The P content of biomass is based on measurements of P concentrations in plant tissues and estimates of the biomass of bole, bark, branches, leaves, and roots of the dominant tree species. The uncertainty in the regression equations describing the biomass components contributes about 16% uncertainty to the estimate of forest biomass; the uncertainty in P concentrations contributes 14% uncertainty. The total P in living biomass on the reference watershed is 274 kg/ha with a coefficient of variation of 21%. Precipitation inputs of P are small (0.04 kg/ha/yr), and the uncertainty in the estimate is a function of the spatial variation in the measurements by multiple precipitation collectors. Streamwater export of P is also small (0.02 kg/ha/yr); uncertainty in streamwater estimates depends on the temporal variation in concentrations and discharge and the timing of sampling collection. Soil pools of P are large and uncertain (1600 kg/ha), because of the heterogeneity of rock volume and soil depth. Internal fluxes of P are dominated by litterfall (4 kg/ha/yr) and root turnover (3 kg/ha/yr), balanced by mineralization and uptake. Analyzing the sources of uncertainty in these ecosystem budget calculations makes it possible to determine where to better allocate measurement and monitoring efforts.