This study investigates temporal changes in the distribution of air dose rates at forest floors from 2012 to 2016 by measuring air dose rates at a height of 10 cm. The study was conducted at four different topography forest sites in Fukushima Prefecture, Japan. At each forest site, the air dose rate was found to have decreased by 7%–22% over time from 2012 to 2016 owing to the movement of radiocesium from organic layers to mineral soil layers in the forest site. However, the spatial distribution patterns of air dose rates did not change at these forest sites over five years. Besides, high correlations between air dose rates and organic plus surface mineral soil inventories were found at these forest sites during most of study years. Therefore, little changes in the spatial distribution of air dose rates could be caused by radiocesium retention at the same location in these forest sites. No statistical correlation between air dose rates and slope gradients was found in the two hilly forest sites with steep slopes above 35°. Accordingly, this study shows that the distribution patterns of air dose rates in the forest floors remained stable depending on the spatial distribution of radiocesium, which formed in the early phase after the Fukushima Daiichi Nuclear Power Plant accident.

本研究通过测量10厘米高的空气剂量率，调查了2012年至2016年森林地面空气剂量率分布的时间变化。这项研究是在日本福岛县的四个不同地形森林地点进行的。在每个森林站点，由于放射性铯从森林站点中的有机层迁移到矿物土壤层，从2012年到2016年，空气剂量率随时间下降了7％–22％。然而，在过去的五年中，这些森林地点的空气剂量率的空间分布格局没有变化。此外，在大多数研究年份中，在这些森林地点发现了空气剂量率与有机加表层矿物土壤清单之间的高度相关性。因此，放射性sites留在这些森林地点的相同位置可能导致空气剂量率的空间分布几乎没有变化。在坡度高于35°的两个丘陵林地中，未发现空气剂量率与坡度梯度之间的统计相关性。因此，这项研究表明，取决于放射性铯的空间分布，林地中空气剂量率的分布模式保持稳定，放射性铯是在福岛第一核电站事故后的早期形成的。