To evaluate the distribution of radiocesium (¹³⁷Cs) among crown positions in trees after the Fukushima Daiichi Nuclear Power Plant accident, we collected foliage and branch samples from different crown positions of four major tree species (Chamaecyparis obtusa, Cryptomeria japonica, Pinus densiflora, and Quercus serrata) from 2011 to 2019 in northeast Japan. We divided the samples into current-year and more than 1-year-old groups (called old foliage and old branches), which sometimes included directly contaminated parts. The ¹³⁷Cs activity concentration in dry foliage and branches was measured using a germanium semiconductor detector. There were complex differences in the relative ¹³⁷Cs activity concentration among species and organ types (i.e., foliage and branches) among crown positions. The relative ¹³⁷Cs activity concentration in current-year foliage was higher in the upper crowns of C. obtusa, but higher in lower crown positions in C. japonica. No differences among crown positions were observed in P. densiflora and Q. serrata. In current-year branches, the relative ¹³⁷Cs concentration in Q. serrata was similar among crown positions but higher in the upper crown in P. densiflora. The concentrations in old foliage and old branches in all species tended to be higher in the lower crown. The factors causing these interspecific and organ type differences among crown positions may be related to the organ turnover rate, dilution effect due to different growth rates, and potassium distribution within the crown. No year-to-year variation was observed in most foliage and branches in all species, except for current-year branches of Q. serrata, old foliage in C. japonica and P. densiflora, and old branches in P. densiflora. Our long-term data on the interspecific and inter-organ patterns of contamination, focusing on variation among crown positions and year-to-year trends, might help to improve the estimation of ¹³⁷Cs deposition and dynamics in polluted forest ecosystems.

为了评估福岛第一核电站事故后树木中树冠位置中放射性铯（¹³⁷ Cs）的分布，我们收集了四种主要树种（Chamaecyparis obtusa，Cryptomeria japonica，Pinus densiflora和2011年至2019年在日本东北部的Quercus serrata）。我们将样本分为当年的和超过1年的组（称为旧叶子和旧树枝），有时包括直接污染的部分。使用锗半导体检测器测量了干燥的枝叶中的¹³⁷ Cs活性浓度。相对而言存在复杂的差异冠位置之间的物种和器官类型（即叶和枝）之间的¹³⁷ Cs活性浓度。相对¹³⁷在当前年叶子铯活性浓度在上部冠更高Ç。扁柏，但在较低的冠位置更高Ç。粳稻。在观察冠位置之间没有差异P。赤松和Q。锯齿状。在当年分支中，Q中的相对¹³⁷ Cs浓度。冠位之间的锯齿状相似，但在P的上冠中较高。赤松。下种树冠中所有物种的老叶和老枝中的浓度往往较高。导致这些冠间位置之间的种间差异和器官类型差异的因素可能与器官周转率，由于不同生长速率引起的稀释效应以及冠内钾的分布有关。除Q的当年分支外，所有物种的大多数枝叶均未观察到年际变化。锯缘青蟹，老枝叶Ç。粳稻和P。Densiflora和P的老树枝。丹参。我们关于种间和器官间污染模式的长期数据（着重于树冠位置之间的差异和逐年趋势）可能有助于改进对受污染森林生态系统中¹³⁷ Cs沉积和动力学的估算。