Root dynamics in an artificially constructed regenerating longleaf pine ecosystem are affected by atmospheric CO2 enrichment

Differential responses to elevated atmospheric CO2 concentration exhibited by different plant functional types may alter competition for above- and be lowground resources in a higher CO2 world. Because C allocation to roots is often favored over C allocation to shoots in plants grown with CO2 enrichm ent, belowground function of forest ecosystems may change significantly. We established an outdoor facility to examine the effects of elevated CO2 on root dynamics in artificially constructed communities of five early success ional forest species: (1) a C, evergreen conifer (longleaf pine, Pinus palu stris Mill.); (2) a C-4 monocotyledonous bunch grass (wiregrass, Aristida s tricta Michx.); (3) a C-3 broadleaf tree (sand post oak, Quercus margaretta ); (4) a C-3 perennial herbaceous legume (rattlebox, Crotalaria rotundifoli a Walt. ex Gemel); and (5) an herbaceous C-3 dicotyledonous perennial (butt erfly weed, Asclepias tuberosa L.). These species are common associates in early successional longleaf pine savannahs throughout the southeastern USA and represent species that differ in life-form, growth habit, physiology, a nd symbiotic relationships. A combination of minirhizotrons and soil coring was used to examine temporal and spatial rooting dynamics from October 199 8 to October 1999. CO2-enriched plots exhibited 35% higher standing root cr op length, 37% greater root length production per day, and 47% greater root length mortality per day. These variables, however, were enhanced by CO2 e nrichment only at the 10-30 cm depth. Relative root turnover (flux/standing crop) was unchanged by elevated CO2. Sixteen months after planting, root b iomass of pine was 62% higher in elevated compared to ambient CO2 plots. Co nversely, the combined biomass of rattlebox, wiregrass, and butterfly weed was 28% greater in ambient compared to high CO2 plots. There was no differe nce in root biomass of oaks after 16 months of exposure to elevated CO2.(.) Using root and shoot biomass as a metric, longleaf pine realized the great est and most consistent benefit from exposure to elevated CO2. This finding suggests that the ability of longleaf pine to compete with sand post oak, a common deciduous tree competitor, and wiregrass, the dominant understory herbaceous species, in regenerating ecosystems may be significantly enhance d by rising atmospheric CO2 concentrations. (C) Published by Elsevier Scien ce B.V.