Abstract
Ohanabatake is a vegetation landscape which is composed mainly of herbaceous plants flowering in June to August. It is found in some particular habitats in the alpine and subalpine zones of the high mountains in central and northern Japan. The Akaishi Mountains (the Southern Japan Alps) also carry the Ohanabatake vegetation on their alpine and subalpine zones. This paper aims to examine the site factors controlling the formation of the Ohanabatake vegetation in the Akaishi Mountains based on the field survey. A total of 47 sites of the Ohanabatake communities are recognized between Kitadake in the north and Tekaridake in the south (Fig. 1). It is also found that these communities are located on the six particular landform types; subalpine leeward gentle slope, alpine leeward concave steep slope, large-scale linear depression within double-ridges, small-scale linear depression, slope along a ravine, and nivation hollow. The physiognomy and species corn-position of Ohanabatake differ from one landform type to the other (Table 1). The present author examines the site factors resulting in the landform-controlled spatial variation of the Ohanabatake vegetation through the analysis of such factors as wind direction and speed, snow cover depth and duration, radiation receipt, etc. As a result, it is concluded that the combined effects of site factors characteristic of respective landform types would produce respective habitat types for the formation of the Ohanabatake communities (Table 5). Consequently, the six landform types can be read as following six habitat types. Type I: Subalpine leeward gentle slope
This habitat type is found below the timberline and is situated on the leeward side of the lowest cots through which strong wind blows. Coupled with the effect of snow accumulation in winter, strong wind restrains the formation of forests or causes the retreat of exsisting forests, producing the conditions favoring the development of herbaceous vegetation, in particular mesic to subhydric tall perennial herbaceous communities (Table 1). This kind of Ohanabatake is often surrounded by Betula ermanaii community.
The slopes are mostly gentle, ranging from 10° to 25° (Fig. 2), and are full of radiation (Fig. 3 and Table 2). So that snow melts early, though there is a deep snow cover in winter. This provides proper moisture for Ohanabatake to grow. As this type of Ohanabatake is rarely observed in other regions, it is one of the characteristic features of the Akaishi Mountains.
Type II: Alpine leeward concave steep slope
This type is formed on concave steep (30°_??_40°) slopes of the leeward side above the timberline on higher peaks with an elevation of c. 3, 000m (Fig. 2). The concave slopes on which Ohanabatake grows are covered with deep snow and tend to cause avalanches, while convex slopes around Ohanabatake are saved from avalanches, and are mantled by Pinus pumila community. As for the moisture conditions, heavy snowfall and lack of sunshine caused by steep gradient delay the snow melt. This prevents soil from desiccation and favors the development of alpine meadow soils (alpine humus soils). In spite of the steepness of slope, the land surface is rather stable.
Type III: Large-scale linear depression-within double ridges-
This habitat is affected by strong winds and is dominated by moist soil conditions. These two factors hinder the formation of forests, even on the slopes below the timberline. In the case of the slopes above the timberline, while convex parts are occupied by wind-blown (dwarf scrub) heath communities, concave parts are covered by snow-patch communities. Below the timberline, concave parts correspond to hydric plant communities of graminoid and pteridophyte, and convex parts to subhydric tall perennial herbaceous communities or wind-blown (dwarf scrub) heath (Table 1).
