Partitioning direct and indirect effects of elevated CO2 on net primary production across climates
Elevated concentrations of atmospheric CO2 are known to affect carbon, water and energy fluxes due to a direct physiological effect via increased carbon assimilation. However, indirect effects through changes of root zone water availability, Leaf Area Index (LAI), canopy structure, or soil biogeochemistry can potentially play an important role in enhancing or hindering the direct effect, but they are impossible to tease apart in experiments. However, the interplay of direct and indirect effects is a possible reason for contrasting reports about the magnitude of CO2 fertilization across different climates, ecosystems, and temporal scales. We present a numerical study to shed light on the differential role of direct (through plant physiology) and indirect (through soil moisture and LAI) effects of elevated CO2 across a number of ecosystems. We specifically ask in which ecosystems and climate indirect effects are expected to be largest. Here, a state-of-the-art ecohydrological model (Tethys-Chloris) is used to quantitatively separate direct from indirect effects. Data and boundary conditions from flux-towers and free air CO2 enrichment (FACE) experiments are used to force the model and evaluate its performance.
Model simulations compare favorably with field observations of various metrics (carbon assimilation, stomatal conductance, LAI, net primary production) and they can generally reproduce the observed total effect of CO2 enrichment, although current accuracy of field measurements precludes robust conclusions for certain variables. Numerical results suggest that indirect effects of elevated CO2, especially through water savings, are very significant and sometime comparable in magnitude to the direct effect. Indirect effects tend to be slightly larger in ecosystems with significant water limitation. The combined effect on carbon fluxes of increasing CO2 from 375 to 600 ppm ranges from 10 to 50% as a function of climate and ecosystem type and has a strong negative correlation with the wetness index. These results are essential to provide a more general understanding of short-term (few years) implications of elevated CO2 and can guide the interpretation of observations where the separation of direct and indirect effects is impossible.