Effects of Crown Shape, Leaf Area Index, and Tree-Planting Interval on Multifaceted Benefits of Urban Trees

Department of Architecture and Building Science Graduate School of Engineering, Tohoku University, Japan

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Abstract

Urban trees, such as street trees, have various effects on their surrounding environment, including solar shading, transpiration, and wind-speed reduction. Many researchers have studied the prediction method and model of these effects. However, the relationship between the morphological parameters of tree crowns and their effects on the thermal and wind environments in pedestrian spaces has not been sufficiently analyzed and quantified. We conducted case studies based on unsteady radiation and conduction simulations by systematically varying the crown shape of trees, leaf area index (LAI), and tree-planting interval, focusing on tree transpiration and thermal radiant environment at pedestrian height. The results showed that trees with Zelkova-type crowns were more effective in mitigating the heat island effect than those with Ginkgo-type crowns. The leaves of Zelkova are concentrated at the top of the crown, leading to increased transpiration. However, Ginkgo-type crowns were better for pedestrians because they provide more shade, resulting in lower ground temperatures and mean radiant temperature (MRT). We also quantified the increase in tree transpiration that occurred with higher LAI and wider planting intervals. Moreover, the increase in latent heat conversion from LAI of 5 to 8 was more gradual than that from LAI of 2 to 5. Based on these results, the tree species whose LAI reaches five or more have better advantage in mitigating urban heat islands. Additionally, this study showed that the planting interval had a greater influence on the thermal radiant environment than the LAI.