What drives drought recovery? A global perspective

Background/Question/Methods:  Understanding the impacts of drought on carbon metabolism is crucial to elucidate how global environmental change will alter the climate regulation ecosystem service provided by terrestrial vegetation. Notwithstanding past and anticipated future changes in drought regime the interplay between hydrologic (amelioration of precipitation deficit) and functional (return to pre-drought levels of carbon metabolism) post-drought recovery is not well understood. Recovery time is however a prime determinant of whether ecosystems revert to their initial state or transition to a new equilibrium. Here we quantify post-drought recovery time of gross primary productivity (GPP) at grid cell to global scales using three reconstructions: MODIS (remote sensing), upscaled FLUXNET (eddy covariance), and an ensemble of Earth system models from the Multi-scale Synthesis and Terrestrial Model Intercomparison Project. Drought is tracked using the multiscalar Standardized Precipitation-Evapotranspiration Index drought with the integration period (the retrospective window used to calculate the metric) varied from 1 to 24-months. We define recovery time as a function of both hydrologic and GPP recovery, i.e., both must attain pre-drought levels for recovery to occur.

Results/Conclusions:  Despite the diverse provenance of the reconstructions, different reconstruction periods, and variable integration lengths consistent patterns emerge across the c. 4 500 000 drought and recovery events cataloged. Post-drought temperature and precipitation conditions are key drivers of recovery time, i.e., deviations from mean climate are more important than the mean state itself. GPP amplitude--the during drought and recovery change in GPP--is also of prime importance: larger amplitudes lead to longer recovery times. Pre-drought GPP, however, functions as a switch; if pre-drought GPP is already depressed there is no effect. However, when the pre-drought GPP baseline is above average, recovery time increases dramatically. We also find that increasing CO2 concentration has acted to shorten recovery times by c. 4 months since 1901. Surprisingly, while recovery time scales with drought severity and drought length, the drought regime itself is of tertiary importance in determining recovery time. More generally, the longest recovery times occur in the tropics and northern high latitudes. Similarly, drought recovery has increased over time; from 1901 to 2010 the areal extent of land ecosystems in recovery has increased up to 50%. These results imply that--as future Anthropocene droughts become more extreme as expected--periods between droughts may become shorter than drought recovery time. This increases the risk of entering a new regime where vegetation never fully recovers and widespread land sink degradation ensues.