Cumulative drought stress and forest functional changes in drought-prone Mediterranean forests

Droughts are a dominant climate stressor in Mediterranean ecosystems, and frequency and intensity of these events are expected to increase. Multi-year long-lasting droughts involving ‘memory’ effects, make short-term or event-specific analysis insufficient to assess ecosystem's responses. We present a remote-sensing framework that combines kernel-density-based phenological anomalies with cumulative sums (Cusums) trajectories to assess long-term functional change in Mediterranean forests of Central Chile. Using MODIS EVI, Moisture Stress Index (MSI) and Evapotranspiration (ET), we generate spatially explicit indicators that capture gradual deviations from the expected phenology and persistent directional shifts.

Our framework revealed persistent negative trajectories preceding the 2010–present megadrought, indicating chronic water stress and progressive loss of resilience. The spatially-explicit indicators highlighted spatially coherent degradation hotspots—northern sclerophyllous and southern deciduous forests—where canopy greenness, moisture, and evapotranspiration declined synchronously. By contrasting pre- and post-2010 relationships between vegetation indices and hydro-climatic variables, we detect a shift of forest-climate interactions consistent with increasing water limitation conditions.

Our results demonstrate how combining KDE-derived anomalies with cumulative change metrics enhance the detection of early and persistent vegetation stress from satellite time series. This provides a sensitive framework to detect early and persistent vegetation stress and to anticipate functional thresholds under continued aridification.