Root dynamics are important for plant, ecosystem and global carbon cycling. Changes in root dynamics caused by rising atmospheric CO2 not only have the potential to moderate further CO2 increases, but will likely affect forest function. We used FACE (Free-Air CO2 Enrichment) to expose three 30-m diameter plots in a 13-year-old loblolly pine (Pinus taeda) forest to elevated (ambient + 200 µL L−1) atmospheric CO2. Three identical fully instrumented plots were implemented as controls (ambient air only). We quantified root dynamics from October 1998 to October 1999 using minirhizotrons. In spite of 16% greater root lengths and 24% more roots per minirhizotron tube, the effects of elevated atmospheric CO2 on root lengths and numbers were not statistically significant. Similarly, production and mortality were also unaffected by the CO2 treatment, even though annual root production and mortality were 26% and 46% greater in elevated compared to ambient CO2 plots. Average diameters of live roots present at the shallowest soil depth were, however, significantly enhanced in CO2-enriched plots. Mortality decreased with increasing soil depth and the slopes of linear regression lines (mortality vs. depth) differed between elevated and ambient CO2 treatments, reflecting the significant CO2 by depth interaction. Relative root turnover (root flux/live root pool) was unchanged by exposure to elevated atmospheric CO2. Results from this study suggest modest, if any, increases in ecosystem-level root productivity in CO2-enriched environments.
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