Origin of Distylium Dry Forest and Occurrence of Endangered Species in the Bonin Islands
Origin of Distylium Dry Forest and Occurrence of Endangered Species in the Bonin Islands
复制标题
小笠原群岛 Distylium 干燥森林的起源和濒危物种的出现
作者：
Y. Shimizu
Distyeland干林是一种低矮的森林或灌木丛，高0.5-8米，生长在小笠原群岛土壤较浅的干燥栖息地。森林，占主导地位的Distyellepidotum，有最高的物种多样性和比例的所有植被类型的小笠原，它包括许多濒危物种。本文研究了小笠原群岛的秩岛列岛的Distyjima-retto干林的分布、种类组成和25种濒危物种的分布格局。研究了森林分布与雾发生的关系。分析了1980年大旱对三岛的危害，以及三岛不同海拔高度的同类物种生境。提出了一个假说来解释绢蒿干林的起源和濒危物种的发生：绢蒿干林起源于群岛曾经比现在高时类似于云雾林的中温带森林;随着岛屿的下沉和随着时间的推移干旱化的趋势，其面积和物种组成一直在下降，产生了许多濒危物种。在最后一次冰河时代之后，海平面上升了约100米，加速了这一趋势，在此期间，群岛的总面积减少到以前面积的三分之一。小笠原群岛位于东京以南约1000公里的太平洋上。这些岛屿从北到南分为三个群，Mukojima，Chichijima和Hahajima-rettos（岛屿群）（图1）。小笠原村的总行政办公室位于小笠原群岛中最大的岛屿千日岛。本文主要研究秩岛列岛的植被。群岛的基岩是第三纪海底火山活动形成的。玻安岩是一种分布广泛的富含MgO、Cr和SiO_2的安山岩。火山角砾岩在某些地方覆盖玻安岩熔岩（黑田1979）。群岛几乎完全被50-100米高的海崖包围。地势较低的土地（丘陵地）位于地势较高的地区，1991年5月2日被接受。2日本东京154世田谷区驹泽1-23-1驹泽大学自然科学系。海崖之上的区域（Tamura and Imaizumi 1981）。气候为亚热带海洋性气候（图2）。年平均气温为22.9°C。与冲绳相比，日本的气温年较差较小，冲绳岛是一个位于几乎相同纬度的大陆岛屿。全年都有一定程度的降雨，尽管年降水量（约1200毫米）不高。降雨集中在5月至6月和7月至9月的雨季。在仲夏，土壤浅的地方会出现缺水现象。
The Distylium dry forest is a low-stature forest or scrub, 0.5-8 m high, growing in dry habitats with shallow soils in the Bonin Islands. The forest, dominated by Distylium lepidotum, has the highest species diversity and proportion of endemics of all vegetation types in the Bonins, and it includes many endangered species. Distribution and species composition of the Distylium dry forest and distribution patterns of 25 endangered species were studied in Chichijima-retto, a cluster of islands in the Bonins. The relationship between the distribution of forest and fog occurrence was investigated. Also analyzed were damage caused by the severe drought in 1980 and the habitats of congeneric species of the three islands at different altitudes. A hypothesis is proposed to explain the origin of the Distylium dry forest and the occurrence of endangered species: Distylium dry forest originated from a mesic forest similar to a cloud forest when the Islands were once higher than they are at present; it has been declining in area and species composition with the sinking of the Islands and the tendency toward increasing aridity over time, producing many endangered species. This trend has been accelerated by a rise of sea level of about 100 m after the last Ice Age, during which the total area of the Islands decreased to one-third of the former area. THE BONIN ISLANDS (Ogasawara-gunto in Japanese) are located about 1000 km south of Tokyo in the Pacific Ocean. The islands are divided into three clusters, Mukojima, Chichijima, and Hahajima-rettos (island groups) from north to south (Figure 1). The head administrative office of Ogasawara Village is located at Chichijima, the largest of the Bonin Islands. This paper is focused on the vegetation of Chichijima-retto. The basal rocks of the Islands were formed by submarine volcanic activity in the Tertiary. Boninite, a kind of andesite characterized by rich MgO, Cr, and Si02 content, is spread widely. Volcanic breccia covers the boninite lava in some places (Kuroda 1979). The Islands are almost completely surrounded by sea cliffs 50-100 m high. Land with low relief (hilly land) occurs on higher I Manuscript accepted 2 May 1991. 2 Department of Natural Sciences, The Komazawa University, Komazawa 1-23-1, Setagaya-ku Tokyo 154, Japan. regions above the sea cliffs (Tamura and Imaizumi 1981). The climate is subtropical and maritime (Figure 2). The annual mean temperature is 22.9°C. The annual range of temperature is small compared with that of Okinawa Island, a continental island located at nearly the same latitude. It rains throughout the year to some extent, although annual precipitation (about 1200 mm) is not high. Rainfall centers around the rainy seasons of May-June and OctoberN ovember. Water deficits occur in places with shallow soils in midsummer.