What was done
The authors reviewed the scientific literature and synthesized current knowledge concerning the responses of various mycorrhizal fungi to elevated CO2.

What was learned
From their literature survey, Staddon and Fitter found that atmospheric CO2 enrichment stimulated root colonization by ectomycorrhizal fungi in five of eight tree species, while it had no stimulatory effect on root colonization in the other three trees.  In most cases, increased root colonization by ectomycorrhizal fungi works synergistically with elevated CO2 to increase plant growth.  In fact, the authors stated that this synergistic relationship is "not surprising," as elevated CO2 often stimulates the fungal-mediated uptake of certain soil nutrients by increasing the flux of carbohydrates into plant roots and the soil.  Moreover, the fungal species may actually serve as a carbon sink for excess carbohydrates that might otherwise accumulate in plant tissues and trigger the down regulation of photosynthesis.

In the case of arbuscular mycorrhizal fungi, atmospheric CO2 enrichment had less of a stimulatory effect on their growth.  Of ten inoculated plant species, for example, four showed a stimulation in root colonization, one exhibited a decrease, and the remainder displayed no change in response to an increase in atmospheric CO2.  However, because there are many species of fungi in each of these two mycorrhizal categories (arbuscular and ectomycorrhizal), and because most plants are typically colonized by multiple fungal species, it is likely that at least one of them will exhibit stimulated root colonization that will synergistically function with elevated CO2 to positively influence plant growth and development.

What it means
Although based on a relatively small number of observations, the results of this review suggest that the rising CO2 content of the air will likely increase, or at the very least maintain, the current growth and colonization of most mycorrhizal fungi associated with plant roots.  In addition, because nearly all of earth's plants form symbiotic relationships with multiple species of mycorrhizal fungi, it is likely that, for each plant, at least one of their associated fungi will respond positively to the increasing CO2 content of the air and act synergistically with it to increase plant growth.