Importance of vegetation dynamics for future terrestrial carbon cycling

Terrestrial ecosystems currently sequester about one third of anthropogenic CO2 emissions each year, an important ecosystem service that dampens climate change. The future fate of this net uptake of CO2 by land based ecosystems is highly uncertain.Most ecosystemmodels used to predict the future terrestrial carbon cycle share a common architecture, whereby carbon that enters the system as net primary production (NPP) is distributed to plant compartments, transferred to litter and soil through vegetation turnover and then re-emitted to the atmosphere in conjunctionwith soil decomposition. However, while allmodels represent the processes ofNPP and soil decomposition, they vary greatly in their representations of vegetation turnover and the associated processes governingmortality, disturbance and biome shifts. Herewe used a detailed second generation dynamic global vegetation model with advanced representation of vegetation growth andmortality, and the associated turnover. We apply an emulator that describes the carbon flows and pools exactly as in simulations with the full model. The emulator simulates ecosystemdynamics in response to 13 different climate or Earth systemmodel simulations from theCoupledModel Intercomparison Project Phase 5 ensemble under RCP8.5 radiative forcing. By exchanging carbon cycle processes between these 13 simulations we quantified the relative roles of threemain driving processes of the carbon cycle; (I)NPP, (II) vegetation dynamics and turnover and (III) soil decomposition, in terms of their contribution to future carbon (C) uptake uncertainties among the ensemble of climate change scenarios.We found thatNPP, vegetation turnover (including structural shifts, wildfires andmortality) and soil decomposition rates explained 49%, 17% and 33%, respectively, of uncertainties inmodelled global C-uptake. Uncertainty due to vegetation turnoverwas further partitioned into stand-clearing disturbances (16%), wildfires (0%), stand dynamics (7%), reproduction (10%) and biome shifts (67%) globally.We conclude that whileNPP and soil decomposition rates jointly account for 83%of future climate inducedC-uptake uncertainties, vegetation turnover and structure, dominated by biome shifts, represent a significant fraction globally and regionally (tropical forests: 40%), strongly motivating their representation and analysis in future C-cycle studies.