BELOW-GROUND ARCHITECTURAL AND MYCORRHIZAL RESPONSES TO ELEVATED CO2 IN BETULA-ALLEGHANIENSIS POPULATIONS

1. Replicate populations of crowded, regenerating stands of Betula all eghaniensis were grown in ambient and elevated (700 p.p.m.) atmospheri c CO2 concentrations in monoliths of forest soil. Early in the second year the seedlings were harvested and detailed measurements of individ ual plant root architectural parameters and ectomycorrhizal colonizati on were made. 2. Comparing the average responses of individual plants within the populations, elevated CO2 had no significant effects on arc hitectural parameters that improve a plant's ability to forage for and acquire soil resources. In contrast, the intensity and magnitude of m ycorrhizal colonization, and whole plant C/N ratios were significantly enhanced with elevated CO2. 3. The allometric scaling relationship be tween total plant biomass and root biomass was not affected by CO2, su ggesting that relative allocation between roots and shoots was not aff ected. However, the allometric scaling relationships between root arch itectural 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 a rchitectural components in relation to plant size within the populatio ns; in elevated CO2 root architectural size (e.g. root length) per uni t biomass was more similar between the smallest and largest individual s within the population than was the case for ambient CO2. 4. Overall, the results of this study suggest that the average individual seedlin g biomass and architectural growth responses within populations of pla nts exposed to elevated atmospheric CO2 levels may be unresponsive, bu t that mycorrhizal responses and interactions among plants within popu lations may be altered significantly. These findings have important im plications for how we make predictions about plant growth responses to elevated CO2 in natural ecosystems. Significant increases in mycorrhi zal 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 l ead to shifts in productivity and plant population structure.