Growing season length enhances black spruce carbon uptake, whereas summer aridity constrains xylem biomass investment: Evidence from wood anatomy and carbon fluxes

Accurately projecting future forest carbon (C) sinks requires insight into how warming and prolonged growing seasons alter C uptake and the formation of woody biomass. To assess climate-growth relationships and C allocation patterns, tree-ring width (TRW) has been widely used as a proxy for woody biomass growth. However, its reliability for tracking C flux dynamics remains unclear. We analysed a 22-year series of eddy-covariance-derived gross primary productivity (GPP) alongside multiple quantitative wood anatomy proxies of biomass growth in a boreal black spruce ( Picea mariana Mill.) forest in Canada. We observed different climate sensitivity in both C uptake, mostly associated with the growing season metrics (GSMs) onset and length, and C allocation to woody biomass, influenced by summer atmospheric and soil aridity. We found that xylem cell number, which primarily determines TRW, showed limited sensitivity to inter-annual GPP variations, suggesting that the variability in cell production may predominantly be influenced by sink-related processes. However, anatomical traits linked to C allocation to the cell wall (e.g. the cell wall area) showed positive associations with seasonal GPP (Pseudo R2 = 0.33, p < 0.01), reflecting a more direct link to current photosynthetic supply (C source-related process). Therefore, cell biomass proxies were more effective than TRW at detecting tree responses to inter-annual shifts in C resources. We demonstrate the potential of a wood anatomical approach for disentangling complex climate influence on forest C sink- and source-related processes. This can provide a more mechanistic basis for improving C cycle models under environmental changes.