Root dynamics in an artificially constructed regenerating longleaf pine ecosystem are affected by atmospheric CO 2 enrichment

Abstract

Differential responses to elevated atmospheric CO 2 concentration exhibited by different plant functional types may alter competition for above- and belowground resources in a higher CO 2 world. Because C allocation to roots is often favored over C allocation to shoots in plants grown with CO 2 enrichment, belowground function of forest ecosystems may change significantly. We established an outdoor facility to examine the effects of elevated CO 2 on root dynamics in artificially constructed communities of five early successional forest species: (1) a C 3 evergreen conifer (longleaf pine, Pinus palustris Mill.); (2) a C 4 monocotyledonous bunch grass (wiregrass, Aristida stricta Michx.); (3) a C 3 broadleaf tree (sand post oak, Quercus margaretta); (4) a C 3 perennial herbaceous legume (rattlebox, Crotalaria rotundifolia Walt. ex Gemel); and (5) an herbaceous C 3 dicotyledonous perennial (butterfly 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, and symbiotic relationships. A combination of minirhizotrons and soil coring was used to examine temporal and spatial rooting dynamics from October 1998 to October 1999. CO 2-enriched plots exhibited 35% higher standing root crop length, 37% greater root length production per day, and 47% greater root length mortality per day. These variables, however, were enhanced by CO 2 enrichment only at the 10–30 cm depth. Relative root turnover (flux/standing crop) was unchanged by elevated CO 2. Sixteen months after planting, root biomass of pine was 62% higher in elevated compared to ambient CO 2 plots. Conversely, the combined biomass of rattlebox, wiregrass, and butterfly weed was 28% greater in ambient compared to high CO 2 plots. There was no difference in root biomass of oaks after 16 months of exposure to elevated CO 2. Using root and shoot biomass as a metric, longleaf pine realized the greatest and most consistent benefit from exposure to elevated CO 2. 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 enhanced by rising atmospheric CO 2 concentrations.