How moss affects urban temperatures: The effects of moss on the thermal dynamics of an urban cementitious surface

Urban surfaces often exhibit higher temperatures than natural ones, increasing heat stress in urban inhabitants. Plants can provide a solution due to their cooling effect. Whereas this has been investigated in vascular plants, this study is the first to investigate the influence of three moss species (Grimmia pulvinata, Ptychostomum capillare, and Brachythecium rutabulum) attached to a cementitious surface on air, surface, and substrate temperatures. They were compared to bare mortar, mortar covered with the climbing plant Hedera helix, and a moss-climber combination under varying weather conditions. Moss was found to affect temperatures in three ways. Firstly, moss increased surface temperatures in direct sunlight, with an average daytime increase of +1.5 °C to +4.1 °C compared to bare mortar. Secondly, moss exhibited insulating properties, dampening the heat flux to and from the underlying substrate. This reduced heat transfer to the substrate during warm days, limiting heat gain, but also reduced heat transfer from the substrate at night, leading to average nighttime temperatures inside the substrate that were higher (+3.0 °C to +3.8 °C) than in the bare samples. Finally, when moss was hydrated, an evaporative cooling effect could be observed, but it lasted only a few hours after watering. These findings suggest that moss could be a net positive in colder seasons or climates, but that under warmer conditions, it is best to keep moss hydrated or shaded during sun-exposed periods. Therefore, a moss-climber combination appears promising, combining the thermal insulation, acoustic and air-quality benefits of moss with the shading effect of climbing plants.