Every leaf growing interrestrial ecosystems is now surrounded by air which is subject to increasing CO2 concentrations. As CO2 is one of the main substrates for photosynthesis, the increase will inevitably have an impact on the physiological processes of plants. The ecophysiology of CO2 uptake and transpiration is briefly reviewed, and the possible yield responses are discussed. It is agreed that the yield will be enhanced as a result of the increase in the CO2 level. However, this improvement may be masked by the same factors that mask the yield improvement obtained through plant breeding. The yield enhancement is therefore difficult to assess by using only historical data. An alternative approach is proposed, which utilizes ecophysiological experiments, tree ring data, and dynamic models of plant ecology
