Responses of a C-3 and C-4 perennial grass to CO2 enrichment and climate change: Comparison between model predictions and experimental data

Ecological responses to CO2 enrichment and climate change are expressed at several interacting levels: photosynthesis and stomatal movement at the leaf level, energy and gas exchanges at the canopy level, photosynthate allocation and plant growth at the plant level, and water budget and nitrogen cycling at the ecosystem level. Predictions of these ecosystem responses require coupling of ecophysiological and ecosystem processes. Version GEM2 of the grassland ecosystem model linked biochemical, ecophysiological and ecosystem processes in a hierarchical approach. The model included biochemical level mechanisms of C-3 and C-4 photosynthetic pathways to represent direct effects of CO2 on plant growth, mechanistically simulated biophysical processes which control interactions between the ecosystem and the atmosphere, and linked with detailed biogeochemical process submodels. The model was tested using two-year full factorial (CO2, temperature and precipitation) growth chamber data for the grasses Pascopyrum smithii (C-3) and Bouteloua gracilis (C-4). The C-3-C-4 photosynthesis submodels fitted the measured photosynthesis data from both the C-3 and the C-4 species subjected to different CO2, temperature and precipitation conditions. The whole GEM2 model accurately fitted plant biomass dynamics and plant N content data over a wide range of temperature, precipitation and atmospheric CO2 concentration. Both data and simulation results showed that elevated CO2 enhanced plant biomass production in both P. smithii (C-3) and B. gracilis (C-4). The enhancement of shoot production by elevated CO2 varied with temperature and precipitation. Doubling CO2 increased modeled annual net primary production (NPP) of P. smithii by 36% and 43% under normal and elevated temperature regimes, respectively, and increased NPP of B. gracilis by 29% and 24%. Doubling CO2 decreased modeled net N mineralization rate (N_min) of soil associated with P. smithii by 3% and 2% at normal and high temperatures, respectively. N_min of B. gracilsi soil decreased with doubled CO2 by 5% and 6% at normal and high temperatures. NPP increased with precipitation. The average NPP and N_min of P. smithii across the treatments was greater than that of B. gracilis. In the C-3 species the response of NPP to increased temperatures was negative under dry conditions with ambient CO2, but was positive under wet conditions or doubled CO2. The responses of NPP to elevated CO2 in the C-4 species were positive under all temperature and precipitation treatments. N_min increased with precipitation in both the C-3 and C-4 species. Elevated CO2 decreased N_min in the C-4 system. The effects of elevated CO2 on N_min in the C-3 system varied with precipitation and temperature. Elevated temperature decreased N_min under dry conditions, but increased it under wet conditions. Thus, there are strong interactions among the effects of CO2 enrichment, precipitation, temperature and species on NPP and N_min. Interactions between ecophysiological processes and ecosystem processes were strong. GEM2 coupled these processes, and was able to represent the interactions and feedbacks that mediate ecological responses to CO2 enrichment and climate change. More information about the feedbacks between water and N cycling is required to further validate the model. More experimental and modeling efforts are needed to address the possible effects of CO2 enrichment and climate change on the competitive balance between different species in a plant community and the feedbacks to ecosystem function.