How does elevated atmospheric CO2 concentration affect vegetation productivity? : a study based on Free Air CO2 Enrichment (FACE) experiments and two generalised vegetation models

Abstract: Increasing levels of CO2 in the atmosphere may substantially increase net primary productivity (NPP) and thereby increase carbon sequestration by terrestrial ecosystems. Realistic Free Air Carbon Enrichment (FACE) experiments tend to show a NPP enhancement of about 25% if CO2 is elevated by 200 ppm. In this study two generalized vegetation models, the Lund-Postsdam-Jena dynamic generalized vegetation model (LPJ-DGVM), and the General Ecosystem Simulator (GUESS), were tested against results from five FACE sites, representing different global vegetation types. The models manage to provide results that were close the observed values in the FACE experiments when presenting the average NPP enhancement for all the sites over all the years. LPJ-DGVM simulated an enhancement in NPP of 29,6%, the GUESS 26,5% compared to the observed 26,5% in the FACE experiments. However the models fail to simulate the interannual variation in the CO2 response. One potential explanation for this is that the generalised representations of soil hydrology in the models do not adequately represent site-specific hydrological patterns and water-availability. The results in this study do not confirm the hypothesis that the NPP enhancement as a result of elevated CO2 is larger under water limitation. Strong critics have been aimed against models without nutrient limitation, like the LPJ-DGVM and GUESS, stating that these oversimulate the NPP effect from elevated CO2. This study indicates that nutrient availability does not crucially limit vegetation response to elevated CO2 productivity.