Naturally revegetated forest governed by mudflow induced sediment heterogeneity

Abstract

We investigated an upstream area of the 1926 Taisho Mudflow that occurred at Mount Tokachi, a volcano in central Hokkaido, Japan, to clarify the relationship between natural forest succession and mudflow-induced sediment characteristics. The study area was divided into three parts, i.e., undisturbed, deposition, and scoured areas, based on disturbance regimes. The deposition area was further divided into three different forest stands for a final total of five forest stand types. We assumed that the mudflow regimes created sediment edaphic heterogeneity and undisturbed and island forests supplied seeds for natural revegetation. The undisturbed forest stand comprised pioneer species, whereas a mosaic forest consisting of almost pure stands characterized by Betula ermanii and Picea glehnii developed in the mudflow. This indicates that each plant species has a characteristic ability to establish and adapt initially and later develop into a mosaic forest according to sediment edaphic conditions, particularly depth, grain size distribution, and water and nutrient gradients. The differences in forest species composition and a 30-year time lag between the development of forest stands at the distal edges and the center explain how the cross-sectional sediment edaphic heterogeneity created by the mudflow regimes affected succession and forest development. Furthermore, plants are specific to sediment depth and texture, as well as moisture and nutrient availability, which play important roles in their growth and development; thus, forest stands with contrasting species and age structures developed in the mudflow.