Plant community structure and threatened plant distribution on stone retaining walls in a coastal rural landscape in western Japan

Stone retaining walls are widespread in the rural landscapes of Japan, yet their ecological roles remain under-studied. We surveyed 27 quadrats (1 m × 10 m) and 24 routes across a coastal rural landscape, recording over 120 species, including seven threatened taxa. Stone retaining walls were largely free of invasive alien species, with 75.8% of species being native, a proportion higher than reported in urban areas, suggesting that exposure to invasives varies across the landscape. Community composition was influenced primarily by site type (farmland or residential walls) and annual sunshine duration. Ruderal plants were abundant on farmland walls, likely reflecting the proximity of farmland as a propagule source. Spatial variation in sunshine duration, which reflects desiccation stress, appeared to shape species composition, whereas wall characteristics such as crevice number and aspect had no significant effect. The seven threatened species recorded exhibited distinct distributions along rainfall and sunshine gradients, independent of site type, with native habitats ranging from farmland to moist mountains and rocky forest edges. These results indicate that stone retaining walls function as microhabitats supporting species from diverse ecological niches. Conservation of these structures should be pursued not only for their cultural heritage value but also for biodiversity, including the persistence of threatened plant species.