Existing natural Japanese cypress (Chamaecyparis obtuse) forests have limitedly distributed in unfavorable locations such as steep areas where logging and transportation are difficult. Exploitation of Japanese cypress has started since over 1,000 years ago because of its valuable quality, consequently have left them in the above-mentioned location. This study aimed at examining edaphic factors controlling the distribution of natural Japanese cypress forests. A 60-ha natural Japanese cypress (Chamaecyparis obtuse) forest exists in Nishi-Odaigahara, Nara Prefecture, Japan, wherein 36 Japanese cypress and 20 Japanese beech (Fagus crenata) dominated forest stands were comparatively examined in terms of soil physical and chemical characteristics. Distribution of Japanese cypress forest delineated from that of Japanese beech by soil depth. When soil depth exceeded over 50 cm, both types of forest were equally distributed, but Japanese cypress became dominant with 84.2% of coverage rate while 15.8% in Japanese beech when soil depth was less than 50 cm. There was no relationship between vegetation cover rate of Japanese cypress and soil depth having 2,000 kPa soil hardness which is considered to be the inhibiting level for root extension. Japanese cypress could extend its distribution even under the shallow soil depth with less than 50 cm. This is due to the characteristic that roots of Japanese cypress are capable to extend horizontally which alleviates interference of soil compact layer on the root growths. On the other hand, the vegetation cover rate of Japanese cypress increased at lower bulk density. This fact implies that Japanese cypress prefers deep soil depth, uncompact soil for their establishment and that Japanese cypress may have competitive relationship with Japanese beech when soil physical conditions are favorable.

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The following study was performed for the purpose of verifying how plant diversity changed by tree species conversion from coniferous plantation to broad-leaved forest. We investigated changes in the composition of plant species over a 30-year period in broad-leaved forest in the warm-temperate zone, which regrew after meteorological damage. The total number of species increased for three years after the disturbance, but subsequently decreased. To analyze the changes in species composition from an ecological perspective, the species were typed according to habitat classification. Lucidophyllous forest species increased throughout the study period. Non-forest type species increased rapidly in the early regrowth stage and then decreased. Overall, in the regrowth broad-leaved forest, the plant species increased markedly in the early regrowth stage due to a temporary increase in non-forest type species caused by disturbance, but these species then decreased with time. To conserve diversity in forest management, we must be aware that plant diversity in regrowth forests depends on non-forest type species.

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We investigated sediment transport rates during 9 months after line and ordinary thinning (scattered tree selection) of 49 years old hinoki (Chamaecyparis obtusa) plantation in order to examine the effects of different thinning practices on surface soil erosion. In the line-thinning site, drastic increase of sediment transport rates was observed immediately after thinning practices at skidding traces, probably due to the low infiltration rate of the surface soil disturbed by repeated skidding. However, the sediment transport rate at skidding traces decreased three months after thinning practices. This might be because unstable sediments were run off by heavy precipitation during the first summer. In the ordinary thinning site, sediment transport rates did not increase significantly after thinning practices, showing lower values compared to those at the logged lines and the skidding traces in the line-thinning site. These results suggested that the ordinary thinning has lower impacts on surface soils, and that the line thinning which promotes aggregated soil erosion should be avoided at the sites of high erosion risks such as steep slopes or volcanic ash deposits for surface soil conservation.

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