DISPROPORTIONAL INCREASES IN PHOTOSYNTHESIS AND PLANT BIOMASS IN A CALIFORNIAN GRASSLAND EXPOSED TO ELEVATED CO2 - A SIMULATION ANALYSIS

1. Elevated CO2 concentrations often lead to increased photosynthetic carbon uptake in plants, but this does nor necessarily result in a pro portional increase in plant biomass. We examined this paradox for gras slands in northern California that have been exposed to elevated CO2 s ince 1992. We evaluated the effects of physiological adjustments on pl ant growth and carbon balance of the dominant species, Avena barbata, using a plant growth model. 2. Without physiological adjustments, an o bserved 70% increase in leaf photosynthesis in elevated CO2 was predic ted to increase plant biomass by 97% whereas experimental measurements suggested 5 and 13% decreases in 1992 and 1993, respectively, and a 4 0% increase in 1994. 3. Simulations with an increase in carbon allocat ion to roots by 29%, or leaf death rate by 80%, or non-structural carb ohydrate storage by 60%, or leaf mass per unit area by 25% each predic ted an approximately 40% increase in plant biomass in 1994 under eleva ted CO2. It follows that greater suppression of the biomass responses to elevated CO2, in 1992 and 1993 resulted from variable combinations of these physiological adjustments. 4. This modelling study concludes that (a) an increase in carbon loss or (b) a decrease in carbon-use ef ficiency or (c) an increase in carbon allocation to root growth will r esult in an increase in biomass growth that is less than that in leaf photosynthesis under elevated CO2. Alternatively, if carbon loss is re duced (e.g, depressed respiration) and/or carbon allocation to leaf gr owth is increased, biomass growth may be stimulated more than leaf pho tosynthesis by atmospheric CO2 concentration. Moreover, this modelling exercise suggests that physiological adjustments may have substantial effects on ecosystem carbon processes by varying ecosystem carbon inf lux, litterfall and Litter quality.