東部ネパールヒマラヤの植生 : 日華区系域の植生帯との関連

抄録

Though the flora of Himalayas has attracted the attention of a number of researchers, the region is still largely unexplored ecologically. In this paper the author studied the zonatioii of vegetation in different successional stages in east Nepal, and the climax zones were compared with that of East Asia. Altitudinal zonation of climax vegetation in east Nepal Altitudinal zones of vegetation in east Nepal were stated based on the investigation during the field trip in 1971 and 1977 (Fig. 1). The following 5 forest zones were recognized in climax state from lower to higher altitude (Fig. 2). 1) The Shorea zone (below 1000 m) This is the zone of subtropical Shorea forests. Though the forests are most extensive in the southern foothills of Himalayas, they sometimes extend into the mountaineous areas along the big valley system such as Arun and Tamur. Shorea robusta is a leading dominant species in the forests of this zone, but besides this there are several other types of forest dominated by e.g. Adina, Dalbergia, Lagerstroemia, Albizzia, Bombax, etc. 2) The Schima-Castanopsis zone (1000-1900 m) This is the zone of warm temperate forests dominated by Schima wallichii and Castanopsis spp. In the lower altitude of this zone the forests are dominated by Schima and Castanopsis indica with scattered trees of Albizzia, while in the higher altitude the Schima-Castanopsis forests are largely replaced by C. tribuloides and/or C. hystrix forests. On the rocky scree slopes the dominant species is replaced by Quercus lanuginosa. The Pinus roxburghii forest is the most extensive type of forest in west Nepal, while in east Nepal this forest is quite restricted to the south-facing steep slope only. 3) The Quercus zone (1900-2500 m) This is the zone of temperate lauro-fagaceous forests. In the easternmost parts of Nepal these forests are dominated by Quercus lamellosa and Q. lineata and are usually mixed with Litsea spp., Machilus spp., and Acer campbellii. In easternmost parts of Nepal Acer campbellii, which usually dominates the upper forest zone, commonly occurred in this Quercus zone. 4) The Acer zone (2500-3000 m) This is the zone of deciduous broad-leaved trees, such as Acer, Magnolia, and Sorbus. In some parts of this zone there are also coniferous forests dominated by Tsuga dumosa. One of the peculiar features of vegetation in this cool temperate zone of Nepal Himalaya is the presence of the evergreen broad-leaved forests, the Lithocarpus pachyphylla forest and the Quercus semecarpifolia forest. 5) The Abies zone (3000-3800 m) This is the uppermost forest zone bounded on the upper end by the forest limit (ca. 3800 m) and corresponds to the subarctic zone. Betula utilis and Rhododendron spp. are the most commonly accompanying species in this zone. Altitudinal distribution of species in different successional stages The above-mentioned zones of vegetation in climax state might be modified by various factors, among which the most strong and extensive disturbing factor for natural vegetation is the anthropogenic ones in east Nepal, e.g. farming, shifting cultivation, grazing by catties, etc. The extensive areas of secondary vegetation could be observed especially in the region lower than an altitude of 2000 m. As a basic information for the comparison of vegetational zonation in various successional stages the altitudinal distribution of species has been worked out separately for the pioneer and grassland herbs, the pioneer shrubs, the pioneer trees, and the climax trees (Fig. 3). In Fig. 3 certain marked differences in the distributional behaviour of the species can be distinguished in each successional stages. In general species distribution overlaps one another in a greater or lesser degree according to their successional status. In the pioneer and grassland herbs the distribution range of each species was quite large and sometimes reached to 2000 or 3000m of altitudinal range. And

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