Trees differ from crops and from each other in their responses to increases in CO2 concentration

Length of exposure, degree of maturity and type of tissue all affect the results obtained in response to elevated CO2 treatment of trees. Seedlings are most responsive and, in many cases, the first few weeks or months of exposure may set the pattern for future growth. Measurements of leaf photosynthesis and respiration are not good predictors for incorporation of carbon into tissue. Seasonal changes in non-structural carbohydrates, emissions of isoprenes from leaves and exudation from roots can 'waste' photosynthate. However, these are difficult or impossible to quantify. Currently, the only generalization that can be made is that growth will be accelerated but the magnitude of this depends on tissue type, nutrition and environmental conditions. The implications of this for a future elevated atmospheric CO2 world are complex. Interactions and competition between species should be incorporated into long-term studies. These studies must, themselves, be incorporated into appropriate models which take into account regional soils and climates for use in prediction of the effects of global climate change on trees and forests.