Full title: Carbon dynamics in European beech and Norway spruce under present and future RCP scenarios

To elucidate how trees adjust their C dynamics under future climate, we conducted a unique growth chamber experiment with tree saplings grown for three years under the present climate conditions (PC), a mitigation scenario (RCP2.6) and a worst-case scenario (RCP8.5). The RCPs were simulated by increased air temperature, vapor pressure deficit, irradiance, CO 2 and O 3 concentrations at hourly resolution. Both species, European beech and Norway spruce, grew more biomass under RCP8.5 than PC and RCP2.6, accompanied by increased partition to belowground. Towards the end of the three-year experiment, the mean residence time (MRT) and mean transit time (MTT) of C along the tree-soil system were assessed in a 13 CO 2 tracer experiment. MRT in leaves was unaffected in both species but decreased significantly in beech stem CO 2 efflux under both RCP scenarios. MTT for beech from leaves to stem CO 2 efflux decreased under RCP2.6. For both species, we found a negative correlation between MTT and maximum assimilation, indicating faster C flux through trees at higher assimilation rates. Conversely, no positive correlation was found between biomass growth and maximum assimilation rates. After three years of growth under the simulated RCP scenarios, we conclude that C turnover would accelerate under future climate conditions. The additional C fixed by photosynthesis will be released back to the atmosphere more rapidly, as a smaller fraction could remain in the biomass. These results support the implementation of faster C turnover in process-based dynamic vegetation models, although confirmation for mature trees is still needed. • Unique growth chamber experiment for 3 years simulating RCP 2.6 and 8.5 scenarios. • 13 C-labeling experiment to trace C flux through acclimated saplings. • Beech and spruce saplings grew more biomass, particularly belowground, under RCP8.5. • Beech and spruce saplings showed accelerated C turnover under future scenarios. • Support for faster C flux through trees being related to increased C assimilation.