Predicting annual trends in leaf replacement and ¹³⁷Cs concentrations in Cryptomeria japonica var. japonica plantations with radioactive contamination from the Fukushima Daiichi Nuclear Power Station accident

Abstract

Sugi (Cryptomeria japonica var. japonica) plantations are the predominant evergreen coniferous forest ecosystems in Japan. Sugi is a unique evergreen tree with no abscission layer in the leaf. However, the lifespan of sugi leaves is unknown. In this study, we described sugi leaves, investigated the age structures of living and newly deceased leaves, and estimated the lifespan and death process of the leaves. The lifespan of sugi leaves was 1–8 years, with an estimated mean of 4.3–5.3 years. Then, we modeled patterns of leaf replacement and cesium-137 (¹³⁷Cs) concentrations and estimated the potential ¹³⁷Cs supply to the forest floor through leaf shedding based on ¹³⁷Cs concentration data from leaves of various ages sampled after the Fukushima Daiichi Nuclear Power Station (FDNPS) accident. Around 90% of leaves that had sprouted before March 11, 2011 (ante -3.11 leaves) were predicted to die within 4 years of the FDNPS accident. Moreover, ~90% of the ¹³⁷Cs in ante -3.11 leaves was predicted to be removed within 3 years of the FDNPS accident. The predicted trends of stand leaf ¹³⁷Cs concentrations were verified with ¹³⁷Cs concentrations measured in four permanent sample plots in Fukushima Prefecture. This study revealed that ¹³⁷Cs translocated from the canopy to the forest floor at a faster rate than the ante -3.11 leaves that had been directly contaminated by the FDNPS accident. Including sugi leaf mortality and replacement in future models will allow for more accurate predictions of the fate and persistence of radiocesium in sugi forests affected by the FDNPS accident.