A time dependent behavior of radiocesium from the Fukushima-fallout in litterfalls of Japanese flowering cherry trees

Radiocesium (134Cs + 137Cs) concentrations, primarily derived from the Fukushima accident in March 2011, were measured in litterfalls and green leaves of Japanese flowering cherry trees (Prunus x yedoensis cv. Somei-Yoshino). The sampling was performed mainly during the defoliation season in 2011 and 2012 using traps to collect litterfalls before contact with the ground. The average radiocesium concentration in litterfalls in 2012 fell to one-third of that in 2011 (0.43 and 1.2 kBq kg-DW−1, respectively). Interestingly, the concentrations in litterfalls collected in late autumn in both 2011 and 2012 (0.68 and 0.19 kBq kg-DW−1, respectively) were significantly lower than those in litterfalls collected in the early autumn (1.7 and 1.1 kBq kg-DW−1, respectively). In addition, the reductions in radiocesium concentrations in the litterfall were nearly synchronous with those in potassium concentrations (p ≤ 0.05). On the contrary, radiocesium concentrations in green leaves were also correlated with potassium concentrations; however, the slopes of the regression lines between the radiocesium and potassium concentrations were very similar in the 2011 litterfalls and the 2012 litterfalls, while the slopes were significantly different between these litterfalls and the green leaves. Consequently, the correlation between potassium and radiocesium was clear but independently observable in each of the litterfalls and the green leaves. It is possible that the reduction in radiocesium concentration occurred as a part of physiological demand, a translocation of potassium from the leaves to the body/twigs.