Thirty years of in situ tree growth under elevated CO 2 : a model for future forest responses?

Rising concentrations of atmospheric carbon dioxide have been predicted to stimulate the growth of forest trees. However, long‐term effects on trees growing to maturity and to canopy closure while exposed to elevated CO 2 have never been examined. We compared tree ring chronologies of Mediterranean Quercus ilex which have been continuously exposed to elevated CO 2 (around 650 μmol mol –1 ) since they were seedlings, near two separate natural CO 2 springs with those from trees at nearby ambient‐CO 2 ‘control’ sites. Trees grown under high CO 2 for 30 years (1964–93) showed a 12% greater final radial stem width than those growing at the ambient‐CO 2 control sites. However, this stimulation was largely 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 high CO 2 grown trees had disappeared. At any given tree age, elevated CO 2 had a relatively greater positive effect on tree ring width in years with a dry spring compared to years with more rainfall between April and May. This indicates a beneficial effect of elevated CO 2 on tree water relations under drought stress. Our data suggest that the early regeneration phase of forest stands can be accelerated in CO 2 ‐enriched atmospheres and that maximum biomass per land area may be reached sooner than under lower CO 2 concentrations. In our study, high CO 2 grown Q. ilex trees reached the same stem basal area at the age of 26 y as control trees at 29 y, i.e. three years earlier (faster turnover of carbon?). Reliable predictions of the future development of forests need to account for the variable responses of trees over their entire lifetime. Such responses to elevated CO 2 can presently only be assessed at such unique field sites.