Mutual Influences of Urban Microclimate and Urban Trees: An Investigation of Phenology and Cooling Capacity

This paper presents an empirical study on urban tree growth and regulating ecosystem services along an urban heat island (UHI) intensity gradient. The UHI effect on the length of the growing season and the association of cooling and shading with species, age, and size of trees was studied in Salzburg, Austria. Results show that areas with a low UHI intensity differed from areas with a medium or high UHI intensity significantly in three points: their bud break began later, the leaf discoloration took longer, and the growing season was shorter. After leaves have developed, trees cool the surface throughout the whole growing season by casting shadows. On average, the surfaces in the crown shade were 12.2 °C cooler than those in the sun. The tree characteristics had different effects on the cooling performance. In addition to tree height and trunk circumference, age was especially closely related to surface cooling. If a tree’s cooling capacity is to be estimated, tree age is the most suitable measure, also with respect to its assessment effort. Practitioners are advised to consider the different UHI intensities when maintaining or enhancing public greenery. The cooling capacity of tall, old trees is needed especially in areas with a high UHI intensity. In the future, species differences should be examined to determine the best adapted species for the different UHI intensities. The present results can be the basis for modeling future mutual influences of microclimate and urban trees.