Concentration levels of radiocesium (Cs-137) in leaves differ among branches. They also change with time. We investigated the spatio-temporal changes of Cs-137 concentration in leaves of Japanese fir (Abies firma) in Fukushima, Japan to understand factors underlying the spatio-temporal changes. We picked up 12 trees (ca 2 to 3 m in height) and monitored the concentration of radiocesium in leaves of upper (ca 2 m from the ground) and lower (ca 1 m from the ground) branches from November 2014 to November 2015.

The Cs-137 concentration in leaves of upper branches was higher than that of lower branches. Light levels of upper branches were higher than those of lower branches and thus the physiological activities of upper leaves must have been higher than lower leaves. This physiological difference between two branches may cause the different concentration levels of leaves. The concentration of radiocesium in leaves peaked in summer and dropped in early spring. This pattern was clear in current-year leaves and subtle in one-year-old leaves. Our data suggest that (1) physiological activities, (2) seasonality, and (3) leaf ages are related to the spatio-temporal changes of Cs-137 concentration in leaves of Japanese fir trees.