The role of temperate trees and forests in CO2 fixation

The global flask network data indicate that the temperate and boreal forests of the northern hemisphere are a significant sink for anthropogenic CO2. Bowen ratio and eddy covariance technology have been used to measure the net CO2 exchange of deciduous and coniferous forest. Some results from an earlier study on spruce with the Bowen ratio technique are presented. New technology that has been developed to measure fluxes continuously by forest stands is described and data are presented to show the net exchange flux of CO2 by temperate forests. These data support the hypothesis that temperate and boreal forests are significant sinks for carbon dioxide. An extensive programme of experimental impact studies is being carried out by a network of 12 laboratories in Europe funded by the European Commission. Parallel studies are in progress in North America and elsewhere. These studies indicate that doubling the atmospheric CO2 concentration results in increases in tree biomass of 30-40%. Interactions with nutrition are particularly significant. If nitrogen is added at a commensurate rate, the overall effect is that trees grow larger more quickly in elevated CO2 than in ambient air but they are essentially very similar in structure and physiology. However, if nutrients are in short supply, developmental and physiological changes occur. Then elevated CO2 causes changes in dry mass allocation to roots, in phenology of bud burst and set, in photosynthesis, in respiration, and in tree water relations. These changes are exaggerated in low nutrition situations. Process-based models have been developed to scale-up from leaf and tree to the stand scale. These models contain explicit description of processes affected by CO2, and are parameterised using the data collected in the impact studies. It is concluded that forests in the temperate and boreal region can effectively contribute to the removal of anthropogenic CO2 from the atmosphere and that tree growth and production of long-lived wood products should be encouraged as a major contribution towards off-setting the greenhouse effect caused by the burning of fossil fuels.