Drought influence on carbon assimilation and water use efficiency in Mediterranean ecosystems

The increase of aridity in Mediterranean region limits the ecosystem ability in sequestering carbon and efficiency in using water. Using 20 years (2000-2020) of satellite-derived observations of gross primary productivity (GPP), canopy greenness (NDVI), crop water stress (CWSI), evapotranspiration (ET), and water-use efficiency (WUE), combined with the Standardized Precipitation-Evapotranspiration Index (SPEI), we aimed to (1) identify the earliest drought-sensitive indicators and ecosystem-specific response times, (2) quantify cumulative sensitivities across vegetation types to establish resilience benchmarks, and (3) characterize the temporal propagation pathways through which thermal and physiological stress (CWSI) co-varies with changes in GPP, water flux, and ecosystem efficiency across vegetation types. CWSI is most sensitive satellite-based drought indicator, responding within 0-1 month across most biomes and detecting physiological stress before declines in GPP or NDVI. Croplands and grasslands showed the strongest and fastest responses (|R_max| up to 0.38 with lags of 1-3 months), whereas forests displayed weaker but delayed responses (lags of 4-8 months). The analysis of cumulative sensitivities revealed prolonged drought integration in semi-arid shrublands (6-10 months), intermediate durations in croplands and grasslands (2-4 months), and shorter durations in forests (1-3 months). These results provide an empirical basis for integrating CWSI into regional drought monitoring frameworks and inform adaptive management strategies for Mediterranean ecosystems under future climate extremes.