Genetic variation of Populus tremuloides in ecophysiological responses to CO₂ elevation

To investigate the genetic variation of trembling aspen ( Populus tremuloides Michx.) in ecophysiological responses to [CO 2 ] elevation, 1-year-old seedlings of four provenances (three families per provenance) from northwestern Ontario were exposed to three [CO 2 ] levels in the greenhouse: ambient (360 ppm), 1.5 × ambient (540 ppm), and 2 × ambient (720 ppm). Biomass and foliage gas exchange were examined after 60 d of treatment. [CO 2 ] elevation significantly increased the rate of net CO 2 assimilation and photosynthetic water use efficiency. The stimulation was generally greater in the 540 ppm [CO 2 ] than in 720 ppm [CO 2 ]. The 720 ppm [CO 2 ] resulted in a 10% photosynthetic down-regulation, but no down-regulation was detected in the 540 ppm CO 2 treatment. The 540 ppm [CO 2 ] (but not the 720 ppm) treatment significantly decreased stomatal conductance and transpiration rate in the provenances from the Great Lakes – St. Lawrence Region but not in those from the Boreal Region. The intercellular to ambient CO 2 concentration ratio (Ci/Ca) was significantly higher under 720 ppm [CO 2 ] than under the other two [CO 2 ]. The CO 2 elevations generally increased the total and root biomass, and the stimulation was greater in the 540 ppm [CO 2 ] than in the 720 ppm [CO 2 ] treatment. The two provenances from the Great Lakes – St. Lawrence region generally had greater biomasses than those from the boreal region, while there were no significant differences between them in the physiological variables. However, we did not find any significant differences between provenances in the responses of biomass to [CO 2 ] treatments.