Relationship Between Distribution and Habitat of Theligonum japonica in Sado Island,Japan

Abstract

　Theligonum japonica Okubo et Makino is a perennial herb that belongs to the family Rubiaceae. It is distributed in scattered patches from Akita to Kumamoto Prefectures in Japan. It is known to grow in moist forests, but few studies have revealed what environmental preferences are responsible for its disjunct distribution. This study conducted distribution surveys, meteorological observations on Sado Island, and cultivation tests to clarify the environmental conditions under which this species grows. The study area on Sado Island was divided into approximately 1 km square meshes, and 381 meshes were surveyed. We found T. japonica within 102 meshes. The result of classification tree analysis showed that T. japonica grows mostly in meshes with shorter daylight hours in summer and lower precipitation in summer influence. Meteorological observations showed that air humidity was high throughout the summer, and fog was observed more frequently at most observation sites. Cultivation tests showed vigorous rooting under conditions of higher air humidity. It is suggested that the population of T. japonica requires core areas where the air humidity is kept high by fog and that this species spreads to the surrounding area through vegetative propagation. In the Japanese archipelago, there are few places where aerial humidity is kept higher by fog in summer; therefore, T. japonica is likely distributed disjunctly.