Warming, elevated CO2 and drought in combination amplify shifts in canopy greenness dynamics in managed grassland

Grasslands are strongly exposed to multiple global changes, including elevated CO₂, warming, and severe drought events. While the individual effects of these global change drivers on the greenness dynamics of grasslands have been comparatively well studied, their combined effects are so far poorly understood. In an in situ multifactor experiment we tested the individual and combined effects of warming (+3° C), elevated CO₂ atmospheric concentration (+300 ppm) and summer drought on the spring phenology and regrowth dynamics following defoliation during summer and late summer of a managed grassland typical for many parts of the Alps. We derived the dynamics of canopy greenness from the time series of green chromatic coordinates using digital repeat photography imagery (phenocams) spanning a period of three years. Among the individual drivers tested, we found that warming strongly advanced spring phenology and accelerated growth, elevated CO₂ accelerated growth, and summer drought had no immediate effects on shifts in canopy greenness but accelerated growth in the subsequent spring. The combination of the three global change drivers caused the most pronounced spring phenological and regrowth dynamics among all treatments, triggering an earlier reduction in canopy greenness during summer and advancing the onset of growth in the successive spring due to a drought legacy effect, which decreased the cumulative growing degree days required for initiating growth. Our findings suggest that in a future climate the combined effects of the three global change drivers will exacerbate shifts in canopy greenness dynamics in managed grassland, which cannot be predicted from the responses from the individual effects of these drivers.