Functional group and aridity regulate impacts of climate change on plant phenology: a meta-analysis.

The interaction between temperature and precipitation greatly affects plant phenology. However, these effects can vary between additive, antagonistic, and synergistic, and it is not clear what determines these different outcomes. One hypothesis is that the effect on plant phenology varies with aridity and plant functional group. Here, we find support for this hypothesis using a global meta-analysis on the timing of leaf-out (2178 values from 55 experimental sites) and flowering (4027 values from 117 experimental sites). We find that, globally, the onset of leaf-out is more influenced by water availability than by temperature, while first flowering is more affected by temperature than by precipitation. On its own, warming advances leaf-out for all functional groups (except in semi-humid regions), whereas warming combined with decreased precipitation delays leaf-out in semi-arid regions. Warming also advances flowering across all functional groups, regardless of changes in precipitation and aridity. We observe synergistic effects of warming and precipitation on leaf-out for forbs in semi-arid regions, while antagonistic effects occur for grasses and sedges except in arid regions. Our findings suggest that considering drought tolerance or resistance of plants across ambient climates is critical for improving our understanding and predictions of how plant phenology responds to climate change.