Long-term consequences of altered tree growth and physiology in the Earth System

Forests cover a third of the vegetated surface of the Earth and interact with global climate. These interactions are being altered by human activities at an unprecedented scale, both in their rate and in their geographical extent. At present, Earth system models (ESMs) struggle to predict how these climate-vegetation interactions will evolve in the future because of varied responses of the carbon cycle in their Land Surface Models (LSMs). This long-standing issue leads to widely varying atmospheric CO2 concentrations and hence varying climate projections under the same scenarios. LSM projections of forest biomass and carbon balance increasingly diverge over longer periods, thus limiting confidence in future climate projections. The goal of this proposal is to develop a new cross-disciplinary framework to constrain climate projections by jointly improving the simulation of forest growth and water use efficiency (WUE; the ratio of photosynthesis to transpiration) at long time scales (decades to century) using novel observational standards for historical growth and physiology derived from tree-ring data. The following three objectives are addressed: (1) advance the mechanistic representation of tree-ring growth and stable isotopes in the global LSM ORCHIDEE, the terrestrial component of the IPSL ESM; (2) use tree-ring data together with eddy covariance flux measurements to improve the capability of the model to simulate the effects of changing climate, CO2 and nitrogen on forest growth and WUE; (3) provide the first climate simulations until 2100 constrained by historical forest responses. The constrained projections will reduce uncertainties on the feedbacks of altered tree growth and physiology on climate. The proposed work will bring into Earth System modelling decades of advances made by dendrochronologists around the world, providing at last a critical long-term constraint for the land surface modelling community as ice-cores are for global circulation models.