Temporal decoupling of carbon source and sink processes in temperate trees: insights from combined measurements of remote sensing and xylogenesis

Uncertainty in forest carbon sequestration arises from limited understanding of source-sink dynamics, especially regarding carbon allocation patterns, temporal decoupling, and species-specific responses to climate variability. Here, based on monospecific stands of two representative species from two of the most important conifer genera in the Northern Hemisphere (Larix and Pinus), we quantified seasonal dynamics of key source-sink processes by combining remote sensing and ground-based measurements, that is canopy gross primary productivity (GPP) via remote sensing, tree wood biomass production (WBP) through microcore analysis, and nonstructural carbohydrate (NSC) pools of aboveground organs. GPP and WBP were decoupled at the intra-annual scale in both species with GPP lasting nearly 6 months and WBP lasting only c. 3 months. GPP was less limited by low temperatures but was inhibited by high temperature stresses. By contrast, WBP only became conspicuous when the temperature exceeded certain thresholds and thereafter increased through the entire temperate range. Larch prioritized growth while pine tended to allocate more carbon to storage during periods of reduced GPP. The findings deepen our understanding of the temporal decoupling of tree carbon source and sink processes, the main climatic drivers, and the underlying physiological mechanisms, particularly the role NSC plays in mediating the source-sink decoupling.