Contemporary patterns and temporal changes in alien plant species richness along an elevational gradient in central Japan

Background and aims - Interest in understanding the factors driving change in alien plant species richness along elevation has been increasing. This knowledge assists in understanding the risk of alien plant invasions and the general mechanisms determining species richness patterns in native flora. In addition, recent land use changes within mountainous regions are expected to affect the species richness of alien plants along elevation. However, few reliable datasets have revealed temporal changes in alien species distribution and richness along elevation. In this study, we evaluated regional species richness pattern of alien plants along an elevational gradient compared with that of native plants and assessed the temporal changes in alien plant species richness along elevation in central Japan. Methods - We evaluated the contemporary patterns and temporal changes in alien plant species richness using herbarium records and linked individual-based rarefaction and extrapolation curves. Key results - We showed that alien plant species richness peaks at lower elevations, whereas that of native plants peaks at higher elevation. The estimated alien plant species richness increased over the last approximately thirty years, particularly at the lower elevations. During 1985-2004, eleven of sixteen alien plant species invaded elevations > 2,000 m, whereas until 1976 only two species had colonised elevations > 2,000 m. Earlier invading alien plants have larger elevational ranges than more recent invaders and most alien plants first colonised elevation < 1,000 m. Conclusions - Artificial land use and road lengths are more important for alien plant species richness than total area. We conclude that high propagule pressure and human disturbance support the establishment of alien plant at lower elevation, and then they extend their range. This is reflected in the current patterns and temporal changes in alien plant species richness. Our results suggested that the risk of invasion by alien plants has been increasing in mountain environments.