Spontaneous vegetation in mandatory green roofs at public educational facilities in Sumida-ku, Tokyo

In Japan, most mandatory green roofs are subject to neglect because of the minimal budget for maintenance. Consequently, spontaneous vegetation (SV) has occurred and replaced existing planting. SV, which is reportedly to increase biodiversity and stabilize the ecosystem in Europe, has the potential to be positively evaluated even in Tokyo. In order to evaluate the SV and devise a way of roof greening with extensive management, we investigated the 14 existing mandatory green roofs in Sumida-ku, Tokyo. Vegetation surveys were conducted from July to August 2022. The results showed that 73 spontaneous species were detected, and they improved green roof biodiversity. Cluster analysis and NMDS results showed that the wilderness in an urban context evolved on the green roofs. We concluded that the mandatory green roofs could supply wilderness to urban residents. In addition, we suggested a supplemental planting of rare and endangered species and the use of moss species in order to utilize the spontaneous vegetation and minimize the cost of roof greening.