VEGETATION PATTERN IN RELATION TO ENVIRONMENTS ON THE LAVA PLATEAUS AROUND MT. TOMURAUSHI IN THE SOUTHERN PART OF THE DAISETSU MOUNTAINS, HOKKAIDO

Various alpine plant communities grow above the forest line (c. 1, 600m) around Mt. Tomuraushi in the Southern part of the Daisetsu Mountains, central Hokkaido (Table 1 and 2). The purpose of this study is to make clear the Character of habitats of the plant communities, in particular alpine meadow (forb communities).<br> In order to examine the environmental conditions governing the distribution of the plant communities in detail, the author established 14 belt transects near the ridges of lava plateaus ranging from Goshikigahara to Koganegahara (Fig. 1) and investigated the structure of plant communities and site factors along the transect. Site factors examined are topography, microclimate and edaphic conditions (Table 3).<br> The distribution of plant communities, including the alpine meadow, is greatly related to the period of snow release. The habitat where the period of snowmelt is early (before mid-June), i.e., the summit of ridges and windward slopes, is characterized by wind-blown and xeric conditions favoring frost action, and thin, immature and oligotrophic soil. Such a habitat usually carries Loiseleuria procumbens community, Primula cuneifolia-Potentilla matsumurae community and Pinus pumila community. The habitat where the period of snow melt is late (July-September), i.e., leeward slopes and concave slopes, is characterized by shorter growing period and thin, immature and oligotrophic soil. This habitat is usually occupied by Primula cuneifolia-Potentilla matsumurae community, Fauna crista-galli-Potentilla matsumurae community, and Phyllodoce aleutica community.<br> The habitat where the period of snowmelt is intermediate (early June-mid-July) is occupied by various kinds of communities according to the different edaphic/moisture conditions and so on, even if snowmelt ends at the same time. The habitat where hydric conditions prevail is occupied by Primula cuneifolia-Carex hakkodensis community and Fauna cnista-gatti community in order of hydric condition. The habitat where mesic conditions prevail is occupied by Anemone narcissiflora var. nipponica community, Trollius kurilensis-Ranunculs acris var, nipponicus community, Geum pentapetalum-Phyllodoce caerulea community, and Sasa kurilensis community.<br> Each of them occupies following respective peculiar topographies or sites. The alpine meadow (Anemone narcissiflora var, nipponica community and Trollius kurilensis-Ranunculs acris var. nipponicus community) is located on the stable and gently sloping surfaces above the forest line, where the effects of soil creep and frost action are relatively weak compared with the surrounding areas. Geum pentapetalum-Phyllodoce caerulea community is located on the earth hummocks and hummock-like mounds. Sasa kurilensis community occupies areas surrounding Pinus pumila community below c. 1, 800m in altitude (Fig. 2-6).<br> In the southern part of Daisetsu Mountains snow release is generally late because of heavy snowfall. Therefore, the snow lies on the leeward steep slopes and depressions till August or September. On the gentle slopes of the plateau, the period of snowmelt is mostly intermediate (early June-mid-July), and drought rarely occurs. Except in some moors and bogs (hydric) and on some ridges (xeric), mesic moisture condition persists during a growing season from June to August.<br> In the southern part of Daisetsu Mountains, the lava plateau with gently sloping surface is extensively distributed above the forest line located around 1, 600m in altitude. This enables alpine meadow to occupy larger areas in this part of Daisetsu Mountains than in any other mountains in Japan.