Optimal carbon partitioning drives sink potential dynamics in large‐scale pine reforestation under climate change

Abstract Reforestation has been proposed as a climate change mitigation strategy to enhance the terrestrial carbon sink. However, its effectiveness remains uncertain, and its potential contribution to increase carbon sequestration may be jeopardized under hotter drought. We studied carbon dynamics under climate change in large‐scale pine reforestation along a compound ecological gradient comprising a wide altitudinal range and four native pine species across a drought tolerance gradient. Combining remote sensing and dendrochronological plots, we calculated proxies of primary productivity, including EVI (Enhanced Vegetation Index), NPP (Net Primary Production) and ABI (Aboveground Biomass Increment) to assess long‐term carbon allocation dynamics using the ratio ABI:NPP. Carbon dynamics (i.e. both NPP and ABI:NPP) were enhanced by water availability and depressed by warming temperatures. Observed variability across climatic and drought‐tolerance gradients coherently support that carbon allocation dynamics were consistent with the optimal partitioning theory. The proportion of carbon allocated to aboveground biomass increased across species with decreasing drought tolerance. Complementary, under warmer and drier site conditions trees invested a lower proportion of biomass aboveground, suggesting greater investment belowground (i.e. roots), as expected under climate change. The non‐linear relationship between carbon allocation and climate showed that forest productivity and aboveground C‐allocation saturated above a precipitation of 750 mm and, most importantly, temperatures below 12°C. Pine woodlands exhibited a notable capacity of acclimation to stress across the wide climatic gradient studied. Trees were able to mitigate temperature‐induced stress by adjusting the carbon allocation ratio according to the optimal partitioning theory. Synthesis and applications . Our results contribute to understanding of carbon allocation dynamics in Mediterranean pine reforestation under climate change. Following the optimum allocation principle, carbon investment to aboveground organs increased dynamically in less drought‐tolerant species and under cooler and more humid conditions. Pines exhibited a remarkable capacity of acclimation to climatic stress expressed by a plastic carbon allocation ratio. Our results provide a comprehensible framework to species selection based on their climatic sensitivity with important implications for reforestation strategies. The observed sensitivity to climate of carbon dynamics helps to better assess the potentiality and limitations of reforestation as a mitigation strategy under climate change. Read the free Plain Language Summary for this article on the journal’s blog .