Simple compartment models can simulate the rapid net transfers of cesium from tree boles to foliage and roots, and to the forest environment. Only 2–4 weeks after inoculating 467 mCi into the dominant tulip poplar trees in a 500-m2 plot, the upward flux of 137Cs to foliage via transpiration was exceeded by losses due to rain leaching and downward translocation via phloem to roots. Fairly high root-soil turnover of cesium (0.003 days 1) must explain the build-up in soil radioactivity, which is greater than rainout or autumn leaf fall can account for. A model with a matrix of constant fractional coefficients of transfer simulated the observable changes during early months. However, periodically varying coefficients are needed to allow for seasonal rhythms of the forest nuclide cycling and dispersal over a period of years.