都市の樹木が極端な高温下でも冷却効果を発揮(City trees provide cooling even in extreme heat)

2025-06-302025-07-05

2025-06-27 スイス連邦工科大学ローザンヌ校(EPFL)

A test was carried out on eight plane trees. © srdjan111 / iStock Photos

スイス連邦森林・雪氷景観研究所(WSL)とEPFLの研究チームは、猛暑下においても街路樹(プラタナス)が周囲の気温を効果的に下げることを実証しました。2023年春、ジュネーブ郊外ランシーに生育する8本のプラタナスに樹幹の樹液流量計を設置。華氏100°F(約38°C)を超えるこの夏の2度の熱波にも、葉の気孔が閉じず、大量の水を蒸散し、蒸散冷却によって都市環境の温度低下に大きく寄与しました。これはこれまで「葉温が30～35℃を超えると蒸散が止まる」という仮説に反するもので、都市の気候モデルやヒートアイランド対策に重要な知見を提供します。また、他の樹種についても同様の冷却能力があるかどうか検証し、都市設計に活かすことが今後の課題とされています。

<関連情報>

異常な夏の熱波にもかかわらず、都市樹木による高い蒸散冷却効果
High transpirational cooling by urban trees despite extreme summer heatwaves

Bachofen C, Peillon M, Meili N, Bourgeois I & Grossiord C
Urban Forestry and Urban Greening Published:2025
DOI:https://www.dora.lib4ri.ch/wsl/islandora/object/wsl%3A40264

Urban trees cool their environment by shading and transpiration (latent heat, LE), thereby alleviating urban heat. LE may be critically reduced during heatwaves, when trees reduce stomatal conductance (g_S) to prevent hydraulic dysfunctions. Recent observations indicate that g_S may still be maintained during heatwaves, but implications for urban heat stress mitigation remain elusive. We recorded sap flow on eight Platanus x acerifolia trees in Geneva to assess LE during 2023, which had two record-breaking summer heatwaves. We repeatedly assessed leaf water potentials at pre-dawn and midday (Ψ_pre, Ψ_mid), g_S, and leaf, canopy, and ground surface temperatures in shaded and sunlit parts (T_leaf, T_can, T_surf). Using ecohydrological modelling (UT&C), we determined the energy budget of the urban square and assessed whether LE and g_S predictions match measurements. Despite air temperatures (T_air) reaching 39.1 ºC, trees continued transpiring up to 37.1 kg h^-1 (LE of 25.3 kW). LE was similar during heatwaves (T_air > 30 ºC) as during cooler periods and compensated approximately 33.3 % of the urban heating by solar radiation (R_S). In contrast, the model predicted a higher decrease of g_S, and LE to 22.8 % of R_S during heatwaves, thereby underestimating actual tree cooling. Despite unprecedented heatwaves, Platanus x acerifolia trees efficiently cooled the urban environment. Measured LE largely surpassed model estimations during heatwaves, hence actual cooling effects of urban trees during heatwaves might be considerably underestimated by current predictions with common stomatal models. Cities with intermittent heatwaves may thus continue to rely on vegetation cooling by transpiration, but further research is needed to determine the best suited tree species to optimise the cooling effect.