trembling aspen (populus tremuloides michx.) is the most widely distributed and probably most genetically varied woody species in north america. four provenances of trembling aspen seedlings from northwestern ontario were grown in the greenhouse to investigate genetic variation of growth and ecophysiological responses to ambient and elevated co2 concentrations. two provenances were from southwest of thunder bay, and the other two were from north shore of lake superior. three families per provenance were grown. leaf gas exchange variables, growth and biomass were measured at 3 and 5 months old (august and october 2001) in the first year and 60 days after bud flush in the second year (april 2002). significant differences between provenances were found in root collar diameter and height in the august 2001 measurement, and in total and stem biomass harvested in april 2002. there were no significant differences in leaf gas exchange or other biomass components expressed between provenances in april 2002. however, the seedlings expressed substantial family differences in leaf gas exchange, growth, biomass and biomass allocation variables. there were positive but low correlations between photosynthetic water use efficiency (wue) and height and total biomass. furthermore, provenance performance was predicted by most monthly climate variables suggesting adaptation of provenances to local climate. high family and single tree heritability estimates of biomass variables were present in the august 2001 measurement. the seedlings were also exposed to three co2 concentrations (ambient, 540 ppm and 720 ppm) in greenhouses for 30 days in the first year and 60 days after bud flush in the second year. other environmental conditions were controlled at optimal. after the first co2 exposure, net co2 assimilation (na), stomatal conductance (gs), intercellular to leaf surface co2 ratio (ci/ca) and transpiration rate (e) were increased by both co2 enrichments, but no provenance differences were found. in the second co2 exposure, na and wue significantly increased in all provenances at both co2 elevations. for the two southwest provenances, g s were significantly decreased by 540 ppm, but not by the 720 ppm treatment. however, g s of the two north-shore provenances did not respond to both co2 elevations. when measured at common co2 level, a 10% down regulation of na was observed for the seedlings in the 720 ppm treatment, but no provenance differences were found. in the final harvest, the total, shoot, stem and root biomass were increased by co2 elevations, while leaf mass and biomass allocations were not. most biomass components were increased by co2 elevations in the southwest provenances, but not in their north-shore counterparts. biomass allocations were not significantly affected by co2 elevations. there were also no provenance differences in biomass allocations in response to elevated co2 , while family differences only existed in stem mass ratio and leaf mass ratio. in conclusion, the two southwest provenances could perform better than the two north-shore ones in the elevated atmospheric ccl environment in future.