to study the physiological and growth responses of eleven ontario provenances of white spruce [picea glauca (moench) voss] to elevated atmospheric co2 concentrations, one-year old seedlings were grown in three greenhouses under co2 concentrations of ambient, 530 ppm, and 700 ppm for a period 90 days. the following physiological traits were measured after 60 and 90 days of treatment: net assimilation rate (a), stomatal conductance (gs), transpiration (e), water-use efficiency (wue), and intercellular to ambient co2 concentration ratio (c|/ca). shoot, root, and total biomass and biomass allocation were also measured after 60 and 90 days of treatment. height and root collar diameter (rcd) data were collected after 0, 30, 60, and 90 days of treatment. net co2 assimilation was significantly enhanced by elevated [co2 ] after 60 and 90 days of treatment despite reductions in photosynthetic capacity. elevated [co2 ] also enhanced wue and decreased gs and e after 60 days of treatment. there was significant down-regulation of photosynthesis in response to co2 concentration elevation. elevated [co2] significantly reduced rubisco carboxylation efficiency (vcmax), rubp regeneration capacity (jmax), and triose phosphate utilisation (tpu), but had no significant effect on leaf respiration (rj) or c 0 2 compensation point. despite its substantial influence on gas exchange, co2 concentration did not significantly affect seedling biomass, biomass allocation, height, or rcd. no significant co2 x provenance interactions were found in the gas exchange measurements. provenance had a significant effect on the height and rcd of the white spruce seedlings. strong correlations existed between gas exchange and monthly climate variables for the 11 ontario provenances of white spruce at ambient and elevated co2 concentrations, suggesting adaptation of individual provenances to local climate. the implications of co2 response of diverse sources of white spruce seedlings on tree improvement programs will probably be minimal. seed sources of white spruce in ontario selected for superior growth characteristics in the present climate will probably perform well in the predicted future climate.