minirhizotron and carbon isotopic methods

• Fine roots constitute a large and dynamic component of the carbon cycles of terrestrial ecosystems. The reported fivefold discrepancy in turnover estimates between median longevity (ML) from minirhizotrons and mean residence time (MRn using carbon isotopes may have global consequences. • Here, a root branch order-based model and a simulated factorial experiment were used to examine four sources of error. • Inherent dlfferences between ML, a number-based measure, and MRT, a massbased meas~re, and the inability of the MRT method to account for multiple replacementS of rapidly cycling roots were the two sources of error that contributed more to the ~isparity than did the improper choice of root age distribution models and sampling bias. Sensitivity analysiS showed that the rate at which root longeVity increases as order increases was the most important factor influencing the disparity between ML and MRT. • Assessing root populations for each branch order may substantially reduce the errors in longevity estimates of the fine root guild. Our results point to the need to acquire longevity estimates of different orders, particularly those of higher orders.