フロラと植生の変貌--房総半島を中心とした生態地理学的考察 (関東平野における第四紀後期の環境変化と文化の発達<特集>)

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1. Present distribution of vegetationLucidophyllous forests in Japan belonging to Camellietea japonica are called “Laurisilvae” by RÜBEL (1930), however, they are very unique and characteristic to the humid warm-temperate Asian region. The horizontal zones of evergreen broad-leaved climaxes or subclimaxes are Machilus thunbergii, Castanopsis cuspidata var. sieboldii and Cyclobalanopsis spp. zones from the coast inland on the Boso Peninsula. In the representative association of lucidophyllous forests there is Castanopsietum sieboldii which is divided into two associations: Bladhio-Castanopsietum sieboldii in the north and Rumohro-Castanopsietum sieboldii in the south.Even in hilly lowlands below 400m in alt, on the Boso Peninsula, there are three vertical zones: 1) the lower hilly zone with Castanopsion, 2) the upper hilly zone with Cyclobalanopsion including Abietum firmae, and 3) the lower montane zone with Tsugion. This phenomenon is based on the shrinkage of the vertical zones. The northern border lines of the climax species remarkably overlap one another, however, those of seral species are diversified due to the difference in temperature response.There are three types of secondary forest: 1) Pinus thunbergii forests along the sea coast, 2) Pinus densiflora forests on sterile, dry habitats, and 3) Quercus serrata forests on fertile, moist habitats. There are also three main types of semi-natural grassland: 1) Imperata cylindrica var. koenigii grasslands along the sea coast, 2) Miscanthus sinensis and 3) Pleioblastus chino grasslands inland. Recently, pioneer communities of naturalized plants are extending their areas.Japan is divided into four climatic zones (subarctic, cool-temperate, warm-temperate and subtropical), but its coastal area is divided into only three zones (cool, warm and subtropical). This is due to 1) the oceanic climate, and 2) coastal species being seral species.2. Present floraThe Boso Peninsula is divided into three floristic areas: Nouth-Boso, South-Boso and Kujukuri areas. Northern plants come into the North-Boso area, southern plants occupy the South-Boso area, and both of them live in the Kujukuri area. The inner zone (North-Boso) and the outer zone (South-Boso and Kujukuri) are separated by the Wakana's zone (Fig. 6). The Crinum line passes through the southernmost part of the Boso Peninsula which is the nothern limit of frostless area and is considered to be the periphery of the subtropical zone.Southern species of tropical and subtropical regions, such as Cynodon dactylon, are distributed along the coast in Kyushu because of its oceanic climate and scarce interspecific competition.Among the 2, 500 species in the flora of higher plants in the Boso Peninsula, about 300 are aliens because man-made or man-modified vegetation covers a very wide area there.3. Past flora and vegetationThe southern border of Fagus crenata forests is the isotherm of 10°C in annual mean temperature. If the annual mean temperature had dropped 8°C during the coldest period of the last ice age, Fagus crenata forests would have covered Kyushu to its southernmost part. Even if the annual mean temperature had dropped 5°C, the Boso Peninsula would have been covered with Fagus crenata forests. Elements of the cool-temperate zone are included in the flora of the Boso Peninsula as relics of the cold past during the Tertiary and the Quaternary. Fossils of those cool floras have been found there.Pinus parviflora and Tsuga sieboldii forests on the ridges of hills are also relics of the cold age due to the shrinkage of vertical zones. During the cold age when Fagus crenata was distributed to the south, the Zoysia japonica pasture grazed by wild animals under the cool-temperate climate was distributed southward simultaneously.

フロラと植生の変貌--房総半島を中心とした生態地理学的考察 (関東平野における第四紀後期の環境変化と文化の発達<特集>)

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