Decadal climate-driven decoupling between gross primary productivity and tree growth in Mediterranean forests

Mediterranean forests are becoming increasingly vulnerable under climate change, as the growing frequency and intensity of droughts and heatwaves amplify physiological stress, reduce productivity, and heighten the risk of large-scale disturbances. However, vegetation activity trends derived from remote sensing may mask divergent responses between photosynthetic activity and growth, which represent a critical early warning signal of forest vulnerability. Therefore, the long-term relationship between photosynthesis and tree growth remains poorly understood at regional scales, especially in Mediterranean areas. To address this challenge, we applied a mechanistic, process-based forest ecosystem model across approximately 2400 km2 of typical Mediterranean forests in southern Italy, encompassing a heterogeneous landscape characterized by diverse stand structures and species dominance. This framework enabled to explicitly trace carbon fluxes from gross primary productivity (GPP) through allocation processes to average tree growth. Using a factorial approach, we identify, over large spatial scales, an emergent pattern of divergence between summer GPP and radial tree growth, amplified in space and time by climate variability over the last two decades and further shaped by forest legacy effects. Our findings also reveal that canopy-level greening can mask structural vulnerability and pre-visual decline across Mediterranean forests. Data show that an apparent long-term trend in photosynthesis decline during summer does not necessarily translate to tree growth decline. Improving our ability to determine if, where and when a key change in forest behaviour will occur, remains essential for designing effective restoration measure and anticipating tipping points in forest resilience under accelerating climate change.