Abstract
Gentiana thunbergii (G. Don) Griseb. is a small winter annual plant that flowers in early spring. Because of habitat development and over collection for horticulture, this plant has become an endangered grassland species. Semi-natural grasslands established on small landscape components, such as levees of paddy fields and agricultural reservoirs, are important alternative habitats for this species. The purpose of this study is to determine the optimal environmental conditions of the alternative habitats of this species in semi-natural grasslands maintained by agricultural practices. The mean soil volumetric water content of the study sites was 50.4%, while there was a significant moderate negative partial correlation between plant population density and soil water content. These findings indicate that although a moderately wet environment is necessary for species growth, excessive wetness is unsuitable. A significant moderate positive partial correlation between number of individuals and relative photosynthetically active radiation (PAR) was found, and relative PAR had a significant fairly strong negative correlation with the height of the surrounding plants. It is considered that the well-lighted environment, maintained by agricultural activities such as regular mowing management contributes to survival of the G. thunbergii population in the levees of paddy fields and the other landscape components in paddy-field areas.
