13 巻 (1996) 1 号 p. 11-23
An eastern extreme stand of Distylium racemosum forest was discovered on Mikura-jima Island (E139° 37'), in the southern Izu Islands. This stand extends c.4 ha in area, and is established at 400 to 550 m a.s.l. in a valley of the northeastern facing slope of the island. Phytosociological study showed that it belongs to the Carici-Castanopsietum sieboldii OHBA 1971, a Castanopsis-type association indigenous to the Izu Islands. It falls into Symplocetosum, a high altitudinal subunit of the association, and finally, into the D. racemosum facies. Symplocetosum is distinguished by many differential species common to Daphniphyllo-Trochodendretum aralioidis OHBA 1971, a wind-exposed scrub established on the summit zone of the island. Consequently, D. racemosum facies has the intermediate flora of both associations. In the survey of individual trees in the transects, D. racemosum showed an inverse J-shaped distribution for DBH size class on a stable site. Moreover, they have many saplings and seedlings in the understory. The maximum size of D. racemosum reaches to 90 cm in DBH and about 20m in height. These facts suggest that this stand has fully matured and has reached a climax stable state. To examine the geographical distribution of D. racemosum forest in Japan, a distribution map was compiled from various sources. Though Mikura-jima Island is about c. 300 km east of the continuous distribution ranges of D. racemosum forests of southwestern Japan, the altitudinal range of D. racemosum forest in this island occupies nearly an equal position as that of southwestern Shikoku and southern Kyushu, which are considered to be the distribution center of D. racemosum forest.