What was done
The authors grew cuttings of two male and two female trembling aspen (Populus tremuloides Michx) trees for about five months in open-top chambers having atmospheric CO2 concentrations of 380 and 765 ppm.  In addition, cuttings were rooted in soils containing low and high nitrogen contents to study the effects of elevated CO2 and soil nitrogen availability on photosynthesis and growth in both male and female individuals of this dioecious forest species.

What was learned
Male cuttings consistently displayed higher rates of photosynthesis than female cuttings, regardless of soil nitrogen availability, and were thus more photosynthetically responsive to atmospheric CO2 enrichment.  Indeed, males displayed photosynthetic rates that were 13% greater than those exhibited by females at ambient CO2 concentrations and rates that were 25% greater than those of females at elevated CO2 concentrations.  In contrast, gender had little effect on rates of dark respiration, which were increased by elevated CO2 by averages of 6 and 32% under low and high soil nitrogen conditions, respectively.  In spite of these increases in dark respiration, large net gains in photosynthetic carbon uptake were still achieved by CO2-enriched cuttings, regardless of gender.  In low nitrogen soils, for example, elevated CO2 increased total cutting biomass by 66 and 22% for male and female individuals, respectively.  In high nitrogen soils, however, atmospheric CO2 enrichment increased the biomass of female individuals more than male individuals (82 vs. 58%, respectively).

What it means
As the CO2 content of the air increases, male and female aspen trees will likely display differential increases in photosynthesis and growth that are somewhat dependent on soil nitrogen content.  Nonetheless, it is expected that regardless of gender, all aspen trees will display enhanced photosynthetic rates and increases in biomass production as the air's CO2 concentration increases.  Thus, it is likely that carbon sequestration in the most widespread tree species in North America will substantially increase with future increases in the air's CO2 content.