Air Quality, Heat Islands & Trees
It seems almost too obvious to construct a list of the ways landscape trees contribute to the quality of life in communities. At every level of organization (state and local governments, environmental groups, agencies, utility companies, neighborhood associations) communities are trying to insure that increasing numbers of trees are planted as a means of mitigating the effects of increased urbanization.
In recent years, the urban heat island effect has gone from a theoretical climate phenomena to a public policy and urban planning issue. As data accumulates about the urban heat island effect communities are becoming increasing aware of the importance of tree planting as a means of combating this phenomena. Temperatures in urban centers are as much as 7 degrees hotter than surrounding suburbs and rural areas.
Researchers estimate that a 1 degree increase in temperature on summer days boost smog production by 3%. It is further estimated that a 1 degree rise in temperature can mean a 2% rise in demand for energy, which can translate into $25 million dollars worth of electrical energy for a city the size of Los Angeles.
By planting trees over 5% of the city (again using a city the size of Los Angeles, approximately 10 million trees) and repainting black roofs and re-coating asphalt with lighter colored material, the drop in temperature would reduce the production of ozone by 10% and reduce energy consumption by $175 million.
If we can lower the temperature by 3%, which computer models suggest is within the range of reductions possible by increased planting of trees, the improvement in air quality would be equivalent to making all of the city's cars electric.
Davis, CA has development guidelines that specify the number and placement of trees in new and renovated parking lots. This ordinance includes provisions for maintaining the planting density, prohibiting tree removal, and replacement of trees that fail for any reason. These guidelines are the results of many years of research on tree location and spacing on reducing heat gain by structures and paved surfaces.
The significance of the urban heat island is not limited to western or southwestern cities. Recently, Chicago started a program to planting roof top gardens, including with trees and shrubs, on city owned buildings. City officials hope to determine the effectiveness of such gardens in reducing the heat generated by large expanses of black tar roofs.
In Sacramento, during the summer months, the temperature differential at 3:00 P.M. between densely treed suburban areas and the city center were as much as 20 degrees centigrade. This cooling effect was attributed to shade as well as transpiration. Transpiration not only cools the leaves of trees but also the surrounding air. Trees then reduce urban temperatures by 1) providing shade and reducing the amount of radiant solar energy heating streets, buildings and roofing and 2) through the cooling effect produced by the transpiration from leaves. Tree placement and selection is important in optimizing the thermal buffering.
Particulates (dirt, dust and other particles that can become suspended in the air) are also a concern to air quality experts. The State of Arizona will spend 12 million dollars over the next 5 years paving heavily traveled dirt roads. The Maricopa Association of Governments (MAG), an organization charged with long range transportation planning for the Phoenix metropolitan area has been reviewing the effectiveness of planting trees along streets and highways to reduce release of particulates from roadways.
With their unique form, color and stature, desert adapted trees already help us create a sense of place in the American southwest. The growing body of research on air quality and the urban heat island effect further demonstrate the impact and importance of landscape trees on the quality of life in communities throughout the world.
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