Meta-analysis of radiocesium contamination data in Japanese forest trees over the period 2011-2013

The fate and dispersion of radiocesium in forests affected by the Fukushima atmospheric fallouts have been efficiently characterized by Japanese scientists thanks to monitoring surveys of radioactive contents in contaminated soil, water, and vegetation samples at numerous sites. In this paper, we carry out a meta-analysis of the field surveys conducted over the period 2011-2013 in evergreen coniferous and deciduous broadleaf forests of Fukushima or neighboring prefectures. The review focuses on contamination data acquired in tree vegetation - about 1500 spatio-temporal measurements of concentrations, inventories and depuration fluxes - with a particular interest for organs that were directly exposed to the atmospheric fallouts and subjected to depuration mechanisms (foliage, branches and outer bark). To reduce the spatial variability between the sites, radioactive data were normalized by the total deposit estimated at each site. Our analysis highlights the overall consistency of field observations despite the variety of experimental protocols, disparate sampling periods, differences in the forest stand characteristics and variability of the atmospheric deposition conditions. Assuming that the sites conformed to the same dynamics (within the range of residual variability), we then derive, discuss, and compare the mean representative evolutions of radiocesium contamination in the two categories of forest. Thanks to a simple mass balance approach, we finally demonstrate that: (i) about 90% of the radiocesium deposit was intercepted by evergreen coniferous vegetation, (ii) 80% of the deposit was gradually transferred to the forest floor in 3years, according to a well characterized depuration kinetics, and (iii) about 4% was readily absorbed by the foliage and translocated to internal organs (inner bark, stem wood and roots) at a rate of about 10^-4d^-1.