Quantifying anthropogenic acceleration of hillslope denudation in granite watersheds: impacts of deforestation on soil coverage in the Tanakami Mountains, central Japan

Hillslope destabilization owing to disappearance of forest cover and tree root system accelerate sediment discharge drastically, resulting in an irreversible state transition in watersheds under a humid temperate climate condition. This study aims to quantify the impacts of deforestation using cosmogenic nuclide method. We measured cosmogenic ¹⁰Be concentration in fluvial sediment collected from outlet of watersheds with different conditions of soil and vegetation coverage, and then reconstructed amount of soil removal in the Tanakami Mountains, central Japan. The study area is underlain by biotite granite with a thick mantle of heavily weathered bedrock (saprolite), which is naturally covered by 1-2 m of soil layers and mixed forest of broad-leaf trees and conifer stands. The present watersheds can be categorized into three types: 1) kept forested and naturally soil-mantled watersheds without significant human impact, 2) deforested and still bedrock-exposed watersheds, and 3) artificially reforested and thin soil coverage watersheds. The concentration of cosmogenic ¹⁰Be in current fluvial sediment show a relatively low constant level in the devastated watersheds (2.1×10⁴－2.8×10⁴ atoms g^-1), whereas the forested soil-mantled watersheds indicate higher varying values (4.9×10⁴－1.2×10⁵ atoms g^-1). Assuming a steady-state denudation, the non-impacted natural watersheds have been denuded at rates ranging from 9.7×10² to 2.5×10³ g^-1 m^-2 yr ^-1. The variety in these denudation rates can be explained by steepness and stages of dissection of each watershed. Based on the empirical relationship between topographic parameters and the denudation rates, we can reconstruct original concentration of detrital ¹⁰Be at an arbitrary watershed even where significant human impact had removed the hillslope materials. Once the soil layer on hillslope have been thoroughly removed, the current fluvial sediment originates from the uppermost part of the saprolite. Thus, the amount of soil loss during the transition can be estimated from the decreased ¹⁰Be concentration observed in the devastated watersheds. In the case of Tanakami Mountains, the thickness of removed material from hillslopes should have reached ~0.3 m for the past few hundred years since initiation of overload by human activities on the forest resources.