The soil seed bank of a Phragmites australis community was studied in Hakone Sengokuhara, Kanagawa Prefecture, central Japan, in the spring of 1992, the summer of 1993 and the autumn of 1994, and the results were compared with the above-ground vegetation studied in the summer of 1994 and the summer of 1995. The autumn sample had the largest number of seed grains and species, and showed a significant difference from the samples of the other two seasons. Kyllinga gracillima and Carex capillacea were the most abundant in all samples. The soil seed bank was very similar to above-ground vegetation in species composition, though they were not always similar in density. Species were classified into the following four categories based on seasonal variation and vertical distribution of buried seeds, and the existence of seedlings. The first group, including Isachne globosa, Astilbe microphylla and so on, consists of species with seeds reserved in upper layers only and no seedlings. This means that they regenerate at a safe site, if their seeds are dropped there by chance. Microstegium vimineum var. polystachyum, Hydrocotyle ramiflora and so on were comprised in the second group characterized by species with seeds reserved in upper layers only and seedlings. These species pass the unsuitable season as seeds that are adapt to severe environment. Kyllinga gracillima, Carex capillacea and so on belong to the third group characterized by seeds reserved in all layers and no seedlings. These seeds remain in the soil as a reserve in case of unpredictable vegetational disturbance. The fourth group consists of species such as Viola verecunda, Lycopus uniflorus and Mosla dianthera that have seeds reserved in all layers and seedlings. These species likely have two kinds of seeds: one contributes to maintainance and dispersion of the population and the other remain as a reserve in the soil in case of disturbance. It appears that plants are usually regenerated vegetatively and that the soil seed bank plays only a small role in the regeneration of a Phragmites australis community. However, seedlings originating from buried seeds must contribute to regeneration when a serious disturbance occurs.
An eastern extreme stand of Distylium racemosum forest was discovered on Mikura-jima Island (E139° 37'), in the southern Izu Islands. This stand extends c.4 ha in area, and is established at 400 to 550 m a.s.l. in a valley of the northeastern facing slope of the island. Phytosociological study showed that it belongs to the Carici-Castanopsietum sieboldii OHBA 1971, a Castanopsis-type association indigenous to the Izu Islands. It falls into Symplocetosum, a high altitudinal subunit of the association, and finally, into the D. racemosum facies. Symplocetosum is distinguished by many differential species common to Daphniphyllo-Trochodendretum aralioidis OHBA 1971, a wind-exposed scrub established on the summit zone of the island. Consequently, D. racemosum facies has the intermediate flora of both associations. In the survey of individual trees in the transects, D. racemosum showed an inverse J-shaped distribution for DBH size class on a stable site. Moreover, they have many saplings and seedlings in the understory. The maximum size of D. racemosum reaches to 90 cm in DBH and about 20m in height. These facts suggest that this stand has fully matured and has reached a climax stable state. To examine the geographical distribution of D. racemosum forest in Japan, a distribution map was compiled from various sources. Though Mikura-jima Island is about c. 300 km east of the continuous distribution ranges of D. racemosum forests of southwestern Japan, the altitudinal range of D. racemosum forest in this island occupies nearly an equal position as that of southwestern Shikoku and southern Kyushu, which are considered to be the distribution center of D. racemosum forest.
There are many abandoned rice fields all over Japan. Most of them were abandoned by set-aside programs that began in 1970. This paper deals with the vegetation of wet abandoned rice fields in Hiroshima Prefecture. From the case study in the Saijo Basin, Higashi-Hiroshima, four plant communities are described : the Juncus leschenaultii community on the first-year fields, and three communities on older fields, the Ischaemum aristatum var. glaucum-Eupatorium lindleyanum community, the Isachne globosa community and the Eleocharis kuroguwai community. Vegetation change on abandoned rice fields is explained based on the species composition of abandoned rice fields communities and natural wetlands communities. Six threatened plant species were found on abandoned rice fields in Hiroshima Prefecture. Old abandoned rice fields have developed into various kinds of wetlands such as swamp woods, Moliniopsis marshes, sedge marshes and reedswamps. Although the old abandoned rice fields are the secondary wetlands, they will be more and more valuable in the future as the wildlife habitats not only in Hiroshima Prefecture but also in many other places in Japan.
A phytosociological survey was carried out in beech (Fagus crenata) and birch (Betula ermanii and B. platyphylla var. japonica) forests on the western side of Mt. Norikura in central Japan with the aim of deriving basic information on the effects of grazing on vegetation. Some observations on basic differences such as changes of architecture and floristic composition of forests were made in communities subjected to grazing and are herein presented along with general information regarding the vegetation of the area. Three types of plant communities were distinguished based on species composition in the area. The major type found in the grazed area was further divided into two communities : one was characterized by a dense shrub layer hindering the growth of the herb layer, and the other showed less dominance of the shrub layer and a luxuriant growth of the herb layer. This reduction of the shrub layer was induced as an initial result of cattle grazing ; nevertheless, the community was basically uniform in species composition. Invasion of new species resulting in a change in floral composition of the forest was found only at the sites with canopy disturbance.