Radiocesium distribution in sugi (Cryptomeria japonica) in Eastern Japan: translocation from needles to pollen

Abstract

We assessed the radiocesium contamination of sugi (Cryptomeria japonica) forests in eastern Japan from November 2012 to February 2013, including 80 sites in Fukushima and 35 sites in other regions (Tohoku and Kanto-Koshinetsu), by measuring the 137Cs concentrations in needles of different ages, male flowers, and pollen. Over a wide geographic area, needles that were present at the time of the Fukushima Dai-ichi Nuclear Power Plant accident contained much higher 137Cs concentrations than needles that emerged after the accident. This result, together with visual analysis of 137Cs distribution using autoradiography, indicated that some of the 137Cs derived from direct fallout remained on the surface of the older needles. Since we also detected 137Cs in younger needles and male flowers, we concluded that 137Cs was translocated toward the tips of sugi needles. The 137Cs concentration in male flowers was higher than and positively correlated with that in the currently growing (2012) needles. Also, a positive relationship was observed between the 137Cs concentration of male flowers and pollen, and they were found to be nearly identical 137Cs concentration. These results indicate the occurrence of acropetal translocation of 137Cs from old needles to young needles, male flowers and pollen. However, the results as related to 137Cs concentration in the needles of three different ages differed from the results of similar studies conducted more than 4 y after the Chernobyl accident. This suggests that, 2 y after the Fukushima Dai-ichi NPP accident, the distribution of 137Cs in the sugi forests has not yet reached a steady state.