Drought legacies delay spring green-up in northern ecosystems

Global warming has increased the frequency and intensity of droughts, causing large impacts on the structure and functioning of terrestrial ecosystems. The direct effect of droughts on autumn senescence is well-documented, but the extent to which the legacy effects influence plant phenology of the following year remains unclear. Using satellite greenness data and long-term in situ observations, we demonstrate that droughts substantially delay the green-up and leaf unfolding of the next spring, particularly following prolonged events with delayed soil moisture recovery. These delays cannot be explained by state-of-the-art phenology models and are strongly linked to postdrought temperature, local climate, drought characteristics and reductions in photosynthesis. Compared to the endogenous memory effects within plants themselves, the exogenous memory effects through changes in environment are five times stronger in drylands and twice as strong in non-drylands. Given projections of increased drought frequency and severity, future advances in spring phenology may be less pronounced than previously anticipated.