In 1996, a long-term CO₂ enrichment study was initiated in a scrub-oak ecosystem at Kennedy Space Center, Florida. The experimental design involved 16 open-top chambers that continuously received either ambient or elevated (ambient + 350 ppm) CO₂ concentrations. The objective of the work presented here was to determine the effect of elevated CO₂ on fine (<5 mm diameter) and coarse (>5 mm diameter) root biomass in this system. Fine root abundance within the experimental chambers was measured throughout the study using minirhizotrons. Biomass of fine roots was estimated from mass/root length values applied to the minirhizotron data. To measure coarse root biomass, ground-penetrating radar (GPR) was used at the end of the study in May 2007, after CO₂ enrichment was stopped and all aboveground vegetation within the experimental plots was removed.

Results/Conclusions

Measurements of root length density (RLD) of fine roots showed stimulation under elevated CO₂ during the early years of the experiment but no effect by the end. Fine root biomass was significantly greater under elevated CO₂ for 8 out of 23 sampling dates between 1996 and 2006. Coarse root biomass estimates were 6,040 ± 507 g m^-2 for the elevated CO₂ plots and 5,105 ± 418 g m^-2 for the ambient CO₂ plots. Overall, 11 years of exposure to elevated atmospheric CO₂ stimulated production of root biomass. This ecosystem experiences periodic fires, and belowground carbon reserves (i.e. coarse roots, lignotubers, and burls) are important for plant re-growth following disturbance. Total root biomass was found to be 4-6 times greater than aboveground biomass in this system, suggesting that as CO₂ increases, additional carbon sequestered from the atmosphere may be stored in belowground plant structures.