In this chapter, we compare the compositions and magnitudes of releases of radionuclides during the Chernobyl and Fukushima accidents and summarize the results of the long-term observations of the radionuclide dynamics in the ecosystem compartments in Chernobyl forests. Due to much larger magnitude of atmospheric release, the area contaminated as a result of the Chernobyl accident is larger; moreover, the near zone of the Chernobyl accident is contaminated with ⁹⁰Sr and other fuel component radionuclides that were not released in any significant amounts during the Fukushima accident.

Similarly to the forest ecosystems in Fukushima, the dominant process at the early stage after the deposition in Chernobyl forests was removal of the intercepted radionuclides from the aboveground forest biomass (foliage, bark) with litterfall and precipitations. In the following period, under certain conditions, the radionuclide concentrations and inventories in the aboveground biomass compartments started to increase till reaching the quasi-equilibrium levels in approx. 10 years after the accident. That was caused by an increase of the radionuclide bioavailable forms in the root-inhabited soil layer. At the late stage after the deposition, ¹³⁷Cs and ⁹⁰Sr in the Chernobyl forest ecosystems are involved into biological cycle: they are absorbed from soil, translocated and accumulated in the tree organs, and removed from the aboveground biomass by the same mechanisms that recycle their chemical analogs and essential plant nutrients K and Ca, respectively.