Multi-Organ Nutrient Imbalances Underpin Drought-Induced Dieback in Scots Pine

The increasing frequency and intensity of hotter droughts are driving widespread forest dieback, yet the role of tree nutritional status in drought-induced growth dieback remains poorly understood. We investigated how nutrient composition across tissues (leaves, wood) relates to water use patterns and growth resilience in rear-edge populations of Scots pine (Pinus sylvestris L.) in Northeastern Spain. Using a multi-proxy approach, we combined analyses of foliar and sapwood nutrient concentrations, stable isotopes (δ13C, δ18O), and dendrochronological indicators across contrasting tree vigor classes. Defoliated trees exhibited pronounced shifts in elemental composition, including depletion of foliar K and increased concentrations of Ca, S, and Fe, alongside higher intrinsic water use efficiency and reduced growth resistance to drought. In contrast, the sapwood elemental composition was less responsive to defoliation but showed stronger associations with isotopic signals and drought resilience, suggesting its integrative role in tree functioning. Coordination of nutrient concentrations between tissues was limited, suggesting organ-specific regulation of nutrient allocation under drought stress. Our results reveal that nutrient imbalances are linked to water–carbon dynamics and drought responses and emphasize the importance of considering multi-organ nutrient dynamics to improve our understanding of long-term nutritional imbalances during drought-induced forest dieback.