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

To accurately estimate stem respiration (RS), measurements of both carbon dioxide (CO2) efflux to the atmosphere (EA) and internal CO2 flux through xylem (FT) are needed because xylem sap transports respired CO2 upward. However, reports of seasonal dynamics of FT and EA 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 EA were observed throughout the experiment, including prior to budburst, indicating that diel hysteresis between stem temperature and EA cannot be uniquely ascribed to diversion of CO2 in the transpiration stream. Low internal [CO2] (<0.5%) resulted in low contributions of FT to RS throughout the growing season, and RS was mainly explained by EA (>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 FT 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.