Species composition and environments of riparian forests consisting of Lagerstroemia subcostata var. fauriei, an endangered species on Yakushima Island

Abstract

The floristic composition and environment of riparian forests consisting of Lagerstroemia subcostata var. fauriei (LSF), an endangered species, was investigated on Yakushima Island. Based on data collected from sample stands located in 35 riparian and 46 slope positions, four main species guilds (an upper evergreen forest guild, a lower evergreen forest guild, a ridge type and a pioneer/wetland guild) were detected using cluster analysis. LSF was identified as a component of the pioneer/wetland guild. TWINSPAN indicated compositional differences between typical evergreen broadleaved forests and secondary or wetland forests. The classification divided the 35 riparian stands into five of the eight types, including three stands classified as evergreen broadleaved forest. This variation in species composition within the riparian forest reflected characteristics of the site environment including watershed area, river width, sedimentation width/river width ratio, and elevation. LSF occurrences were limited to the stands on riverbed sediments and the foot-slopes of small watersheds (area<ca. 500ha), indicating the importance of sedimentation for the establishment of riparian forests consisting of LSF.