Temporal and spatial patterns of internal and external stem CO2 fluxes in a sub-Mediterranean oak

To accurately estimate stem respiration (R_S), measurements of both carbon dioxide (CO₂) efflux to the atmosphere (E_A) and internal CO₂ flux through xylem (F_T) are needed because xylem sap transports respired CO₂ upward. However, reports of seasonal dynamics of F_T and E_A are scarce and no studies exist in Mediterranean species under drought stress conditions. Internal and external CO₂ fluxes at three stem heights, together with radial stem growth, temperature, sap flow and shoot water potential, were measured in Quercus pyrenaica Willd. in four measurement campaigns during one growing season. Substantial daytime depressions in temperature-normalized E_A were observed throughout the experiment, including prior to budburst, indicating that diel hysteresis between stem temperature and E_A cannot be uniquely ascribed to diversion of CO₂ in the transpiration stream. Low internal [CO₂] (<0.5%) resulted in low contributions of F_T to R_S throughout the growing season, and R_S was mainly explained by E_A (>90%). Internal [CO₂] was found to vary vertically along the stems. Seasonality in resistance to radial CO₂ diffusion was related to shoot water potential. The low internal [CO₂] and F_T observed in our study may result from the downregulation of xylem respiration in response to a legacy of coppicing as well as high radial diffusion of CO₂ through cambium, phloem and bark tissues, which was related to low water content of stems. Long-term studies analyzing temporal and spatial variation in internal and external CO₂ fluxes and their interactions are needed to mechanistically understand and model respiration of woody tissues.