What was done
Idealized ecosystems representative of regenerating longleaf pine communities of the southeastern USA were constructed within large soil bins and placed within open-top chambers receiving atmospheric CO2 concentrations of 365 and 720 ppm for about 18 months to study the effects of elevated CO2 on root dynamics and growth in this important forest community.  The five species included in the regenerating forest stands were longleaf pine (Pinus palustris Mill.), sand post oak (Quercus margaretta), a C4 bunch grass called wiregrass (Aristida stricta Michx.), a C4 perennial legume called rattlebox (Crotalaria rotundifolia Walt. Ex Genmel), and an herbaceous C3 perennial called butterfly weed (Asclepias tuberosa L.).

What was learned
Elevated CO2 increased total aboveground biomass in longleaf pine and sand post oak by approximately 20 and 50%, respectively.  However, atmospheric CO2 enrichment had no effect on the aboveground biomass produced by the three non-woody herbaceous species.  Hence, at the ecosystem level, elevated CO2 increased total aboveground biomass by an average of 35%.

In the belowground environment, elevated CO2 increased root biomass of longleaf pine by 62%, which was a much larger response than that experienced by its aboveground components.  In contrast, however, sand post oak, which was very responsive to elevated CO2 aboveground, exhibited no significant CO2-induced changes belowground in terms of root biomass; while the three herbaceous species displayed an average CO2-induced reduction in root biomass of 28%.

At the whole-community level, CO2-enriched plots displayed 37% greater root length production per day and 47% greater root length mortality per day at soil depths between 10 and 30 cm than was observed in control plots exposed to ambient CO2 concentrations.  Between the soil surface and a depth of 10 cm, however, elevated CO2 had no significant effects on these dynamic root parameters.

What it means
As the air's CO2 content continues to rise, the ability of longleaf pine trees to compete for soil moisture and nutrients in regenerating stands of longleaf pine savannahs will likely be enhanced, due to large preferential CO2-induced increases in the trees' root systems.  This phenomenon will likely give the longleaf pine a competitive edge over its primary competitors -- sand post oak and wiregrass.  In the words of the authors, "such competitive shifts suggest that longleaf pine savannahs may flourish in a future CO2-enriched world."  And if that happens, the trees will likely become a major repository of sequestered carbon.  Sand post oak should also do well as the air's CO2 content increases, significantly enhancing the carbon sequestering power of the total community, which should experience a shift from savannah towards forest.