|During this century, the form and character of
cities and towns have changed dramatically. Fifty to one hundred years ago
urban areas were different; cities were smaller and skylines rarely rose above
eight to ten stories. Automobiles were
|During this century, the form and character of
cities and towns have changed dramatically. Fifty to one hundred years ago
urban areas were different; cities were smaller and skylines rarely rose above
eight to ten stories. Automobiles were not the major method of personal
transportation, and the ratio of open space to buildings was dramatically
different. Even the lifestyles of urban residents have changed.
Cities are dynamic, with ever-changing land uses. This is particularly true today. In planning the urban forest, it must be assumed that cities and towns will continue to change. The likelihood of changes and the character of potential environments should be carefully considered. Tree selection must assume that the urban forest will be dynamic, if it is to respond to the changing conditions of the community.
Space for Trees
The stereotypical image of the "city" as a place of continuous tall buildings and intensive, pedestrian -oriented streets is accurate for only a small percentage of urban areas. Most urban environments consist of automobile oriented, low density land uses and recently constructed buildings.
Cities are actually more spread out than the stereotype suggests, with a higher proportion of open space to buildings. However, the presence of open space does not necessarily imply more room for the urban forest. The majority of open space is devoted to transportation corridors and car storage. Space is almost always a limiting factor in some way. It could be a conflict between vehicles, pedestrians and trees for space at ground level. Above ground, the canopies of trees interfere with overhead utilities and views. Beneath the ground, tree roots compete with utilities and paving.
In urban environments, natural areas with continuous vegetation are replaced by discontinuous and fragmented vegetation. The space available for trees occurs in small pieces rather than large ones. In some areas, such as downtown cores, fragmentation of planting sites is extreme. Greater continuity can be found in residential neighborhoods, stream corridors, steep slopes or around other extensive land forms.
There appears to be a trend toward increasing density in many new residential and commercial developments. This threatens a corresponding reduction in the space available for trees.
Environmental conditions tend to be more varied in urban areas than in less developed areas. For example, asphalt surfaces surrounding one site make it hot and dry. But around the corner, buildings cast shadows that cut temperatures and minimize evaporation of soil moisture. Light, wind, temperature, and soil conditions can change abruptly from one spot to another.
Urban conditions are frequently more severe for the growth of plants than in natural environments. On the other hand, conditions could also be better due to irrigation or lack of competition.
Seasonal variation may also be extreme compared to more natural areas. A site may be hotter in the summer and colder in the winter. Or, it may be drier in the summer and wetter in the winter due to soil compaction and the presence of paved surfaces. However, the city as a whole generates heat that usually keeps temperatures warmer in all seasons than more natural areas. This is called the "urban heat island" effect.
While it's not possible to generalize about the severity of urban conditions, one thing is certain. Conditions vary considerably, and it's wise to assess each site individually. The best way to identify site conditions is to tour on foot. Be sure to consider changes that are planned in the vicinity, even if plans are not yet final. The construction of streets and buildings, rechanneling surface water, and routing utility corridors can all affect the ability of a site to provide for the needs of trees. While it's best to anticipate urban evolution, it's safe to assume that plants and people will always have to adapt to unforeseen changes.
One goal of urban forest management is to create and maintain the maximum amount of visual and biological diversity. Preserving the uniqueness of different areas involves selecting trees that compliment the activities occurring there. This may require drawing on a broad range of plant species to create a distinct character. The urban landscape can be divided into four broad land use categories: natural areas, parklands and campuses, residential property, and fragmented spaces. These categories are based primarily on activities that take place in them and the mood created by those activities.
Natural areas are relatively undisturbed. The typical urban resident also considers them to be "natural," because of their size and location. These ar