Tohru Araya

Naturally revegetated forest governed by mudflow induced sediment heterogeneity

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.ZusammenfassungDer obere Bereich des 1926 abgegangenen Schlammstroms Taisho, der in Zentralhokkaido im nördlichen Japan gelegen ist, wurde untersucht, um die Beziehung zwischen natürlicher Pflanzensukzession und der durch einen Schlammstrom erzeugten Sedimentheterogenität zu klären. Der Wald im Untersuchungsgebiet wurde anhand des Prozessregimes in einen ungestörten Bereich, das Anrissgebiet und einen Ablagerungsbereich gegliedert. Der Ablagerungsbereich wurde in drei Waldbestände eingeteilt, so dass sich insgesamt fünf Bestandstypen ergaben. Es wurde angenommen, dass der Schlammstrom edaphische Heterogenität verursachte und dass sowohl der ungestörte als auch der inselförmige Waldbestand Samen für die natürliche Wiederbesiedlung lieferte. Der ungestörte Waldbestand setzte sich aus verschiedenen Pionierarten zusammen, wobei sich auf dem Schlammstrom ein Waldmosaik aus fast reinen Betula ermanii und Picea glehnii Beständen entwickelte. Das deutet darauf hin, dass jede Pflanzenart ein bestimmtes Vermögen hat sich erst anzusiedeln und sich dann später in ein Waldmosaik zu entwickeln, was von den edaphischen Bedingungen, insbesondere Bodenmächtigkeit, Korngrößenverteilung, und Wasser- und Nährstoffgradienten, abhängt. Die unterschiedliche Artenzusammensetzung des Walds und eine 30-jährige Entwicklungsverzögerung der Waldbestände im mittleren Bereich im Vergleich zum Randbereich verdeutlichen, wie die durch den Schlammstrom erzeugte edaphischen Heterogenität die Sukzession und Waldentwicklung beeinflusste. Zudem hängen Pflanzen sowohl von Sedimentmächtigkeit und—textur als auch vom Feuchtigkeits- und Nährstoffangebot ab, was eine wichtige Rolle für deren Wachstum und Entwicklung spielt; dementsprechend entwickelten sich Waldbestände mit unterschiedlichen Arten und Alter auf dem Schlammstrom.

Kinematics Study on Topography of Landslide Instability

The topographic model of Varnes (1958) about the relation between topography and movement is well known. This model, however, cannot explain how asymmetry of a landslide forms; which was often observed. To research this problem, 59 points of landslide disaster areas in the Kinki district were surveyed. Accumulating the data of the topography of landslide instability just after a disaster, i.e. scarp, graben, crack, the typical landforms were picked up to inquire about movements which make these landforms. Consequently it was concluded that the asymmetry of landslide in form is formed by the difference in the deformation type (tension and shear) or by the extent of the rotation of the landslide movement in plain view. Moreover the forms of landslides were classified into I, II, III, IV by these factors.

Using Dendrochronological Analysis In A Study On History of Landslide Movement

In general, landslide mass can be divided into small blocks depending upon their activities. The objective of this study is to clarify the historical movement of these blocks using dendrochronological analysis. Firstly, we investigated microtopography and inclination of trees (Abies sachalinensis) planted on gentle slope of the landslide. Secondly, we examined compression woods of tree rings formed in the inclined stems. In terms of compression wood, date of initiation, change in forming direction, and increase in areal extent are important factors to analyze historical movement of landslide.The landslide investigated is formed by two blocks which indicate different directions and histories of mass movement, although tree ring analysis clarified extensive movements from 1972 to 1973 and from 1986 to 1987 in both blocks. Ring samples in which compression woods are formed in different directions were found at the central area of each block, whereas samples showing extensive compression wood were found near the crack within each block. These results suggest frequent and complex movements in a small scale within a block.