Species composition, life-form and disseminule-form spectra of riparian forest were investigated in the Tanzawa Mountains, central Japan. The results showed that the gradient of vegetation species composition corresponded mainly to stand age. Pioneer trees such as Alnus and Euptelea appeared throughout all stages, pioneer herbs appeared in the early stages, and late successional trees such as Acer and Zelkova appeared in the late stages. Cornus controversa and Prunus jamasakura which are animal-dispersed pioneer trees, appeared only in the late stages. Percentage occurrence of annual and biennial herbs and anemochores decreased with stand development, while that of tall trees and endozoochores increased with stand development. The frequent appearance of animal-dispersed pioneer trees in the late stage was a marked characteristic of riparian forest in the Tanzawa Mountains. Such characteristics may be affected by the surrounding vegetation and the size of the watershed.
The relationship between forest community types and soil surface stability was studied on a steep slope of diluvial upland along the Sagami River in Kanagawa prefecture. Vegetation was classified phytosociologically into three communities: Zanthoxylum ailanthoides, Zelkova serrata and Quercus serrata communities. The surface geology, microtopography, soil profiles and soil erosion were investigated to decide the habitat conditions at the three communities. The findings showed that the type of forest community corresponded to the microtopography; the Q. serrata community was found on the upper side slopes, while the Ze. serrata and Za. ailanthoides communities occurred on the lower side slopes and in dells, respectively. The soil at the Q. serrata community had experienced no disturbance and little erosion history. By contrast, the soil at the Ze. serrata community showed signs of past disturbance, and there was more erosion than at the Q. serrata community site. The site supporting the Za. ailanthoides community had been affected by a landslide about ten years previously and consequently, much more soil had been eroded than at the other sites. These results suggested that each community site had a distinct soil type and level of surface soil stability, due to the soil erosion rate. Hence it was considered that the most influential factor determining the distribution of vegetation types was soil surface stability. Seedlings of Q. serrata appeared to be vulnerable to soil disturbance, such as that on lower side slopes, and Ze. serrata seemed to be able to grow from germination under disturbed conditions. Furthermore, Ze. serrata did not show superior survival competition with Q. serrata. Therefore the Q. serrata and Ze. serrata communities were thought to show a tendency to occur on suitable upper side and lower slopes, respectively.