Warming, elevated <scp> CO ₂ </scp> and drought in combination accelerates plant phenological shifts in managed montane grassland

Abstract Rapid increases in the atmospheric concentrations of CO 2 and other greenhouse gases are accelerating global warming, which in turn favours the occurrence of severe drought events. While the individual effects of warming, elevated CO 2 and drought on certain phenological stages, such as the onset of flowering, have been reasonably well‐studied, many other stages remain less explored, particularly in response to the combination of multiple global change drivers. Using a multifactorial experiment in a managed montane grassland, we investigated the individual and combined effects of warming (+3°C), elevated CO 2 (+300 ppm) and summer drought on the phenology of grassland species, including grasses, legumes and forbs, during three growing periods in spring, summer and late summer defined by the three seasonal harvests, following a common agricultural management practice in Central Europe. We found that warming advanced the start of peak growth and flowering phenology during spring, elevated CO 2 advanced flowering and summer drought inhibited full flowering and advanced senescence during regrowth after the first harvest. The combination of warming and elevated CO 2 had strong synergistic effects on peak growth during spring. During summer, after the first mowing, we observed either additive or antagonistic effects on regrowth phenology. The combination of warming, elevated CO 2 and drought advanced senescence in most species compared to all other treatments during summer. Overall, our findings reveal distinct and diverse effects of both individual and combined global change drivers on the phenology of mountain grassland species, underscoring that phenological shifts are pivotal indicators of global change. Furthermore, our study suggests that the studied global change drivers in combination exert a stronger influence on phenological shifts than the individual drivers alone. Synthesis . Warming, elevated CO 2 and drought, individually and more strongly in combination, cause shifts in spring phenology and alter the timing of phenological stages of grassland species during their regrowth after cutting in summer. These shifts may alter species interactions, with consequences for ecosystem structure and function.