Street trees are a subset of urban trees, representing species with growth forms best suited to cultivation along pedestrian walkways and vehicle thoroughfares. Most cities and towns of the world are landscaped with street trees and shrubs. The trees enhance the appearance of residential, commercial or industrial property areas and offset, to a small degree, the loss of natural vegetation cleared for construction of buildings, streets and roads. Street tree plantings generally take place on public rights-of-way and are the responsibility of the local government public works department. Tree plantings on private properties abutting streets can be complementary to public landscaping.
Within the legal limits of incorporated towns and cities, a master street plan for landscaping may be in effect which specifies the types of trees which can be planted in public spaces, as well as on private property according to property zoning codes. Permits are often necessary to plant or remove trees. Regulations may prohibit the cultivation of noxious and potentially-invasive tree and shrub species. The over-all objective of these urban forestry rules is to create an attractive and fairly homogeneous urban landscape to please both residents and visitors.
The key issues facing street tree cultivation are climate, soil and water. Typically, street trees represent a mixture of native species, and introduced species known to be adapted to the local urban area’s climatic regime. In the establishment of street trees, it is essential that planting holes are dug deeply and widely enough so that root growth is not impaired, and the hole filled with friable soil to assure that trees can quickly become established after transplanted. Restricted root growth is one of the major reasons for street tree failures. Water from rainfall or irrigation is the third key factor. Because the trees are growing in areas where the surrounding soil is covered with impermeable materials, the soil is greatly limited in the capture of rainfall and runoff.
Numerous configurations of street tree planting exist. Most common are individual trees in cutouts in sidewalks, the soil surface sometimes covered with a metal grill; rows of trees planted in the grassy strip between the sidewalk and roadway; trees or shrubs in medians between opposing lanes of vehicle traffic; clusters of trees and shrubs in the center of traffic circles; in median barriers at intersections; in barriers used for street closures to vehicular traffic and so on.
Street trees are multifunctional. In addition to their aesthetic impact on the urban landscape, they provide shade to sidewalks and create slightly cooler microclimates for pedestrians, may act as beneficial windbreaks in certain situations and also function to reduce road noise pollution. Median plantings in divided roadways reduce oncoming traffic headlight glare. Trees also provide environmental services by creating arboreal habitat for birds, insects and small mammals. Street trees also improve air quality through their absorption of atmospheric carbon dioxide and the giving off of oxygen. Drawing carbon dioxide from the atmosphere helps slow global climate change.
The TreeWorld Blog “Urban Trees” provides guidelines for selection of appropriate street trees and the individual plant descriptions also identify those which are recommended.