Tree growth and carbon balance in a temperate forest exposed to elevated carbon dioxide for eight years

Anthropogenic changes in the atmosphere have focused attention on the global carbon cycle. Increases in atmospheric CO2 concentrations strongly influence plant carbon uptake and because forests account for the bulk of terrestrial productivity the future role of forests in the carbon cycle is a subject of current scientific interest. This research investigates processes related to the carbon balance of a forest exposed to elevated CO 2. Difficulties in predicting the response of forests to elevated [CO 2] arise from the spatial scale of forests and long-lived nature of trees. I conducted this research in a Pinus taeda forest exposed to elevated [CO2] using Free Air Carbon dioxide Enrichment technology for 8 years. Growth was stimulated in the dominant canopy species in every year of the experiment with no evidence of a temporal decline in the response. Inter-annual variation in the growth response of P. taeda to elevated [CO2] was explained by the combination of temperature and rainfall during the active growing season. Suppressed trees, exposed to elevated CO2 in the forest understory showed a strong stimulation in photosynthesis throughout the experiment but there was no evidence of any growth enhancement in any year or cumulatively over 8 years. Initial tree size and their location relative to larger neighbors strongly determined the growth rates of understory trees. Ecosystem carbon gain caused by increased growth of the dominant tree species under elevated CO2 may be partly offset as the rate of CO2 diffusing from P. taeda stems was found to correlate with annual tree diameter increment. The interpretation of this result was complicated by the significant contribution of soil derived CO2 to the measured stem efflux. Isotopic measurements indicated that stem CO2 efflux was a mixture of both soil CO2 and autotrophic respiration. At least over a period of eight years tree growth in this young, actively growing forest was stimulated by elevated CO 2. The magnitude of the stimulation depended on inter-annual variation in weather and also on canopy position.