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 fine root responses of aspen to elevated CO2 and soil nitrogen supply.

What was learned
Trees subjected to atmospheric CO2 enrichment possessed roots that were thicker and longer than those of trees growing in ambient air, regardless of soil nitrogen supply.  However, the fine root biomass of the CO2-enriched trees was enhanced by 65% on the nitrogen-rich soils, but it was only increased by 17% on the nitrogen-poor soils.  In addition, fine root production and turnover rates were significantly enhanced by atmospheric CO2 enrichment, but only at high soil nitrogen.  Thus, elevated CO2 significantly and positively impacted root and fine root production in aspen trees, particularly when soil nitrogen levels were high; and this phenomenon increased the input of carbon to the soils beneath the trees.

What it means
As the atmospheric CO2 concentration rises, it is likely that aspen, which is the most widely distributed tree species in North America, will exhibit significant increases in fine root growth rates, turnover rates, and biomass production, especially when soil nitrogen supplies are non-limiting to growth.  When soil nitrogen is limiting, however, it is likely that thicker and longer roots will still result, thereby increasing the absorptive root area capable of searching out and obtaining soil nitrogen, which can positively feedback to further enhance the growth of roots and other organs.  Thus, it is likely that greater belowground growth will occur in aspen stands as the CO2 content of the air continues to rise, which will effectively increase carbon inputs to soils, which in the words of the authors, "can be substantial," especially "if soil nitrogen is non-limiting."