Elevated Atmospheric CO2 Influences on Longleaf Pine Grasslands

Research Team: Dr. Hugo Rogers (Agricultural Research Service), Dr. Robert J. Mitchell (Scientist), Dr. Brett Runion (Agricultural Research Service), Dr. Steve Prior (Agricultural Research Service), and Seth Pritchard (Postdoctoral Associate - Agricultural Research Service)

Current rise in atmospheric carbon dioxide (CO2) is projected to double that of pre-industrial levels within the next century. Carbon dioxide is the primary connection between the biosphere and the atmosphere. Through plant stomata, CO2 moves along a gradient in concentration (higher in the atmosphere, and lower inside the leaf) to chloroplasts within the leaf where it is eventually fixed into sugar through the process of photosynthesis. While increasing CO2 concentrations may lead to climate change including global warming, the increase in atmospheric CO2 has generally been shown to increase photosynthesis, water use efficiency (a measure of how much carbon is fixed per unit water loss), and growth (particularly that of roots). However, much of the work studying CO2 has been done at the scale of leaves or individual plants. Individual species differ in their morphological and physiological responses to increased CO2, thus potentially altering competitive relationships and making community responses more difficult to predict. Through funding by the Environmental Protection Agency and Department of Energy (EPSCOR and NIGEC programs, respectively) to the USDA Soil Dynamics lab at Auburn University and the Jones Ecological Research Center, we began a series of experiments investigating how elevated CO2 influenced competition among plants in a common garden experiment that was representative of the longleaf pine ecosystem.

We selected an assemblage of species common to longleaf pine that represent major functional guilds: longleaf pine (an evergreen conifer), wiregrass (a C4, perennial bunchgrass), sand post oak (a broadleaf tree), rattle box (a nitrogen-fixing legume), and butterfly weed (a C3-perennial herbaceous forb). Planting densities were similar to that found in longleaf pine savannas. We hypothesized that photosynthetic (and water use efficiency) responses of individual species would predict how community response was regulated by altered competition. However through time, it was found that evergreen species (longleaf pine) responded in growth much more than would have been predicted by their photosynthetic increases in elevated CO2 environments. Wiregrass, also an important component to the pyrogenic feature of longleaf pine ecosystems also increased biomass more than would have been predicted by photosynthetic response of individuals. By increasing the components of the system that are needed for frequent fire, disturbance can continue to function in maintaining plant species diversity in higher CO2 environments.

Additional Information:

Pritchard SG, Davis MA, Mitchell RJ, Prior SA, Boykin DL, Rogers HH and Runion GB. Root dynamics in an artificially constructed regenerating longleaf pine ecosystem are affected by atmospheric CO2 enrichment. J Exp Bot 2001; 46:55-69.

Davis MA, Prichard SG, Prior SA, Mitchell RJ, Runion GB, Rogers HH. Competitive responses of a model early successional longleaf pine ecosystem. J Ecol.

Runion GB, Mitchell RJ, Green TH, Prior SA, Rogers HH, Gjerstad DH. Longleaf pine photosynthetic response to soil resource availability and elevated atmospheric carbon dioxide. J Env Qual 1999;28:880-7.

Runion GB, Entry JA, Prior SA, Mitchell RJ, Rogers HH. Tissue chemistry and carbon allocation in seedlings of Pinus palustris subjected to elevated atmospheric CO2 and water stress. Tree Phys 1999;19:329-35.

Entry JA, Runion GB, Prior SA, Mitchell RJ, Rogers HH. Influence of CO2 enrichment and nitrogen fertilization on tissue chemistry and carbon allocation in longleaf pine seedlings. Pl Soil 1998;200:3-11.

Prior SA, Runion GB, Mitchell RJ, Rogers HH, Amthor JS. Effects of atmospheric CO2 on longleaf pine: productivity and allocation as influenced by nitrogen and water. Tree Phys 1997;17:397-409.

Prior SA, Pritchard SG, Runion GB, Rogers HH, Mitchell RJ. Influence of atmospheric CO2 enrichment, soil N and water on needle surface wax formation in Pinus palustris (Pinaceae). Am J Bot 1997;84:1070-77.

Rogers HH, Prior SA, Runion GB, Mitchell RJ. The influence of CO2 on carbon partitioning in plants. Pl Soil 1997;187:229-48.

Runion GB, Mitchell RJ, Rogers HH, Prior SA, Counts TK. Effects of resource limitations and elevated atmospheric CO2 on ectomycorrhizae of longleaf pine. New Phyt 1997;137:681-9.

Funded by: Environmental Protection Agency Experimental Program to Stimulate Competitive Research (EPSCoR) and Department of Energy National Institute for Global Environmental Change (NIGEC)