Habitat Characteristics of a Chasmophyte, Spiraea thunbergii, in the Upper Reach of the Tama River

In Japan, the ecology of the shrubs that grow in rocky stretches in dingle is poorly understood. One of the shrubs, Spiraea thunbergii (Rosaceae), which grows naturally in rocky stretches along rivers, is endangered regionally in Osaka and Hiroshima. To learn more about the shrubs growing along rocky stretches and to consider conservation strategies for this species, the habitat characteristics of S. thunbergii were investigated along a rocky stretch of the Tama River in the Sawai area (70 km from the river mouth, Ome City, Tokyo). Individual shrubs were measured and their habitat was surveyed, including their relative elevation from the water surface, distance from the river, microhabitat, and local moss cover. Many individuals grew in a mossy, rocky stretch of the river. Smaller individuals tended to grow on the moss. Approximately 70% of the shrubs grew between 0 and 2 m from the water surface. Therefore, it is very likely that mosses serve an important role in the growth of S. thunbergii, especially smaller plants.