Abstract
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.
Highlights
Cities are the most important living space of humans, at least as measured by the proportion of the population living in them
In order to assess the extent to which three different urban heat island (UHI) intensities influence the length of the growing season and, tree physiognomy, the results of the phenological monitoring in spring and autumn were compared with respect to the three defined categories
The present study has shown that, on the one hand, UHI is not uniform throughout the urban area and that the different intensities affect the phenological development of trees
Summary
Cities are the most important living space of humans, at least as measured by the proportion of the population living in them. By 2030, global urban population is projected to increase to 70%. This growing urbanization is causing huge changes in the urban environment [1]. One result of these transformations is the urban heat island (UHI) effect, which is characterized by higher air and surface temperatures compared to the rural environment. Factors contributing to this effect include, for example, the coverage of natural surfaces with material that absorbs the solar radiation more strongly and the industrial heat output. The projected increase in the average annual temperatures suggests an intensification in the UHI effect [3]
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