TRIPS 2.0: Toward more comprehensive modeling of radiocaesium cycling in forest

Because internal transfers can play a key role in radiocaesium persistence in trees, a reliable representation of radiocaesium recycling between tree organs in forest models is important for long-term simulations after radioactive fallout in Chernobyl and Fukushima. We developed an upgraded 2.0 version of the initial TRIPS ("Transfer of Radionuclides In Perennial vegetation System") model involving explicit differentiation between tree organs (i.e., foliage, branches, stemwood and bark). The quality of TRIPS 2.0 was evaluated by testing model outputs against independent datasets for pine stands in Belarus and Ukraine. Scenarios involving "hot particle" deposits in forest remained challenging, but in all other scenarios generally positive verification results for soil and tree compartments indicated that the TRIPS 2.0 model adequately combines the major relevant processes. Interestingly, the response of stemwood contamination to changes in radiocaesium availability in soil, as determined by soil conditions, was shown to be more sensitive than for other tree compartments. We recommend the conceptual tree discretization of TRIPS 2.0 for generic forest modeling for two reasons: 1) regardless of different soil conditions, there was concurrent good agreement between simulations and data for individual tree compartments (foliage, branches, stemwood and bark), and 2) the measurements necessary to estimate internal tree transfers are easily accessible to usual field monitoring in forest biogeochemistry (for details, see Goor, F. & Thiry, Y., 2004. Science of the total environment, 325(1-3), 163-180).