Although the climatic environment has not been quantified, it is commonly believed that subtropical dry scrub is distributed in the dryer areas of Chichijima Island in the Ogasawara (Bonin) Islands, the northwestern Pacific. To clarify the hydroclimatic conditions of subtropical dry scrub, year-round atmospheric and soil moisture observations were carried out in an area of subtropical dry scrub at Hatsune-yama Point in the eastern part of Chichijima Island. In the summer of 2001, the forest height and cover ratios of all scrub and forest species, and soil moisture, were measured at twelve sites on the island, including Hatsuneyama Point, along an east-west transect across the ecotone between subtropical dry scrub and secondary mesic forest. The subtropical dry scrub was not distinguished from the mesic forests by year-round dry atmospheric conditions; annual precipitation was larger and potential evaporation weaker at Hatsune-yama Point than at Chichijima Observatory on the western side of the island. At the former, however, the lack of precipitation and the very high potential evaporation extremely depleted soil moisture, to less than 20 volumetric %, during the summer, just after the rainy season. This severe depletion of soil moisture occurred only in the areas of subtropical dry scrub, and, consequently, formed a dry-wet gradient of soil moisture across the ecotone between the subtropical dry scrub and the secondary mesic forest. Since forest height was significantly correlated with this dry-wet gradient of soil moisture, the degree of depletion of soil moisture during summer just after the rainy season presumably affects forest structure on Chichijima Island. Based on these results, we conclude that subtropical dry scrub is distributed only in areas with seasonally dry edaphic conditions. This demonstration of hydroclimatic conditions using observational data implies that edaphic heterogeneity determines the vegetation pattern on Chichijima Island.
