Issues and perspectives for investigating root responses to elevated atmospheric carbon dioxide

A thorough assessment of how plants and ecosystems will respond to increasing concentrations of atmospheric CO₂ requires that the responses of root systems and associated belowground processes be understood. Static measures of root-to-shoot ratio have not been satisfactory for describing the integrated responses of plants to CO₂-enriched atmospheres, but research with a process orientation has suggested that elevated CO₂ can stimulate root growth or root activity and provide a positive feedback on plant growth. There are, however, critical questions concerning the relevance of root data from short-term studies with potted plants when scaling to questions about plants in the field. Data on root responses to CO₂ enrichment in the field are fragmentary, but they allow us to more clearly define research questions for further investigation. Three perspectives for analyzing the significance of root responses as a component of the overall response of the terrestrial biosphere to increasing atmospheric CO₂ are suggested: (1) roots as a platform for nutrient acquisition and a mediator of whole-plant response to CO₂; (2) carbon storage in roots as a component of whole-plant carbon storage; and (3) effects of CO₂ enrichment on root turnover and the implications for carbon storage as soil organic matter. The relative importance of these different perspectives will vary depending on the ecosystem of interest and the larger-scale issues being considered.