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

To accurately estimate stem respiration (R), measurements of both carbon dioxide (CO) efflux to the atmosphere (E) and internal CO flux through xylem (F) are needed because xylem sap transports respired CO upward. However, reports of seasonal dynamics of F and E are scarce and no studies exist in Mediterranean species under drought stress conditions. Internal and external CO 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 were observed throughout the experiment, including prior to budburst, indicating that diel hysteresis between stem temperature and E cannot be uniquely ascribed to diversion of CO in the transpiration stream. Low internal [CO] (<0.5%) resulted in low contributions of F to R throughout the growing season, and R was mainly explained by E (>90%). Internal [CO] was found to vary vertically along the stems. Seasonality in resistance to radial CO diffusion was related to shoot water potential. The low internal [CO] and F 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 CO 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 CO fluxes and their interactions are needed to mechanistically understand and model respiration of woody tissues.