City trees cool down ‘heat islands’ in urban areas
By Christopher M. Ferlin
In downtown Tucson, large 10-story buildings surround the pedestrians walking beside the small two-lane streets that lie below the crowded structures. Congress Street near City Hall has a tree planted every 40 feet or so along the sidewalk, while other areas have none.
A relative lack of city trees, along with the increased development that comes with a high population density, contributes to a problem called the urban heat island effect. This phenomenon is the increased temperature within cities when compared to surrounding areas. According to the U.S. Environmental Protection Agency, a city of about 1 million people typically has an annual average air temperature roughly 2 to 5 degrees Fahrenheit higher than nearby rural areas, with the biggest differences registering at night.
City of Tucson officials have taken notice of this problem and over the years have been trying to develop plans to reduce this effect.
“When I look at the Urban Heat Island, for me on the landscape outdoor side, it’s about trees,” said Irene Ogata, Tucson’s urban landscape manager. Ogata is leading efforts to plant new trees and to preserve mature trees around the city. Mature trees have a larger canopy and can be more effective than smaller trees, she noted. All trees help cool the surface to varying degrees with their shade and by evaporating water.
Trees as cooling systems
All trees participate in the natural process of transpiration, which is the process of water evaporation from stomata located on the leaves. Along with the normal evaporative processes from other surfaces, this is known as evapotranspiration. Water has a high heat capacity that enables it to absorb a lot of heat. Then, when it evaporates, it takes that heat with it. This process is similar to the human body sweating to remove heat.
Evaporative coolers use the same concept. In these “swamp coolers,” air pulled in by a motor is passed though evaporative pads that are saturated with water. The resulting air that is pushed into the air ducts leading to the house is cooler than the outside air temperature. The moisture has the ability to cool the air by up to 30 degrees Fahrenheit depending on the percent humidity.
Trees are also evaporative coolers, but because the air is not constrained in the open environment where trees grow, the cooling effect typically is not as drastic as in a swamp-cooled home. Like roofs, trees also decrease the amount of sunlight that can reach the ground by providing a canopy to absorb or reflect the sun’s rays. Increasing the number of trees in the city, then, can help cool it, especially during hot, sunny days.
Tucson adding greenery
One of the best examples of what the City of Tucson is doing is converting old parking lots into “green” areas and adding more trees to parking lots, Ogata said.
She pointed to the area in front of Tucson City Hall as an example of a parking lot that became a park (see photos at top). The land between City Hall and Alameda Street used to be a parking lot covered with blacktop that was not only unpleasant to look at but did not take advantage of an area that people readily walk. In the 1990s, the Tucson-Pima Arts Council and “countless volunteers” worked to remove some of the unattractive concrete and blacktop, with city authorization. In its place, they put mostly natural soil with now-lush vegetation. Benches in shaded areas allow pedestrians and nearby workers to stop and take a break from the harsh Tucson sun.
Other areas around Tucson have undergone similar alterations. For instance, Geronimo Plaza on University Boulevard near Park was converted in the mid-1990s from a congested parking lot for consumers shopping in Main Gate Square to one of the nicest green areas surrounding campus. A central fountain is fringed by trees that provide a cool environment for customers of local restaurants and businesses. Not only is this more aesthetically pleasing than a parking lot, but it also improves storm water infiltration into the soil.
If designed correctly, the new areas will only remove a minimal amount of parking spaces while providing areas with tree cover. For instance, she noted that Safeway at Broadway and Campbell added trees to its existing parking lot when it expanded in 2006. Using some of the area for planting also decreases the amount of heat-collecting blacktop on the surface.
The City of Tucson currently has about 1 to 2 percent tree cover, according to Alison Meadow, who is an assistant research scientist in the Department of Soil, Water and Environmental Science. Meadow was responsible for the analysis of the urban forest data collected using a program called iTree while interning at the Office of Conservation and Sustainable Development in Tucson. The software program, offered free by the U.S. Forest Service, uses input to estimate tree cover and other factors.
The program has a percent-error of about 10 percent, which means Tucson likely has a maximum of 11 to 12 percent canopy coverage. According to The Trust for Public Land, Tucson had 3.1 percent “park” coverage of vegetation in 2011. The American Forests organization recommends a 25 percent canopy cover in the Southwest.
The iTree program also estimates energy savings, amount of air pollution removed, and amount of energy saved. The level of tree cover in Tucson has the potential to save 2,224 megawatt-hours of energy along major streets and routes, according to the program, Meadow indicated. This is the equivalent to the energy used in 172 households each year.
“Those are a small subset of all the trees in Tucson that would make up the full canopy cover,” Meadow added. “We haven't completed a full survey of all trees in the city; I would imagine that the energy savings will prove to be exponentially higher when we do.”
According to a separate survey by Meadow, 1,199 trees can be found in the downtown area – an average of one tree every 98 feet. If all the available tree planting locations were filled, pedestrians would enjoy the aesthetics of seeing one tree every 82 feet on average.
“We have an immediate capacity to increase by about one hundred to two hundred trees just in the downtown area,” said Meadow.
This increase could make downtown Tucson more enjoyable for both residents and tourists while helping to curb the heat island effect.
“The best ways to reduce the urban heat island effect is to build low to the ground, paint rooftops with light color such as white, and increase the amount of vegetation and shade,” said Michael Crimmins, an associate professor and extension climate specialist for the Soil, Water and Environmental Science department at the University of Arizona.
Crimmins, a specialist in climatology and geography, recommends increasing vegetation cover in order to reduce the amount of sunlight hitting the heat-retaining surfaces of the city and being absorbed. Other practices such as white roofs and decreasing building area by constructing structures low to the ground also assist in reducing heat absorption.
Building low to the ground can help reduce the urban heat island in several ways. If the building is low to the ground, then the potential for surrounding tree canopy shading the building increases, which in turn reduces the amount of solar radiation coming into contact with the building. Lower buildings also expose less surface area to the sun than taller buildings, and are less likely to block cooling breezes.
Although government projects can help to decrease the urban heat island effect, individuals can do their part too. Planting trees around residential properties can cool the local environment, increase the aesthetic appearance, and reduce the extent of the urban heat island.
Christopher Ferlin is an environmental science student in the Department of Soil, Water and Environmental Science at the University of Arizona