LiDAR-derived canopy structure explains ¹³⁷Cs concentrations in throughfall in Fukushima plantation forest

Following the Fukushima Daiichi nuclear power plant accident, approximately 1.8 PBq of Cesium-137 (¹³⁷Cs) got deposited in forested areas-2600 km² received more than 100 kBq/m² of ¹³⁷Cs. In Fukushima's predominantly cedar plantation forests, 60-90 % of the ¹³⁷Cs deposition was intercepted by the forest canopy. Previous studies have confirmed the gradual migration of cesium from tree canopies to the forest floor. However, these investigations have typically focused on plot-level trends, overlooking variations within the canopy itself. Our study aims to address this gap by employing terrestrial Light Detection and Ranging (LiDAR) scan to elucidate the relationship between ¹³⁷Cs activity/flux in throughfall and canopy characteristics post-accident. By defining a conical effective impact zone above each sampler and employing voxelization, we developed a robust and quantifiable method for assessing the impact of canopy volume on ¹³⁷Cs flux. We identified two distinct patterns: high penetration with low ¹³⁷Cs activity, and low penetration with high ¹³⁷Cs activity. Analyzing various rainfall events revealed that prolonged light rain often resulted in higher concentrations. Point cloud volume analysis within a 5° range conical zone above rain gauges indicated that larger canopy volumes correlated with reduced throughfall, leading to higher ¹³⁷Cs activity (R² = 0.308-0.578). Conversely, larger canopy volumes also increased ¹³⁷Cs flux (R² = 0.1879-0.7496). Hydrogen and oxygen stable isotope composition suggested significant canopy evaporation during extended periods of light rainfall, resulting in concentrated and elevated ¹³⁷Cs levels. This precise canopy quantification aids in understanding ¹³⁷Cs source allocation and exposure in forest ecosystems, providing a basis for radiation dose quantification and health risk assessment.