Climate Controls on Carbon Dioxide Fluxes in a High‐Elevation Grassland

Climate change significantly influences natural and semi-natural ecosystems at global scale. This impact is further amplifiedin alpine environments, owing to a set of climate feedback mechanisms (i.e., elevation-dependent warming). Due to the shortsnow-free season and the consequent adaptation of alpine plants to fast growth, high-elevation grasslands mainly act as carbonsinks during the vegetative season, with primary productivity usually exceeding ecosystem respiration. We combined timeseriesof field data (CO2 fluxes and environmental variables), with weather, vegetation and remotely-sensed land-cover data from theNivolet area (Gran Paradiso National Park, western Italian Alps), to identify the factors controlling diurnal ecosystem respira-tion and primary productivity in such environments. We found that growing-degree-days-since-snowmelt (GDD0—a measure ofcumulative heat available for plant growth since the snow has melted) exerted a strong control on both respiration and primaryproductivity. The effect of GDD0 on productivity is both indirect, mediated by the seasonal trend in greening (proxied by vegeta-tion indices), and direct, possibly reflecting the importance of cumulated heat on vegetation height. While greening had limitedeffect on respiration, GDD0 had a strong effect, supporting the view that ecosystem respiration was mainly temperature-relatedand microbial-driven. After accounting for the effect of greening, we found that scarce winter cumulative precipitation had pos-itive impacts on productivity and respiration, suggesting a possible role of reduced snowpack on the cycling of organic matter.Other variables (i.e., solar irradiance, air temperature, soil moisture, vegetation composition and topography) either supportedour expectations or showed no significant effect. Our results contribute to identify the causal relationships between climate anddiurnal carbon cycling, improving our understanding of how climate change affects alpine ecosystems in these high-elevationenvironments