Landscape Architecture / Feb 2008 / by Linda McIntyre
Treeconomics
Greg McPherson and the Center for Urban Forest Research tell us what a city's tree canopy is worth. It's more than you might think.
He wrote his thesis on effect of tree shade on temperature inside and outside houses. While teaching courses in urban forestry and horticulture at the University of Arizona's School of Renewable Resources in Tucson, McPherson got involved with a nonprofit tree planting group.
Tree planting efforts shouldn't focus on the numbers of trees. "That's kind of a false goal that could lead to putting trees in the wrong place."
Trees by the Numbers. Trees can improve the urban environment in a variety of ways, including:
- Reducing air temperature. Trees and other plants on building sites can lower air temperatures; McPherson has measured differences of 10 degrees Fahrenheit between a Tucson park and a desert area.
- Lowering heating costs. Trees planted to function as windbreaks reduce wind speed and resulting air infiltration by up to 50 percent,reducing heat loss for potential annual heating savings of 10 to 12 percent.
- Lowering cooling costs. A 1993 CUFR study using data from Atlanta concluded that shade and lower air temperatures from three 25-foot-tall trees, two on the west side of a house and one on the east, could reduce cooling costs by 34 percent, and later research by the center showed that a strategically planted tree can save 100 kilowatt hours in annual electricity use.
- Storing carbon dioxide. A typical tree will reduce atmospheric carbon dioxide by about 200 pounds annually over a 40-year period. A recent study by McPherson and colleagues found that Los Angeles' Million Trees program would reduce atmospheric carbon dioxide by about one million tons over 35 years, equivalent to taking 7,000 cars off the road every year.
- Reducing hydrocarbon emissions. Parked cars emit hydrocarbons for gasoline evaporating out of leaky fuel tanks and worn hoses; the emission are a significant component of urban smog, comprising as much as 20 percent of the total inventory of emissions. Shaded parking reduces these emission by lowering air temperature to one to three degrees Fahrenheit, gasoline temperature four to eight degrees, and temperature inside the car by as much as 40 degrees. California funds tree planting in parking lots as an air quality improvement measure because of this impact.
- Intercepting rainfall. Studies simiulating uban trees' effects on stormwater runoff have reported reductions of 2 to 7 percent. The crown of a mature trees can store 50 to 100 gallons of water during large storms. This interception and storage function reduces runoff volume and delays the onset of peak flows.
- Increasing property values. Consumer research suggests that buyers are willing to pay 3 to 7 percent more for houses with trees on the property; one comprehensive study found that each large tree in a front yard increased sales price by about 1 percent. Other studies have shown that shoppers prefer venues with trees, making more frequent and longer trips to such destinations, paying more for parking and spending up to 12 percent more for goods and services.
To get a clear economic picture, these benefits have to be offset by costs for tree planting, maintenance, and infrastructure damage by roots and debris.