonally variable climatic contexts, but weakened where baseline winter snow accumulation was greater. Together, our findings show that short-term productivity responses in mountain grasslands reflect a balance between direct energetic constraints and indirect pathways through integrated canopy functional properties, and that this balance varies along macroclimatic gradients.