Japan has steep terrain formed by active crustal movement and high natural rate of erosion. Land form and land-forming processes strongly influence spatial pattern of forests in Japan. Because erosion caused by water is one of the main factors of land-forming process, catchment, the unit of hydrological cycle, is appropriate for evaluation of the spatial structure of forests, especially for mountainous areas in Japan. In this study, we examined spatial distribution of tree species and above ground biomass for a 306 ha catchment on Mts Tanzawa, central Japan, where erodible steep slopes dominate.

In hilly regions of Japan, habitat segregation of tree species relating landform has been detected: Disturbance tolerant species were distributed on unstable grounds around valleys, while late-successionals were located around ridges where ground-surface was stable (Sakai and Ohsawa 1994). However, we hypothesized that trees could be classified into more groups in mountains catchments because of more complex topography.

Species names and DBH of trees (≧ 5 cm) and soil depth were investigated at 105 quadrats (10 × 10 m). Topographical indices were extracted from DEM by using GIS. Those data were subjected to CCA and GLM to analyze topographical distribution of trees within the catchment.

Results/Conclusions

Twenty-nine of tree species, which occurred in ten or more quadrats, could be divided into five groups based on similarities in spatial distribution of species. Characteristics of those groups were explained by elevation, convexo-concave (similar to ridge or valley) and disturbance intensity (soil depth and slope angle) with CCA. In this mountain catchment, there were habitat variations for both ridges and valleys, and thus forest structure was more complex than Japanese typical hilly regions, where higher elevation sites consist with ridges and lower intensity of disturbances, and lower elevation sites are vice versa.

Above ground biomass of trees was larger on convex grounds that expressed in coarse scale (380×380m), owing to distributional pattern of large trees, such as Fugus crenata. However, this tendency was not so clear, because topographical preference was different among species, e.g., F.crenata (1st in biomass) tended to be larger on large ridges, Abies firma (2st in biomass) did on small ridges, and Carpinus japonica (3rd in biomass) was independent from the convexo-concave.