Because of their prominent role in the global carbon balance and their possible carbon sequestration, the response of trees to elevated CO2 has been intensively studied and their major physiological responses have been examined.
However, despite our understanding of the short term response of photosynthesis to elevated CO2, little is known about the response of photosynthesis to long term enhancement under elevated CO2. An analysis of leaf level photosynthetic responses indicates an average enhancement of about 36% in coniferous species and 63% in broad leaf species.
While over the short term an increase in CO2 has been shown to stimulate trees photosynthesis, in the longer term, an acclimation process down-regulating the photosynthetic activity has often been reported and studied. Averaged across all observations available for trees, there appears to be a 21% down regulation in photosynthetic potential for trees grown at elevated CO2 concentrations compared to the ambient-grown controls. This phenomenon has mainly been attributed to source-sink problems and storage capacity. The down regulation examples reported in the literature can nearly always be related either to poor nutritional conditions or to constraints in root growth. Furthermore, no down regulation is usually found if the trees are soil rooted or provided with sufficient nitrogen: this suggests that the stimulatory effect of CO2 on photosynthesis can be sustained throughout months and years of increased CO2 exposure. Branch bag experiments on adult trees will illustrate this statement as well as other experimental results of the European ECOCRAFT research group.
Changes in the balance between rates of uptake of CO2 and inorganic nutrients, specially nitrogen, result in concomitant changes in the composition (especially C:N ratio) of leaves: A reduction in leaf N is commonly detected.
Changes in respiration rate may be an important factor affecting the entire carbon budget of a tree under elevated CO2. A direct, instantaneous and reversible effect of elevated CO2, as well as a long term effect on the dark respiration rate has been documented in several temperate trees. Some discrepancies which remain in the literature may be related to the poor understanding of growth and maintenance costs in trees.
Key words: elevated CO2, photosynthesis, acclimation, respiration.
Correspondence: Marianne Mousseau, Laboratoire d'Ecologie Végétale, URA CNRS 1492, Batiment 362, Université Paris-Sud, 91405 Orsay Cedex, France
Telefax: +33-1-69417938