Using a biogeochemistry model in simulating forests productivity responses to climatic change and [CO₂] increase: example of Pinus halepensis in Provence (south-east France)

Tree-ring chronologies provide long-term records of growth in natural environmental conditions and may be used to evaluate the impacts of climatic change and [CO2] increase on forest productivity. This study focuses on 21 Pinus halepensis forest stands in calcareous Provence (in the south-east France). Changes in productivity are simulated using the global biogeochemistry model BIOME3, that we have adapted to run with chronological data. Tree-ring data (width and density) were used to estimate, for each stand, an observed series of changes in productivity. Simulated and observed productivity changes are then compared to validate the chronological biogeochemistry model BIOME3C. Variations in productivity were well reconstructed at 15 sites. After this validation, BIOME3C was used to simulate forest productivity changes for a 2 x CO2 scenario. The 2 x CO2 climate used as input was obtained using results from Meteo-France's ARPEGE atmospheric general circulation model (ALCM), downscaled to local meteorological stations. Productivity increases moderately for all stands (from 17 to 24%) when climatic changes alone were taken into account. The main factor responsible for this increase is a reduction in summer drought severity. Productivity increases highly for all stands (from 72 to 86%) when the physiological fertilising effect of the [CO2] increase is considered separately. When both climatic changes and the [CO2] increase were taken into account, productivity increases highly, from 107% (for Moustier) to 141 % (for La Ciotat). The direct fertilising effect of [CO2] increase has a greater influence on the forest stands productivity than the indirect climatic changes effect. These results also exhibit the importance of the synergy between the effects of climate change and [CO2] increase, as the increase in productivity resulting from the combined effects are more than the sum of the individual CO2 and climate effects. Although the detected effects of global change during the 20th century were slight, acceleration of these changes is likely to lead to great changes in the future productivity of P. halepensis forests. (C) 2003 Elsevier B.V. All rights reserved