Tree-ring isotopic imprints on time series of reproductive effort indicate warming-induced co-limitation by sink and source processes in stone pine.

This article investigates shifting source–sink dynamics in stone pine (Pinus pinea L.), a masting gymnosperm with high reproductive costs, under warming-induced drought in the Spanish Northern Plateau. Using 60 years (1960–2016) of data on cone yield (CY), tree-ring widths, and stable carbon (Δ¹³C) and oxygen (δ¹⁸O) isotopes, the study reveals a transition from predominantly sink-limited reproduction—where cone setting three years before seed rain governs yield—to a co-limitation by both sink and source processes involving current-year assimilate supply during kernel filling. Climate models show that early-period CY was mainly driven by winter–spring conditions three years prior, while recent decades exhibit combined effects of climate in both years three and one before seed rain, reflecting increased drought stress and source limitation. These findings suggest that ongoing climate warming may intensify source limitations on reproduction, potentially affecting stone pine regeneration, forest structure, and the economic value of its edible seeds.