We studied the distribution and habitat of Chrysosplenium species in the upper Watarase river basin. Sørensen’s index was used to evaluate distributional overlaps among the species on two nested watershed scales; branch -1 2-scale （10⁰-10² km²） and second^-order watershed scale （10 -10 km²）. Seven Chrysosplenium species were distributed in the survey area. Among the seven species, two species （C. grayanum and C. kamtschaticum） were only sparsely distributed. The distribution of C. grayanum was biased towards lower elevations and that of C. kamtschaticum was biased towards higher elevations in the study area. The other five species （C. album var. stamineum, C. pilosum var. sphaerospermum, C. echinus, C. macrostemon, C. flagelliferum） were abundant in the study area. They were distributed in similar habitats; that is, habitats with similar temperature, stability of the ground surface （as indicated by altitude and slope, distance to the nearest valley, river bed slope of the nearest valley, and drainage density）. The distributions of these five species overlapped on both scales. These results indicated that C. grayanum and C. kamtschaticum had different ecological breadth in the upper Watarase basin. Chrysosplenium album var. stamineum, C. pilosum var. sphaerospermum, C. echinus, C. macrostemon, and C. flagelliferum may have similar ecological breadth in terms of climatic conditions and stability of the ground surface in this study region and on the studied scales. These five species showed similar trends of co^-occurrence on two nested scales in the upper Watarase river basin, which observed on a forest or micro-4-3 topographical scale（ 10⁴-10³ km²） in previous studies. In this study, we confirmed this phenomenon was maintained up to the branch scales.

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Robinia pseudoacacia, an alien species, also occupied the highest terraces and occasionally invaded C. arbutifolia communities. The height and DBH^-class distributions of C. arbutifolia communities implied that they may change into R. pseudoacacia stands when invaded, which persist due to rapid propagation by root suckers. Salix serissaefolia^-S. sachalinensis communities below the high^-water line persist when there is sufficient disturbance from flood waters. However, dams on the upper reaches now mitigate flooding in the Azusa River, reducing the establishment of new riparian C. arbutifolia woodlands. Consequently, R. pseudoacacia stands are likely expand to the detriment of natural riparian vegetation.

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[in Japanese]

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[in Japanese]

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Ailanthus altissima, a deciduous tree native to China, is known to invade open and disturbed sites and has been naturalized in many regions of Japan. I examined the species composition and stand structure of A. altissima community established at deforested sites under severe feeding pressure by sika deer （Cervus nippon） in the northern part of Hyogo Prefecture, Japan. These deforested sites were formed by clear^-cutting secondary oak forests and artificial Japanese cedar forests. Many A. altissima individuals of ≥1.5 m in height were found at the deforested sites, but none was found at neighboring sites covered by secondary oak forests and artificial Japanese cedar forests. Many native plant species, but not A. altissima, were browsed on by sika deer, indicating that A. altissima is unpalatable to sika deer. Native pioneer tree species （e.g., Mallotus japonicus and Aralia elata） that are strong competitors of A. altissima showed very low dominance in the A. altissima community.

This was the case in the community with low dominance of A. altissima, established at the deforested sites.

These findings suggest that browsing on native plant species by deer greatly contributed to the establishment of the A. altissima community. The height class distribution of A. altissima individuals of ≥1 m in height was unimodal in the A. altissima community, indicating that the regeneration of this species is discontinuous. However, 2880 A. altissima individuals per hectare （<1 m in height） were found in the understory of the community, most of which were ramets that originated from roots. The establishment of such a ramet bank may contribute to the regeneration of the A. altissima community to some extent. The maximum height of A. altissima trees was 15 m; the relationship between height and stem diameter at breast height for A. altissima showed that the height of the species has still not peaked in the study area. Thus, the A. altissima community is likely to continue growing and existing for more than a century.

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In cool temperate forest zones in eastern Japan, certain forb species like Cimicifuga simplex in light^-snow regions grow on slopes of beech forests, while in heavy^-snow regions its habitat is rather limited to valleys adjacent to beech forests. This research study reveals reasons for this difference in forb species habitats. As key factors, the study focuses on differences in the lighting environment during the leafing season as well as the state of tree leaf litter sedimentation on the forest floors in both light^-snow and heavy^-snow environments. Regardless of the amount of snow, the volume of tree leaf litter sedimentation on the forest floor was large on slopes of beech forest and small in valleys. In heavy^-snow beech forests, the number of layers of litter sedimentation as well as the density of litter compaction within a certain thickness of sedimentation were larger compared to light^-snow regions. In terms of the lighting environment during the summer season, no difference was observed on slopes of beech forest between light^-snow and heavy^-snow environments. However, over the spring season in light^-snow regions, the lighting environment of the forest floors was good during the leafing seasons, because foliation begins with the herbaceous layer and leaf expansion of canopy trees comes later. On the other hand, in heavy^-snow regions, the lighting environment of the herbaceous layer was poor during the leafing season, since foliation starts from canopy trees as snow covering the floor remains longer. In valleys, foliation of herbaceous layer preceded that of shrubs even in heavy^-snow regions. This phenomenon was attributed to the fact that lingering snow in the valley floor covers the shrubs alongside the valley, which in turn delays foliation of the shrubs. Two factors are considered reasons restraining the growth of forb species in beech forests in heavy^-snow regions: 1） the large number of layers and the high density of litter sedimentation at the forest floor constrict the establishment and germination of forb species, and 2） the poor lighting environment from spring to early summer （which are supposed to be productive seasons） is due to lingering snow that makes the crown canopy of beech forests foliate before forb species. However, the study found that forb species are able to grow in valleys in heavy^-snow regions, because the volume of litter sedimentation is small and herbaceous layer begins and completes foliation before shrubs do.

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Worldwide, field boundaries are one of the most important habitats for biodiversity. The diversity of these field boundaries is, however, rapidly declining as a consequence of land^-use change. Semi^-natural grassland on rice paddy field levees （Keihan or Aze in Japanese）, which is one of the types of field boundary that contains many grassland plants, is also under serious threat due to farmland consolidation and abandonment. The objectives of the present study are to review the relevant literature, to clarify the factors that affect biodiversity, to present conservation measures, and to suggest the direction of future studies. Before 1990, the vegetation on paddy levees had not been considered as a type of semi^-natural grassland. Thereafter, however, the importance of traditional paddy fields as a habitat for biodiversity was indicated, and the detrimental effects of farmland consolidation and abandonment were reported. After 2000, numerous studies reported on topics such as conservation strategies, comparisons of biodiversity on the lowermost hillside slopes, and soil seed banks. Traditional terraced paddy fields play a significant role in the conservation of biodiversity, pest control, and environmental conservation, and are also of high cultural value. Hypotheses on the diversity of the grasslands on paddy levees differ according to scale. Large^-scale hypothesis, such as those relating to historical factors, have not been well discussed. The vegetation on paddy field levees is affected by the surrounding grasslands, to which they are been historically closely related. Therefore, when attempting to determine the establishment of paddy levee flora, it is important to clarify the history of the paddy fields and their surrounding grasslands. In order to clarify the history of paddy fields, the following approaches can be adopted: researching historical data, surveys of the agricultural landscape, and analyses of pollen, plant opals, charcoal particles, and plant residues from archaeological sites. Vegetational studies, biogeography, and population genetics can also be of value in clarifying the establishment of paddy levee flora. The nature of the vegetation on paddy levees depends on various factors, including habitats, environmental conditions, and management regimes; however, the relationships between the vegetation and these various factors have yet to be clarified. The area of paddy fields is decreasing as a consequence of the abandonment of cultivation. The species richness of the vegetation on paddy levees is rapidly decreasing and its composition is rapidly changing. In order to improve the problems associated with abandonment, it is necessary to identify hotspots that include rare species, to increase the efficiency of cutting machines and methods, to obtain external assistance, and to introduce grazing. Farmland consolidation changes the vegetation of paddy levees. For the improvement of problems resulting from consolidation, the reuse of topsoil and establishment of seed sources are suggested, along with modification of the agricultural engineering of consolidation. Further measures for conservation include the sowing of seeds, transfer of seedcontaining hay, transfer of seed^-containing soil, and removal of exotic plants in the consolidated fields. In future investigations, the compilation of a representative list of grassland species and a comparison of species richness will be necessary. It is also necessary to collect and analyze past information on paddy levee vegetation, to urgently investigate the present vegetation on paddy levees, to establish conservation strategies, and to conserve various semi^-natural grasslands including paddy levees.

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