Below-ground architectural and mycorrhizal responses to elevated CO2 in Betula alleghaniensis populations.

1. Replicate populations of crowded, regenerating stands of Betula alleghaniensis were grown in ambient and elevated (700p.p.m.) atmospheric CO₂ concentrations in monoliths of forest soil. Early in the second year the seedlings were harvested and detailed measurements of individual plant root architectural parameters and ectomycorrhizal colonization were made. 2. Comparing the average responses of individual plants within the populations, elevated CO₂ had no significant effects on architectural parameters that improve a plant's ability to forage for and acquire soil resources. In contrast, the intensity and magnitude of mycorrhizal colonization, and whole plant C/N ratios were significantly enhanced with elevated CO₂. 3. The allometric scaling relationship between total plant biomass and root biomass was not affected by CO₂, suggesting that relative allocation between roots and shoots was not affected. However, the allometric scaling relationships between root architectural parameters and plant biomass, and between fine root biomass and woody root biomass were significantly altered by elevated CO₂. For all of these relationships, elevated CO₂ reduced the 'size bias' of architectural components in relation to plant size within the populations; in elevated CO₂ root architectural size (e.g. root length) per unit biomass was more similar between the smallest and largest individuals within the population than was the case for ambient CO₂. 4. Overall, the results of this study suggest that the average individual seedling biomass and architectural growth responses within populations of plants exposed to elevated atmospheric CO₂ levels may be unresponsive, but that mycorrhizal responses and interactions among plants within populations may be altered significantly. These findings have important implications for how we make predictions about plant growth responses to elevated CO₂ in natural ecosystems. Significant increases in mycorrhizal infection rates and architecture-biomass allometries suggest that below-ground competitive interactions within plant populations may be reduced in elevated CO₂. Alterations in competitive interactions may lead to shifts in productivity and plant population structure. [ABSTRACT FROM AUTHOR]