Using a new approach involving one-time measurements of radiocarbon ( 14 C) in fine (14 C-derived mean ages represent the time C was stored in the plant before being allocated for root growth, plus the average lifespan (for live roots), plus the average time for the root to decompose (for dead roots and mixtures). Comparison of the 14 C content of roots known to have grown within 1 year with the 14 C of atmospheric CO₂ for the same period shows that root tissues are derived from recently fixed carbon, and the storage time prior to allocation is 14 C-derived estimates of long mean ages can be reconciled with faster estimates only if fine-root populations have varying rates of root mortality and decomposition. Our results indicate that a standard definition of fine roots, as those with diameters of