The impact of rising CO2 on ecosystem production

A fundamental property of green plants is that the rate of photosynthesis is dependent in the ambient CO₂ concentration. There is overwhelming experimental evidence that this effect increases plant production in most C₃ plants: hundreds of experiments with many species show that plant growth increases an average 30% to 40% for a doubling of the present normal ambient CO₂ concentration (Kimball, 1986). External environmental factors, such as temperature and the availability of nutrients, modify this response. The greatest stimulation of photosynthesis and growth can be expected to occur at high temperatures and much smaller responses at low temperature. Factors which restrict growth, such as low nutrients, will reduce but usually do not eliminate the stimulation of production with increasing CO₂ even when nitrogen is severly limiting. There are also reports of direct effects of ambient CO₂ concentration on dark respiration which show that there is an immediate reduction in the rate of CO₂ efflux or O₂ consumption when the CO₂ around plant tissues is increased. There have been very few longterm field studies of the effects of increased CO₂ on whole plants and ecosystem processes but the data from these studies are consistent in showing an increase in plant production with an increase in CO₂ concentration of the ambient air.