What was done
The authors grew cuttings of four quaking aspen genotypes for five months at CO2 concentrations of 380 or 720 ppm and low or high soil nitrogen in open-top chambers located in Michigan, USA, to determine the effects of elevated CO2 and soil nitrogen availability on photosynthesis and growth.  Two of the genotypes were "early-fall" varieties, which drop their leaves a few weeks before "late-fall" varieties, which comprised the other two genotypes in this study.

What was learned
Elevated CO2 significantly increased net photosynthesis regardless of soil nitrogen, until after budset in the fall, at which time photosynthesis dropped for all trees irrespective of treatment or genotype.  In spite of the significant increases in whole-tree photosynthesis, cuttings grown in elevated CO2 displayed no discernible increases in aboveground growth within the five-month study period.  However, elevated CO2 significantly increased fine root length belowground and increased root turnover rates at high soil nitrogen, but not low nitrogen, by increasing root production, as opposed to shortening life span.  This observation suggests that in the presence of high soil nitrogen, cuttings grown in elevated CO2 transferred greater amounts of carbon into the rhizosphere beneath them.

What it means
As the CO2 content of the air increases, different aspen genotypes will probably all exhibit increased rates of photosynthesis.  Thus, the increasing CO2 content of the air will not likely act as a selective factor among aspen genotypes, and existing genetic diversity should therefore be preserved.  The additional carbohydrates produced from this process can be utilized above- and belowground to increase overall biomass and survival.  If utilized belowground, as was the case in the present study, root growth and development can be increased, thus allowing aspen trees to gather greater amounts of water and nutrients from the soil to support the enhanced biomass production that commonly occurs in trees exposed to higher levels of atmospheric CO2.  In addition, increased carbon allocation belowground can stimulate biological activities within the rhizosphere surrounding plant roots.