What was done
Six genotypically different aspen (Populus tremuloides) cuttings were grown in open-top chambers for 2.5 growing seasons in Michigan, USA, at atmospheric CO2 concentrations of 350 and 700 ppm.  In addition, the trees were grown on soils containing adequate and inadequate supplies of nitrogen.  In this particular paper, the authors report results pertaining to the biomass production and tissue nitrogen dynamics of young aspen trees in response to elevated CO2 and soil nitrogen supply.

What was learned
Total tree biomass was influenced more by soil nitrogen than it was by atmospheric CO2 concentration.  Increasing soil nitrogen, for example, resulted in a 223% enhancement in biomass, whereas increasing the CO2 concentration resulted in a smaller 32% enhancement.  With respect to interactions among these variables, the CO2-induced increase in biomass was twice as great in high-nitrogen than in low-nitrogen soils (38 and 16%, respectively), indicating that the CO2-induced growth response was limited by low soil nitrogen.

Although atmospheric CO2 enrichment decreased tissue nitrogen concentrations by an average of 18%, the total amount of nitrogen in the trees actually increased by 13% in the trees grown in the high soil nitrogen regime.  However, there was no such increase in total nitrogen content in trees grown in the low soil nitrogen regime.  Thus, the greater CO2-induced increase in biomass for trees grown in high soil nitrogen resulted in greater fine root production that facilitated the enhanced total uptake of nitrogen from the soil in that treatment.

With respect to genotype, there were significant genotype x soil nitrogen interactions, wherein late-senescing clones had greater tissue nitrogen contents and total biomass than their early-senescing counterparts.  However, there were no significant genotype x CO2 interactions, indicating that atmospheric CO2 enrichment affected both types of clones (and all six genotypes) in a similar manner.

What it means
As the CO2 content of the air rises, it is likely that aspen, which is the most widely distributed tree species in North America, will exhibit significant increases in growth, with the largest increases occurring on high-nitrogen soils.  However, even on low-nitrogen soils, aspen clones should significantly increase their biomass, as indicated by the 16% increase observed in this study.  Thus, regardless of soil nitrogen availability, aspen trees should grow faster and larger as the atmospheric CO2 concentration continues to increase.  And these phenomena, in the words of the authors, "present the possibility that Populus tremuloides dominated forests will reach both commercial and ecological maturity over a shorter period of time, lessening the commercial rotation age and accelerating the rates of ecosystem development."  Moreover, because the rising CO2 content of the air will likely affect aspen genotypes similarly, it will help maintain the genetic diversity that exists within natural aspen populations.