Immediate and carry-over effects of late spring frost and growing season drought on forests in the Northern Hemisphere

Forests are increasingly exposed to extreme global warming-induced climatic events. However, the immediate and carry-over effects of extreme events on forests are still poorly understood. Using eddy covariance data from 34 forest sites in the Northern Hemisphere, we analyzed the responses of ecosystem gross primary productivity capacity under light saturation (GPP2000) of forest ecosystems to late spring frost (LSF) and growing season drought. The immediate and carry-over effects of frost and droughts on needle-leaf (NF) and broadleaf (BF) forests were analyzed. Path analysis was applied to reveal the plausible reasons behind the varied responses of forests to extreme events. The results show that LSF had clear immediate effects on the GPP2000 of both NF and BF. However, GPP2000 of NF was more sensitive to drought than that of BF. There was no interaction between LSF and drought in either NF or BF; instead, drought effects were masked by the LSF effect in NF. Path analysis further showed that the response of GPP2000 to drought differed between NF and BF, mainly due to the difference in the sensitivity of canopy conductance. Moreover, LSF had a more severe and long-lasting carry-over effect on forests compared to drought. These results enrich our understanding of the mechanisms of forest response to extreme events across forest types.