Heat Islands, Air Quality & 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 ever-increasing urbanization.
In recent years, the urban heat island effect has gone from a theoretical climate phenomenon to a public policy and urban planning issue. As data accumulates about the urban heat island, communities are becoming increasingly aware of the importance of tree planting as a means of combating an array of urbanization issues and improving livability in communities of all sizes. Temperatures in urban centers are as much as 7 degrees F hotter than surrounding suburbs and rural areas.
Trees and vegetation lower surface and air temperatures by providing shade and through evapotranspiration. Shaded surfaces, for example, may be 20–45°F (11–25°C) cooler than the peak temperatures of unshaded surfaces. Evapotranspiration, alone or in combination with shading, can help reduce peak summer temperatures by 2–9°F (1–5°C). In studies of three large cities, changes in the ambient temperatures were computer modeled to estimate the indirect cooling effects of trees in reducing air-conditioning energy use. For these cities, cooling-energy savings were calculated. The study considered planting an average of four shade trees per house, each with a canopy cross section of 25’, and estimated net annual dollar savings in energy expenditure of $6.3 M, $12.8 M, and $1.5 M for Baton Rouge, Sacramento, and Salt Lake City, respectively. Clearly, local climate impacts the amount of the actual savings. If we can lower the temperature by 3 degrees F, 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.
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, 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. Davis, CA has development guidelines that specify the number and placement of trees in new and renovated parking lots. The 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.
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 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 the release of particulates from roadways.
Dr. Greg McPherson wrote, in his publication Desert Southwest Community Tree Guide: “Benefits, Costs, and Strategic Planting”, “Desert Southwest communities can promote energy efficiency through tree planting and stewardship programs that strategically locate trees to save energy and minimize conflicts with urban infrastructure. These same trees can provide additional benefits by reducing storm water runoff, improving local air, soil, and water quality, reducing atmospheric carbon dioxide (CO2), providing wildlife habitat, increasing property values, calming traffic, enhancing community attractiveness and investment, and promoting human health and well-being.”
When developing strategies to address the urban heat island in the southwestern deserts, the goal should be to maximize canopy size and shade generated with the minimum amount of resources (water, fertilizer, seasonal maintenance) expended. The same objectives should apply to the long-term maintenance of the shade canopy. Within any discussion of increasing urban tree canopy shade is a more difficult question of what is the best and most resource efficient means to meet this goal. Conditions unique to desert community heat islands are not easily comparable to regions where heat is not as extreme and the requirement for conserving water use in the landscape is not a primary, long-term goal. For the desert heat island, any cost/benefit analysis must include consideration of how much resource (water) is required to, not only, generate a given volume of shade but also to maintain it over the longer term. Growth rate is another consideration or how much time, and correspondingly how much resource(s), is required to produce the desired shade canopy. With their adaptation to our conditions, desert tree species generate and sustain optimal shade quickly and with minimal irrigation, relative to non-desert species. 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. With their unique forms, colors and stature, desert adapted trees already help us create a sense of place in the American southwest and are an essential and resource appropriate option to mitigate regional unban heat islands.
In a future article we will discuss research on the wide range of personal health effects from improvements in community tree plantings.