Trait–environment interactions explain variability in alpine soil organic carbon

Alpine ecosystems are globally relevant carbon (C) reservoirs, with grassland soils containing substantial amounts of soil organic carbon (SOC). Ongoing warming and declining snow cover are promoting shifts in plant community composition, greater biomass production, and upward expansion of vegetation into open and rocky areas. These changes are expected to particularly affect SOC dynamics at high elevations, where vegetation is patchy, and soils currently hold very little C. To investigate how environmental gradients and plant functional traits relate to SOC storage, we conducted over 160 coupled vegetation and soil surveys between 2,000–3,000 m a.s.l. on different geologies throughout the Swiss Alps. We applied a structural equation modelling (SEM) framework to assess how topo-climatic and soil conditions directly affect SOC, and how plant traits mediate these effects. We expected community-weighted plant traits to account for a substantial proportion of SOC variability because vegetation composition strongly reflects local microclimate and controls the type and quantity of organic matter entering the soil. We further hypothesized that topo-climatic factors influence SOC primarily through functional vegetation composition. We found that SOC is best predicted by vegetation cover, plant height, specific leaf area (SLA), and the proportion of graminoid species, all of which were positively associated with SOC.  As expected, the effect of temperature on SOC is primarily indirect, operating through its influence on plant functional composition, whereas soil pH exerts a direct negative effect on SOC. Together, these factors explained a large share of the total variation in alpine SOC stocks. Furthermore, SLA and size-related traits co-varied with total soil N stocks, suggesting potential positive feedback, where nutrient enrichment and acquisitive strategies promote greater biomass inputs and further facilitate SOC accrual. By disentangling these factors, we show how plant functional traits and diversity contribute to SOC accumulation in mountain ecosystems and how alpine greening may affect future C storage above the current vegetation line.