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 expresse d at several interacting levels: photosynthesis and stomatal movement at the leaf level, energy and gas exchanges at the canopy level, photo synthate allocation and plant growth at the plant level, and water bud get 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 bioch emical, ecophysiological and ecosystem processes in a hierarchical app roach. The model included biochemical level mechanisms of C-3 and C-4 photosynthetic pathways to represent direct effects of CO2 on plant gr owth, mechanistically simulated biophysical processes which control in teractions between the ecosystem and the atmosphere, and linked with d etailed biogeochemical process submodels. The model was tested using t wo-year full factorial (CO2, temperature and precipitation) growth cha mber data for the grasses Pascopyrum smithii (C-3) and Bouteloua graci lis (C-4). The C-3-C-4 photosynthesis submodels fitted the measured ph otosynthesis data from both the C-3 and the C-4 species subjected to d ifferent CO2, temperature and precipitation conditions. The whole GEM2 model accurately fitted plant biomass dynamics and plant N content da ta 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 va ried with temperature and precipitation. Doubling CO2 increased modele d annual net primary production (NPP) of P. smithii by 36% and 43% und er normal and elevated temperature regimes, respectively, and increase d NPP of B. gracilis by 29% and 24%. Doubling CO2 decreased modeled ne t 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. g racilsi soil decreased with doubled CO2 by 5% and 6% at normal and hig h 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 temperat ures was negative under dry conditions with ambient CO2, but was posit ive under wet conditions or doubled CO2. The responses of NPP to eleva ted CO2 in the C-4 species were positive under all temperature and pre cipitation 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. T he effects of elevated CO2 on N_min in the C-3 system varied with prec ipitation 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, precipitatio n, temperature and species on NPP and N_min. Interactions between ecop hysiological processes and ecosystem processes were strong. GEM2 coupl ed these processes, and was able to represent the interactions and fee dbacks that mediate ecological responses to CO2 enrichment and climate change. More information about the feedbacks between water and N cycl ing is required to further validate the model. More experimental and m odeling efforts are needed to address the possible effects of CO2 enri chment and climate change on the competitive balance between different species in a plant community and the feedbacks to ecosystem function.