Simulated responses of Pinus halepensis forest productivity to climatic change and CO2 increase using a statistical model

Tree ring chronologies provide long-term records of growth in natural envir onmental conditions and may be used to evaluate impacts of climatic change and CO2 increase on forest productivity. This study focuses on 21 Pinus hal epensis forest stands in calcareous Provence (in the south-east of France). A chronology of net primary productivity (NPP) both for the 20th century a nd for each stand was estimated using tree ring data (width and density). T he response of each stand to climate in terms of NPP was statistically mode lled using response functions. Anomalies between estimated NPP and NPP reco nstructed by response functions were calculated to evaluate the fertilising effect of CO2 increase on tree growth. The changes in anomalies during the 20th century were attributed to the effect of CO2 increase. A multiplying factor (beta) linking CO2 concentration and stand productivity was then cal culated, on the basis of the trend observed during the 20th century. In thi s study, the value of the beta factor obtained under natural conditions (be ta = 0.50) is consistent with those from controlled CO2 enrichment experime nts. Both response functions and the beta factor were used to predict NPP c hanges for a 2 X CO2 scenario. The 2 X CO2 climate was obtained using predi ctions from Meteo France's ARPEGE atmospheric general circulation model (AG CM) downscaled to Marseilles meteorological station. NPP increased signific antly for nine stands solely when the climatic effect was taken into accoun t. The main factors responsible for this enhancement were increased winter and early spring temperatures. When the fertilising effect of the CO2 incre ase was added, NPP was significantly enhanced for 14 stands (i.e. NPP enhan cement ranged from 8% to 55%). Although the effects of global change were s lightly detectable during the 20th century, their acceleration is likely to lead to great changes in the future productivity of P. halepensis forests. (C) 2000 Elsevier Science B.V. All rights reserved.