Improvement of Terrestrial Ecosystem Model in terms of High CO2 Response

Plants respond to variations of ambient CO2 concentration directly through stomatal aperture and photosynthetic biochemistry, followed by a series of indirect processes such as allocation and growth. At the ecosystem scale, changes in biomass, litter production, and heat and water budgets, result in changes in vegetation dynamics and biogeochemistry. Although many experimental and modeling studies have explored the nature of plant's high CO2 responses, it is still difficult to make an accurate long-term projection (i.e., including acclimation) under increasing CO2 conditions. We are improving our terrestrial ecosystem model (VISIT) on the basis of experimental and ecophysiological modeling studies. For example, theoretical optimization of canopy-scale nitrogen utilization allows us to make better prediction of leaf area index under increasing CO2 conditions. Coupled carbon-nitrogen-water cycling framework of the model enables us to evaluate ecosystem-scale dynamics in response to elevated CO2. The global model simulations using the VISIT model, on the basis of historical and projected scenarios of atmospheric CO2 and climate (e.g., IPCC SRES and CMIP3 data), imply that plant's water-use efficiency would increase because of stomatal closure and photosynthetic increment. We have also conducted a meta-analysis across a wide range of plant functional types with respect to actual leaf area index and canopy nitrogen metabolisms. Finally, we discuss remaining issues in terms of plant high CO2 responses such as changes in respiratory metabolisms, mortality, and underground processes. Our works are directed to better representation of atmosphere-biosphere interactions by ecosystem and earth-system models.