Seven-year-old ponderosa pine (Pinus ponderosa Dougl. ex P. Laws.) saplings and one- and two-year-old ponderosa pine seedlings of a Sierra Nevada and a Rocky Mountain seed source, respectively, were exposed to CO(2)-enriched atmospheres in an outdoor open-top chamber facility for 2.5 years. Seedling growth (main stem diameter, height, volume) increased with increasing CO(2) concentration, though the two populations exhibited different patterns of response. By the beginning of the last growth season, however, the trees under the highest CO(2) concentrations showed signs of stress that included accelerated needle abscision, chlorosis, and apparent alteration of tolerance to heat. The stress response is at least partly attributable to elevated foliar temperatures resulting from CO(2)-induced stomatal closure, which in turn lowered transpirational cooling of needles.