The meltdown and subsequent hydrogen explosion at the Fukushima nuclear power plant in Japan in March 2011 dispersed tremendous amounts of radionuclides into humans, forests, and other objects [1
]. The fallout was primarily in the form of radiocesium, which is the most damaging to forest trees; cesium-134 and cesium-137 have been estimated at 1.8 × 1016
and 1.5 × 1016
Bq, respectively [2
]. These radiocesiums were dispersed by the northwest wind-flow and deposited on trees in significant concentrations over approximately 200,000 hectares of rural land, more than 90% of which is forested. The dispersed radiocesium landed in aerosol form on the outer surface of the bark and migrated directly into entire tree bodies through translocation to the sapwood and heartwood [4
]. Although radiocesium is recycled among other forest components—such as living plant tissues, leaf litter, mulch, and soil—once it enters the heartwood, there is no more turnover, and it continues to accumulate [6
]. This has made it very difficult to remove radiocesium from living tree plants, though we attempted to use a foliar spray of potassium solution applied by hand and by radio-controlled helicopter in a small forest, as well as the direct injection of potassium solution into tree trunks over several years in order to reduce radiocesium through the use of potassium fertilization [7
], see Figure S1
. None of these approaches, however, succeeded in decreasing the radiocesium levels in Fukushima forest trees.
To date, no permissible level has been set for wood products in Japan, although it is now illegal to sell any timbers in Fukushima with more than 1000 cpm on its surface, as measured by a NaI(TI) scintillation counter [8
]. Unfortunately, such blanket regulatory rules do not accurately reflect the amount of radiocesium in wood: 100 cpm in the heartwood is equivalent to 1055 Bq/kg and 650 cpm in the bark to 33,400 Bq/kg, as shown in Figure S2
. However, 1000 Bq/kg in wood has been set as the legal limit in Ukraine and other countries affected by the Chernobyl disaster. Even today, more than 30 years after the Chernobyl disaster, a biomass energy company in Sweden continues to monitor radiocesium levels in the wood and bark from various Swedish forests because it is difficult for the systems in their plants to capture radiocesium fly ash from biomass sources with levels higher than 1000 Bq/kg [9
]. Since cesium-137 has a long half-life (30.17 yr−1
), the radiocesium will remain in the trees throughout their entire lives.
There is no method to treat contaminated wood. Here we employ the ponding method of wood impregnation to remove radiocesium from tree stems through water flow [10
]. This procedure has traditionally been used to increase the water-permeability of water-rich stem wood. Initially, freshly cut stems are placed in a pond, river, or sea in order to introduce bacteria that can degrade the pit membranes in tracheids or vessels over the course of several years. The stems are then positioned vertically to allow the water to drain out, a process through which the bacteria can also flow out. This is also an effective method for drying stem wood without warping or cracking it [13
]. It should be noted that the pit membrane as a target is composed of a primary wall, mainly xyloglucan and cellulose.