Origin of Distylium Dry Forest and Occurrence of Endangered Species in the Bonin Islands

Abstract

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.