Shade trees along city and village streets are attractive, but they also serve some very important environmental purposes. Trees chosen for the urban landscape must meet several important criteria if they are to succeed in an environment that includes stress factors never encountered in their native habitat. It's a rare tree that rises above it all successfully for long enough to touch the branches of its cousins across the street.
The Urban Forest
Not long ago, the urban forest consisted of towering old maples and elms, reaching across quiet streets in residential neighborhoods. Dutch elm disease, maple blight and four-lane arterial streets have changed that. Today's city tree may be fortunate to end up in a neighborhood of green lawns, but it has numerous relatives growing through holes in pavement along wide boulevards surrounded by towering buildings.
Natural Air Conditioning
The function of shade trees in cities and towns is to beautify and provide shade, but they do much more for the inhabitants. Their shade also creates cool islands that can counteract the urban heat island created by large expanses of pavement and rooftops. According to the U.S. Department of Energy, a healthy young tree can duplicate the cooling effect of "10 room-size air conditioners running 20 hours a day."
In the late 1960s, NASA scientist Bill Wolverton, was looking for a way to rehabilitate areas destroyed by agent orange when he discovered that certain plants consumed volatile organic chemicals (VOCs) like formaldehyde, benzene and trichloroethylene. Large trees absorb carbon dioxide, keep the carbon and discard the oxygen as part of their vascular respiration. We also know that urban tree leaves filter dust and particulate matter from the air and remove common combustion pollutants such as ozone, carbon monoxide and sulfur dioxide. Perhaps they can remove VOCs and other carcinogens as well.
Trees reduce storm water run-off by holding soil and absorbing ground water. Trees soak up dirty water from soil and return clean water to the environment, through respiration, as "dew." By keeping water from running into storm drains, trees help keep it in place, recharging aquifers--the areas of soil and rock below the surface that filter water as it drains into water tables and wells.
To accomplish the benefits of the urban forest, trees must be chosen with specific criteria in mind. Native trees tend to do better because they are hardy and disease-resistant and because their soil, moisture, light requirements match local conditions. Certain non-native varieties like the Norway maple, however, are more pollution-tolerant than other varieties of the same species. High-maintenance trees like catalpa and magnolia should be reserved for parks; trees for public areas should be low-maintenance, pollution-tolerant species like Chinese elms, Ginkgoes and oaks. Although the city budget controls purchases, state foresters, university extension programs and the Arbor Day's Tree City program can help city foresters discover resources for reforesting.