The Direct Effect of Increased CO2on Gas Exchange and Growth of Forest Tree Species

CO2 enrichment of the atmosphere is now well documented and its effect on the growth of world forests is being questioned by the scientific community. The direct effects of increased CO2 on tree species are reviewed: the different experimental approaches are described, as well as the principal results already obtained. Short-term experiments have shown an increased photosynthetic rate, as predicted by leaf models. In longer experiments this increase is reduced after a few weeks or months by mechanisms that remain to be found. Elevated CO2 seems to decrease the dark respiration rate, but the results are still controversial. Biomass partitioning in elevated CO2 is clearly related to the mineral supply of the trees: An increase in root investment in elevated CO2 is related to a poor mineral status. The mineral content of trees grown in elevated CO2 is generally lowered compared to controls. No general rule has yet been found for the effect of increased CO2 on leaf area development. The paper emphasizes large areas of ignorance: the reasons for the different responses of different species, which may be related to their developmental strategies, are largely ignored. Much experimental effort is needed to parameterize all the physiological processes which are susceptible to change with an increase in atmospheric CO2, leading to a change in forest tree growth.