Impact of climate change and atmospheric carbon dioxide concentration on the growth of planted forests

The impacts, if any, of climate change on planted forests will be apparent either as progressive change in the suitability or otherwise of particular areas for forestry, or through changes in the growth and yield of forests as a result of changing conditions. It is not possible to predict with any confidence the influence of climate change on the location of planted forests. Uncertainties about the effects of the expected temperature rises on physiological processes and growth, and about the influence of possible changes in rainfall patterns on water balances, limit our ability to do more than make informed guesses. There is strong experimental evidence, from studies on young trees, that increased atmospheric carbon dioxide (CO2) concentrations lead to increased growth. Studies with complex physiological models lead to the same conclusion for forest stands. However, all the research that has been done has used relatively high CO2 concentrations-usually double present levels, with no acclimation-so the results cannot be accepted as necessarily reflecting future forest responses. Furthermore, forests-even planted forests-are complex ecosystems within which there are many interacting processes, and there have been no long-term studies on the effects of increased CO2 concentrations on them. Nutrient cycling and availability are important factors that may determine the long-term responses to increasing atmospheric CO2 concentrations and dry mass production by trees. This paper reviews the basis of several well-recognised forest growth simulation models, and presents results obtained when these models were compared and used to study climate-change scenarios for sites in Australia and Sweden. The results vary because of the different structures and assumptions of the models, but generally indicate that we can expect growth increases in the short-term, when plantations are young, but that the long-term outlook is uncertain.