Measurements of downward-erosion rates in rill and interrill areas were carried out in a small bare land area of the subtropical Okinawa island (Figs. 1 and 2). The bare land is composed of BC layer of red soil derived from phyllite. Interrill-erosion rate in the first-order divide segment has no consistent relationship to the divide segment length and the divide segment gradient (Fig. 4 and Table 1). This indicates that the erosive action is almost uniform in the first-order divide segment of interrill area. Rill-erosion rate, Er, increases with increasing total rill length, and the rate of increase becomes lower as total rill length increases (Fig. 8 and Table 2). Figure 9 shows that Er also increases parabolically with an increase in the product of rill catchment area and rill gradient. On the basis of the assumption that rill discharge is proportional to rill catchment area, the result shown in Fig. 9 suggests that Er per unit total stream power decreases with increasing total stream power in a downstream direction.
In the 1980's, more than 60% of the Japanese live in the Densely Inhabited Districts in Japan. While the natural environments in urban districts have been deteriorated since the rapid economic growth period (1960's-early 1970's) and still it remains very difficult to solve environmental problems. Since the 1960's many Japanese cities have enacted municipal ordinances to conserve tall and/or aged trees from the depletion caused by various developments in urban districts. The aim of this study is to make clear the existing conditions of natural environments by using the inventory of preserved trees in Japanese urban districts. 21 cities, from Sapporo in Hokkaido to Kagoshima in Kyusyu, were selected for this research. 5, 460 preserved trees (hozonju in Japanese) of the 21 selected cities were collected as an index for the accumulation of tall and/or aged trees that can give us some amenities and that have been familiar with local residents for generations. This research is a preliminary study on the development of method using data of preserved trees as an index for the accumulation of the remaining trees in urban districts. The research also intends to find out basic tree species, locations, and the present conditions of the trees in urban districts of contemporary Japan. The following are major findings: 1. The 5, 460 trees located in the 21 cities can be classified into 164 species, of which only 19 species are flowering trees (144 trees). 2. The most basic species are Zelkova serrata (Keyaki), Ginkgo biloba (maidenhair tree), Cinnamomum camphora (camphor tree), Pinus (pine), Celtis sinensis persoon, Castanopsis cuspidata, llex rotunda, Aphananthe aspera, Podocarpus maerophyllus, Cryptomeria japonica (Japanese red cedar), and Machilus thunbergii. 3. The total ratio of the four most popular species of Zelkova serrata, Ginkgo biloba, Cinnamomum camphora and Pinus to the whole is as high as 50.2 %. The top 11 species comprise 73% of the whole. 4. These 11 species are consisted of 4 evergreen trees, 3 needle-leaf trees and 4 deciduous trees, and so Japanese cities as a whole can be said to show a variety in urban landscapes. 5. Zelkova serrata and Taxus cuspidata (Japanese yew) are inclined to be distributed in no them Japan, and Cinnamomum camphora is generally distributed in southwestern Japan, while Ginkgo biloba shows nationwide distribution. 6. The tallest tree is a Zelkova serrata in Morioka (36m) and the thickest one is also a Zelkova serrata in Kanazawa (7.8m around at the height of 1.5 m). Both cities escaped from war damages during World War II in their Densely Inhabited Districts. 7. Although urban districts are more or less man-made environments, it is suggested that the present vegetative landscape is under direct influence of climate or other natural characteristics of each region. 8. The trees concerned are owned by private residents, Shinto shrines, Buddhist temples, companies, and local governments. The ratio of trees owned by residents is 39% and this high ratio of trees located on private residential lands is a characteristic to the green areas in Japanese cities together with the high ratio of religious precincts. And the location of the trees is related to the historical development of cities. 9. 328 trees in the list of 5, 460 preserved trees had already been absolved by December 1, 1984. Reasons were the death by sickness and decay or cutting-down in the process of urban developments. The results suggest that many other trees or green areas having been maintained for generations in urban districts have suffered from or will suffer from deterioration. 10. The commonest species that has been absolved for the reason of sickness or decay is Pinus with a ratio of 53% out of the total.