Quantifying Direct and Indirect Effects of Elevated CO2 on Ecosystem Response

Abstract:

Increasing concentrations of atmospheric carbon dioxide are expected to affect carbon assimilation, evapotranspiration (ET) and ultimately plant growth. Direct leaf biochemical effects have been widely investigated, while indirect effects, although documented, are very difficult to quantify in experiments. We hypothesize that the interaction of direct and indirect effects is a possible reason for conflicting results concerning the magnitude of CO₂ fertilization effects across different climates and ecosystems. A mechanistic ecohydrological model (Tethys-Chloris) is used to investigate the relative contribution of direct (through plant physiology) and indirect (via stomatal closure and thus soil moisture, and changes in Leaf Area Index, 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. Data and boundary conditions from flux-towers and free air CO₂ enrichment (FACE) experiments are used to force the model and evaluate its performance. Numerical results suggest that indirect effects of elevated CO₂, through water savings and increased LAI, are very significant and sometimes larger than direct effects. Indirect effects tend to be considerably larger in water-limited ecosystems, while direct effects correlate positively with mean air temperature. Increasing CO₂ from 375 to 550 ppm causes a total effect on Net Primary Production in the order of 15 to 40% and on ET from 0 to -8%, depending on climate and ecosystem type. The total CO₂ effect has a significant negative correlation with the wetness index and positive correlation with vapor pressure deficit. These results provide a more general mechanistic understanding of relatively short-term (less than 20 years) implications of elevated CO₂ on ecosystem response and suggest plausible magnitudes for the expected changes.