Knowledge about the effects of the rise in atmospheric CO
2
concentration on trees and forest is assessed and, the converse, the possible impact of forests on the atmospheric CO
2
concentration is discussed. At the cellular scale, much is known about the role of CO
2
as a substrate in photosynthesis, but only little about its role as an activator and regulator. At the leaf scale, the response of CO
2
assimilation to CO
2
concentration has been described often and is well represented by biochemically based models, but there is inadequate information to parametrize the models of CO
2
-acclimated leaves. Growth and partitioning to the roots of seedlings and young trees generally increases in response to a doubling in atmospheric CO
2
concentration. Experimental results are very variable, because of the differing length of the experiments, the artificial conditions and the artefactual constraints. At larger scales, direct measurements of responses to increase in atmospheric CO
2
are impractical but models of canopy processes suggest that significant increases in CO
2
assimilation will result from the rise in atmospheric concentration. Inferences from the increase in amplitude of the seasonal oscillation in the global atmospheric CO
2
concentration at different latitudes suggest that forest is having a significant impact on the global atmospheric concentration, but it seems unlikely that expansion of the forest resource could effectively reduce the increase in atmospheric CO
2
.
Atmospheric carbon dioxide: a driver of photosynthetic eukaryote evolution for over a billion years?