Multi-proxy analysis confirms the tight coupling of carbon assimilation and allocation, with divergent NSCs strategies in two boreal forest species

Understanding the link between photosynthesis and carbon allocation to woody biomass remains a critical gap in predicting forest responses to climate change due to the pervasive lack of comprehensive carbon-based data at the whole-stand level. We employed an integrated approach combining micrometeorological techniques (Eddy Covariance, EC), process-based and biogeochemical modelling, tree ring width (TRW), and quantitative wood anatomy to assess changes in carbon fluxes and allocation dynamics over mature stands of black spruce (Picea mariana Mill.) and jack pine (Pinus banksiana Lamb.) from 1999 to 2021 in Canada. We used Gross Primary Production (GPP) from EC to calibrate and validate GPP simulations from the 3D-CMCC-FEM model, incorporating tree ring width (TRW) and wood anatomical traits, such as cell wall area (CWA), as proxies for carbon fixation.

Our findings demonstrated that the forest ecosystem model effectively captured GPP at daily, monthly, and annual scales, strongly correlating with EC-based estimates (P < 0.001). Both stands revealed a strong association between observed and modelled GPP and CWA, highlighting that CWA better reflects carbon assimilation in woody biomass than TRW. Species-specific differences in non-structural carbohydrates (NSCs) dynamics were also evident, as model simulations indicated that Pinus banksiana actively utilized NSCs for growth, while Picea mariana relied on NSCs as a buffer under cold conditions. This multi-proxy approach enhanced our understanding of carbon dynamics and temporal and spatial carbon flux pathways. Our findings provide critical insights into carbon allocation strategies, contributing valuable knowledge for refining climate change models in boreal ecosystems.