Multifactorial diagnosis of drought‐induced decline in Mediterranean conifer forests

Drought‐induced tree mortality is a complex process involving interdependent shifts in tree water, carbon and nutrient relations, alongside adjustments in growth and trait variability. Anticipating dieback under increasing drought severity requires mechanistic insights into the underlying physiological processes. We applied a tree‐level, multi‐proxy approach to identify factors associated with vigour decline in mortality hotspots of Mediterranean Pinus sylvestris , Pinus pinaster and Pinus halepensis forests located in north‐eastern Spain. Tree‐ring metrics, foliar carbon and oxygen stable isotopes (δ 13 C, ∆ 18 O) and elemental composition and soil properties were integrated to assess drought impact on growth ( DGIi ), intrinsic water‐use efficiency (iWUE), ∆ 18 O‐inferred stomatal conductance (G ∆18O ) and nutrient status of trees. Declining trees had consistently higher DGIi than non‐declining trees. In P. sylvestris and P. pinaster , elevated DGIi was linked to reduced G ∆18O and increased iWUE, indicating more severe drought stress in declining trees. Declining P. sylvestris and P. halepensis also exhibited pronounced nutrient imbalances—particularly in P, K, Zn, Fe, S and Ca—and all species showed disrupted Fe:Cu ratios. Differences in vigour across species were also associated with variation in needle size and morphology, tree height and soil pH, with declining P. sylvestris and P. halepensis displaying contrasting adjustments in leaf mass per area. Synthesis . Together, these results highlight that interactions among hydraulic impairment, nutrient imbalance, foliar senescence and unfavourable soil conditions amplify climatic stress in the studied populations of Mediterranean pines. Such potential feedbacks may accelerate crown defoliation and ultimately contribute to tree death under increasing drought severity.