Regional Differences in CO2 Fertilization Effects

Free Air Carbon Enrichment (FACE) experiments indicate increased atmospheric CO2 enhances plant photosynthetic rates (the CO2 fertilization effect). These experiments also indicate other factors, such as water and nutrient availability, can limit the effects of CO2 fertilization. To isolate regional differences in CO2 fertilization, we ran global simulations using observed concentrations of CO2 from the global flask network and the National Centers for Environmental Prediction (NCEP) reanalysis (1958-2002) as input to the Simple Biosphere Model, version 3 (SiB3). SiB3 predicts photosynthetic rates using the Ball-Berry stomatal conductance equation, which allows SiB3 to respond to variability and trends in observed CO2 concentration. SiB3 also responds to other factors that limit or influence the photosynthetic rate, such a relative humidity and soil moisture. Using SiB3, we isolated those regions with the strongest and weakest CO2 fertilization. In general, soil moisture limited the effects of CO2 fertilization: dry and semi-arid regions showed the smallest CO2 fertilization, while moist regions showed the largest.