Shun-ichi Kikuchi

Structure and composition of riparian forests with special reference to geomorphic site conditions along the Tokachi River, northern Japan

The structure and composition of riparian forests were examined along the Tokachi River, northern Japan. Both the hydrogeomorphic gradient and the temporal gradient were analyzed in attempt to explain the present pattern of riparian forests. The stability of floodplain surfaces was estimated on the basis of the elevation above the riverbed and the distance from the river channel. The characteristics of the substratum on which trees were established were also examined by excavation of buried sediment. The results indicated that soil moisture and organic content increased while the size of particles in the substratum decreased with increased elevation and distance from the river channel. Gradient analysis was employed to examine the distribution of dominant species, such as Alnus hirsuta, Toisusu urbaniana, Populus maximowiczii, Picea jezoensis and Abies sachalinensis. The relative dominance of each could be arrayed across the elevation gradient. Although the three broad-leaved pioneers dominated bars and floodplains near the river channel, their modes shifted from lower to higher elevation and amplitudes of distribution curves decreased in the following order: A. hirsuta, T. urbaniana, P. maximowiczii and conifers, which were located on the highest floodplains. Sites could be divided into three classes in terms of stability. There were fewer species at active sites, which favored the three pioneer species, but species richness and diversity increased with stand age. Semi-active and stable sites were more diverse with the establishment of conifers and other broad-leaved trees, which included upland species. However, species richness peaked and then decreased after trees reached 50 to 60 years of age. The growth of dwarf bamboo and the development of conifer-dominant stands impeded the establishment of other species, thereby reducing species richness and diversity in mature stands. Chronologically, floodplains could be differentiated into high- and low-frequency zones of flood disturbance, with pioneer species occupying the former, and late successional species found largely in the latter.

Some methodological developments in the analysis of sediment transport processes using age distribution of floodplain deposits

Abstract Floodplain areas are primary storage sites for river sediment. In the Saru River, ages of floodplain surfaces were examined by tree ring analysis, and vertical and horizontal configurations of floodplain deposits were measured by field and aerial-photo surveying. A flood in 1992 provided a good opportunity to examine depositional and erosional processes of floodplain sediment before and after the flood. Predominant disturbances were observed in the unconstrained, wide reaches where floodplains develop. This event indicated that eroded areas of floodplain deposits in each age class linearly increased with sediment volume and that the proportion of the total area eroded decreased exponentially with increase in the age of sediment. We constructed basic equations expressing continuity of age distribution in order to analyze river sedimentation in a time series according to the results of the 1992 flood. The floodplain disturbance rates determined by this analysis showed similar temporal changes with sediment transport rates monitored at the Iwachishi Reservoir. A sharp increase in sediment discharge was seen after 1962 associated with the historical maximum rainfall in the temporal analysis of floodplain sediment. The time series approach presented here is useful for evaluating the speed of sediment waves and the cumulative impact of sedimentation in a river basin.

Stand and self‐thinning dynamics in natural Abies stands in northern Hokkaido, Japan

Stand dynamics and self-thinning were analyzed in relation to the dynamics of above-ground biomass in natural Abies sachalinensis stands growing on sand dunes in northern Hokkaido, Japan. This was done in order to examine wave-type regeneration in the stands. Fifty-two plots were established in almost pure Abies stands that ranged from saplings to the mature and collapsing growth stages. Above-ground biomass and tree height reached asymptotic levels prior to the collapsing phase, unlike wave-regeneration Abies stands in central Japan and North America. Stand density was high in the young growth stages, but the self-thinning rate, that is, the density decrease per biomass growth in the study stands was greater than in wave-regeneration stands in central Japan, as indicated by a large self-thinning exponent (−1.26 by reduced major axis regression). The range of tree height distribution was very narrow, and the stands’ vertical structure was typically single-layered. The slenderness ratio of trees was large, except in young stands. In mature and collapsing stands, advanced seedling density increased markedly. These stand and tree characteristics were considered to be correlated with the wave-type regeneration in the study stands, and it is assumed that prevailing winds affect tree mortality.

Structure and composition of riparian forests with reference to geomorphic conditions

A riverbed is a route through which materials from, hillslopes are swept downstream by water flow. During a flood event, large amounts of materials are transferred downstream without deposition. In contrast, when the power of the water flow has weakened after a flood bed load materials are deposited in the riverbed. Therefore, riverbeds are dynamic sites, in which the surface and vegetation are disturbed frequently and simultaneously (Kikuchi 2001). Hydrogeomorphic processes (e.g., erosion and deposition of sediment) and associated channel changes are the predominant disturbances that, allow vegetation to become established along riverbeds (Nakamura et al. 1997). Consequently, the structure and species composition of riparian forest stands reflect the intensity and frequency of disturbance.

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.

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.