Urban heat island effects on moss gardens in Kyoto, Japan

Urban heat islands (UHIs) represent a serious problem for urban biodiversity and landscapes. The impact of UHIs on Japanese gardens are of particular concern because these gardens use moss as a ground cover, and moss can be strongly affected by UHIs due to their sensitivity to drought stress. We studied 17 historical gardens in Kyoto, Japan, and propose an effective management strategy for moss gardens that takes into account the influence of UHIs on these gardens. The study sites included gardens covered by large patches of hygrophilous moss, gardens covered by mosses including small patches of hygrophilous moss, gardens covered by relatively drought-resistant moss, and gardens with small patches of mosses. We measured temperature and relative humidity within the gardens and determined the surrounding land-use types. A comparison of microclimates showed significant differences among garden types. Hygrophilous gardens had the lowest temperature and highest relative humidity, whereas gardens with small patches of moss had the highest temperature and lowest relative humidity. Using structural equation models, we determined that urbanization within 1000 m from the gardens increased temperature and decreased relative humidity, strongly affecting the type of garden. These effects could be attributed to the sensitivity of mosses to changes in microclimate caused by UHIs. Taken together, the results indicate that urban planning—e.g., increasing green space—within 1000 m of moss gardens can maintain those gardens by mitigating the influence of UHIs.