Diameter-driven variation in wood CO2 efflux across the stems and crowns of three temperate broadleaf tree species.

A major constraint on modelling woody-tissue CO₂ efflux is the scarcity of datasets that adequately represent its variability and underlying drivers. We intensively sampled CO₂ efflux along the vertical profile of nine temperate broadleaf trees across three species and found substantial longitudinal variation. We partitioned the flux into growth and maintenance-associated components; accordingly we use 'efflux' for measurements and 'respiration' only for the derived components. Expressing efflux by wood volume, surface area and sapwood volume each offered insight into the underpinning of the flux. However, efflux was most strongly associated with surface area, particularly in medium and large branches, and across both growth and maintenance respiration. In small branches, where sapwood dominates, this surface area scaling weakens, suggesting maintenance respiration shifts from surface area- to volume-based scaling, while growth respiration remains associated with surface area. Sapwood depth, tree ring width and the relative contributions of growth and maintenance respiration explained much of the variation in efflux, though additional biological and physical factors, such as CO₂ diffusion from sap and variation in sapwood parenchyma, likely also contribute. These findings represent an important step towards reducing uncertainty in the spatial scaling of woody-tissue CO₂ efflux by linking within tree variation in respiration to underlying anatomical and growth-related controls.