The mechanisms that control fine root lifespan are poorly understood. Simulation modeling of nutrient uptake in a soil environment provides a good tool for examining how soil nutrient availability interacts with root function and root efficiency. Theoretically, a root should live until it reaches maximum efficiency. Here, we define root efficiency as the lifetime phosphorus (P) gain divided by the lifetime carbon (C) cost. Fine root morphology and physiology differed significantly between fast- and slow-growing species of Aceraceae (maple) and Quercus (oak). Fine roots of fast-growing species had higher P uptake and respiration rates. We also found that the roots of fast-growing species had higher construction costs and smaller diameter. Using the root efficiency model of Eissenstat and Yanai (1997), we compared the relationship between P uptake and root C cost in a simulated field situation where diffusion of P to the root surface and the formation of P depletion zone limited P uptake. At all P levels, slow-growing species were less efficient than fast-growing species at nutrient capture (44-61% less). Maximum root efficiency in the slow-growing species occurred 20-47 days later than in the fast-growing species. The influence of mycorrhizae, root age, root herbivory and root pathogens were not included in these simulations and their potential influence will be discussed.