Among the benefits of urban trees is a direct impact on the physical environment. Trees provide shade and evaporate water, and as such they lower the local temperature. They also absorb a substantial amount of rainfall, thereby mitigating risks of flooding, and their roots, leaves, and bark may be able to remove a variety of chemicals from the air, soil, and water. Via photosynthesis, trees incorporate carbon dioxide, a greenhouse gas and important contributor to climate change. And trees can contribute significantly to noise mitigation in urban landscapes.
These benefits of trees create substantial utilitarian value. In addition to reducing costs of heating and cooling and to improving air quality, adding trees to an urban environment has been shown to generate higher property values, and to stimulate retail activity.
Trees modify their environment, but they also can serve as barometers (or, perhaps, thermometers) of that environment. The growth and morphology of trees reflects the stresses to which they are subjected and incorporates the influences of heat, drought, metals in the soil, and gaseous air pollutants. As a specific example, the date of autumn leaf drop of urban gingko trees, which lose most of their leaves in a single day, has come progressively later over the past few decades in the face of warming climate.
Beyond their utilitarian benefits, urban trees also contribute to aesthetics. Humans have a deep inborn affinity to features of the natural world, a phenomenon denoted “biophilia.” Urban trees help local residents, including children, to connect with the natural world, and landscapes with trees tend to be judged as more pleasing than those without. Adding trees to an urban landscape contributes to individual stress relief, stronger community social ties, improved health, and reduced crime.
The specific choice of tree species for urban environments must incorporate a variety of factors. The size of the tree relative to available space, risks posed by the tree (from falling limbs or fruit, for example), and aesthetic factors all come in to play. Large trees might provide more environmental benefit but also create more hazard. One might suppose that native species — those that occurred in this region before European settlement — would be best able to thrive in the local environment and would provide more resources to local fauna. However, neither of those suppositions is necessarily true.
The urban environment may differ substantially from the undisturbed native landscape, and non-native species, or horticulturally-bred cultivars, may better tolerate conditions of exposure, water availability, temperature, and other factors. Moreover, many animals are able to take advantage of non-native trees for shelter, food, and other resources.
In addition to these general benefits of trees, individual people can become quite attached to individual trees. The place of the tree in the urban landscape, the provision of shade and display of autumn colors, the recreational value of the tree and its role in attracting wildlife — all of these can create strong connections.
Taken together, the values provided by urban trees far outweigh the costs of planting and maintaining them. Urban planners are advised to consider both the utilitarian and the aesthetic benefits of adding trees to the landscape.
David Goldstein is a Wright State University Emeritus Professor of Biology.