Cumulative and Legacy Effects of Droughts on Global Photosynthetic Phenology

DroughtDrought exerts large impacts on plant phenology for photosynthesis (i.e., the start and end of the photosynthetic season, SOS/EOS), negatively influencing the length of the photosynthetic season (LOS) and global carbon uptake. While the effects of concurrent drought on SOS and EOS have been widely studied, the cumulative and legacy effects of droughts on plant photosynthetic phenology remain largely unknown. Here, based on SOS, EOS, and LOS derived from global remote-sensing gross primary production (GPP) and the standardized precipitation and evapotranspiration index (SPEI) across various timescales, we comprehensively quantified the cumulative and legacy effects of droughts on global photosynthetic phenology during 2001–2020. We determined the cumulative drought effects by the maximum correlations between plant phenological timing and cumulative SPEI over 1–12 months (rmax-cml), while the drought legacy effects were evaluated by the maximum correlations between phenological timing and 1-month SPEI at several preceding months (rmax-lag). The results showed significant drought legacy effects on SOS, EOS, and LOS in 48.87%, 42.01%, and 45.91% of global vegetated areas, respectively, and cumulative effects in 37.44%, 28.10%, and 20.36% of vegetated areas, respectively. The cumulative drought effects manifested over short timescales, advancing both SOS and EOS, leading to divergent changes in LOS. In contrast, the legacy effects may persist over extended periods and strongly advance EOS, consequently shortening LOS. Among biomes, grasslands exhibited the most rapid and pronounced responses to cumulative and legacy drought effects, while deciduous broadleaf forests showed the weakest responses. Moreover, we found that the rmax-cml and rmax-lag were largely driven by hydrological conditions, whereas the corresponding timescales were mainly influenced by thermal factors and background phenology (e.g., the peak rate of plant greening). Overall, these findings indicate that drought poses strong cumulative and legacy effects on global ecosystems and the carbon cycle by significantly altering the photosynthetic phenology, which is crucial for the development of earth system modelEarth system models to accurately predict global phenology and carbon cycle under climate change.