Global response trajectories of ecosystem light use efficiency to flash drought

Climate change is increasing the frequency, intensity, and duration of extreme drought events, raising growing concerns about their impacts on terrestrial carbon cycling. Flash droughts can rapidly suppress ecosystem photosynthesis, yet their impacts on light use efficiency (LUE), a functional measure of carbon gain per unit absorbed radiation, remain poorly understood. Here, we quantified the response trajectories and intensity of global ecosystem LUE to flash drought using flux-tower measurements, satellite observations, and LUE model simulations. A change-point framework was developed to quantify the relative strength of post recovery compared with the initial decline (Δk), with larger values indicating stronger recovery relative to the initial decline. Consistent with flux-tower observations, satellite-derived LUE revealed that ecosystem responses to flash drought were predominantly recovery-oriented, with 93.5% of pixels exhibiting positive Δk. Despite this widespread recovery, recovery strength weakened significantly over time, resulting in a marked decline in global mean Δk (−0.0086 yr−1, p < 0.05) from 2002 to 2015. Compared with satellite observations, the LUE models (VPM, EC-LUE, and MODIS17A2) broadly reproduced ecosystem response trajectories but underestimated response intensity, primarily due to their limited ability to capture the rapid initial decline in LUE during flash-drought onset. Attribution analysis further showed that LUE response intensity was primarily controlled by flash-drought duration, atmospheric water demand, radiation supply, and evapotranspiration. These findings reveal a widespread but weakening capacity of ecosystems to recover photosynthetic functioning following flash drought, highlighting increasing risks to the stability of terrestrial carbon cycling under continued climate warming.