In addition to dispering into the soils and out into the atmosphere, CO2 produced by tree roots can travel up the stem. The presence of this newfound pathway suggest that scientist have underestimated the actual respiration rates of below-ground plant tissues. In this project we aim at more fully understanding this process. We will therefore quantify the belowground respiration in different tree species by measuring both internal (in the xylem sap) and external (from the soil) CO2 fluxes at annual time-scale and we will investigate the fate of root-respired CO2 using 13C labeling.