The concentration and efflux of tree stem CO_(2) and the role of xylem sap flow

The accurate assessment of actual tree stem respiration and its relation with temperature plays a considerable role in investigating the forest carbon cycle.An increasing number of research reports have indicated that tree stem respiration determined with the commonlyapplied chamber gas exchange measuring system does not follow expectations regarding temperature relationships.This method is based on the nowadays widely-accepted theory that the respired CO_(2) in a tree stem would all diffuse outward into the atmosphere.However,it neglects partial CO_(2) that is dissolved in the xylem sap and is carried away by the transpirational stream.Scientists have started to realize that the respired CO_(2) measured with the chamber gas exchange method is only a portion of the total stem respiration(CO_(2) efflux),while the other portion,which is sometimes very substantial in quantity(thought to occupy maybe 15%-75%of the total stem respiration),is transported to the upper part of the stem and to the canopy by sap flow.This suggests that the CO_(2) produced by respiration is re-allocated within the stem.Accordingly,the change in CO_(2) efflux could be reflected in the rates of sap flow in addition to its dependence on temperature.Proper methods and instruments are required to quantify the internal and external CO_(2) fluxes in the trunk and their interaction with related environmental factors.