30 YEARS OF IN-SITU TREE GROWTH UNDER ELEVATED CO2 - A MODEL FOR FUTURE FOREST RESPONSES

Rising concentrations of atmospheric carbon dioxide have been predicte d to stimulate the growth of forest trees. However, long-term effects on trees growing to maturity and to canopy closure while exposed to el evated CO2 have never been examined. We compared tree ring chronologie s of Mediterranean Quercus ilex which have been continuously exposed t o elevated CO2 (around 650 mu mol mol(-1)) since they were seedlings, near two separate natural CO2 springs with those from trees at nearby ambient-CO2 'control' sites. Trees grown under high CO2 for 30 years ( 1964-93) showed a 12% greater final radial stem width than those growi ng at the ambient-CO2 control sites. However, this stimulation was lar gely due to responses when trees were young. By the time trees were 25 -30 y old the annual difference in tree ring width between low and hig h CO2 grown trees had disappeared. At any given tree age, elevated CO2 had a relatively greater positive effect on tree ring width in years with a dry spring compared to years with more rainfall between April a nd May. This indicates a beneficial effect of elevated CO2 on tree wat er relations under drought stress. Our data suggest that the early reg eneration phase of forest stands can be accelerated in CO2-enriched at mospheres and that maximum biomass per land area may be reached sooner than under lower CO2 concentrations. In our study, high CO2 grown Q. ilex trees reached the same stem basal area at the age of 26 y as cont rol trees at 29 y,i.e. three years earlier (faster turnover of carbon? ). Reliable predictions of the future development of forests need to a ccount for the variable responses of trees over their entire lifetime. Such responses to elevated CO2 can presently only be assessed at such unique field sites.