Individual and Interactive Effects of Elevated CO2, Warming and Drought on the Phenology of Mountain Grassland

Mountain grasslands are strongly exposed to multiple global changes, including elevated CO₂, climate warming and the increased occurrence of drought events. While the individual effects of these global change drivers on the phenology of grasslands have been comparatively well studied, there is a lack of understanding of their interactive effects. In a multifactor global change experiment on a managed montane grassland typical for many parts of the Alps, we tested the individual and combined effects of elevated CO₂ (eCO₂; +300 ppm), warming (eT; +3°C) and severe summer drought on phenology. We derived the canopy-level phenological transition dates from Green Chromatic Coordinates (GCC) time series calculated from phenocam images. Preliminary results reveal that warming, individually and when combined with elevated CO₂, led to an early spring advancement and that drought accelerated senescence, more strongly under future (eCO₂ + eT) compared to ambient conditions. Our preliminary findings suggest non-additive effects of interacting global change drivers on the phenology of mountain grassland, with cascading consequences for grassland productivity.