Energy Efficiency

A study of Chicago’s urban forest found that increasing tree cover by 10 per cent (an additional three trees per building) would reduce total heating and cooling energy use by 5 to 10 per cent . At a national level, researchers estimate that planting three additional trees for each building in the United States could save more than US$2 billion in energy costs annually (McPherson, 1994; Akbari et al., 1988).

Tree windbreaks have been found to reduce residential heating costs by 10-15 percent, while shade and evapotranspirational cooling from trees have been found to reduce cooling costs by 20-50 percent (Dwyer, 1993; Laverne and Lewis, 1995).

A study of benefits and costs of tree planting in Chicago found that the projected value of trees (e.g., pollution reduction, energy saving, property value) is nearly three times greater than the projected costs (McPherson, et. al., 1995).

A recent study found that planting shade trees could reduce the need for power plants. Data from California shows that 50 million shade trees planted in strategic, energy-saving locations could eliminate the need for seven 100-megawatt power plants (McPherson and Simpson, 2001).

Research reports savings of between 10 and 15 per cent on winter heating costs thanks to trees acting as windbreaks, and cooling cost reductions of between 20 and 50 per cent in summer due to shade and cooling through evapotranspiration (Heisler, 1986).

On hot summer days, a tree can act as a natural "evaporative cooler" using up to 100 gallons of water a day and thus lowering the ambient temperature (Kramer and Kozlowski, 1960).

Several investigators have documented dramatic (30 - 50%) differences in cooling-energy use between houses on landscaped and un-landscaped sites (Akbari, 2002).

Computer simulations using standard building and tree configurations for cities across the U.S. indicate that shade from a single well-placed, mature tree (about 25-ft crown diameter) reduces annual air conditioning use 2 to 8 percent and peak cooling demand 2 to 10 percent (Simpson and McPherson, 1996).

The ambient air temperature difference between an urban heat island and a vegetated area can be as much as 2-10 degrees F. The temperature measured directly above man-made surfaces can be as much as 25 degrees F hotter than the air temperature beneath a forested area (Akbari et. al., 1992; Simpson and McPherson, 1996).

Trees in Davis, California parking lots reduced surface asphalt temperatures by as much as 36°F, vehicle cabin temperatures by over 47°F, and fuel-tank temperatures by nearly 7°F. (Scott et. al., 1999).

In laboratory research, visual exposure to settings with trees has produced significant recovery from stress within five minutes, as indicated by changes in blood pressure and muscle tension (Ulrich, 1984).

Researchers have found that in housing areas with more trees, rates of domestic violence are lower than in otherwise identical housing areas with fewer or no trees. Residents from buildings with trees reported using more constructive, less violent ways of dealing with conflict in their homes (Kuo and Sullivan, 1999).

Symptoms of children with Attention Deficit Disorder (ADD) are relieved after contact with nature. The greener the setting, the more the relief. By comparison, activities indoors such as watching TV, or outdoors in paved, non-green areas leave ADD children functioning worse (Taylor, et. al., 2001).

When 250 residents of Detroit were interviewed concerning their preference of trees in urban areas, eight out of ten respondents stated that trees would have an influence on the choice of a place to live. Ninety percent of the respondents believed that trees increase property values in excess of ten percent (Getz, et. al., 1982).

Hurricane Hugo devastated Charleston, South Carolina, in 1989. Little was spared: homes, churches, power lines, and the urban forest were all heavily damaged or destroyed. 200 residents were asked to identify the single most special physical feature of Charleston damaged or destroyed by Hugo. People identified the urban forest more often than any other aspect of Charleston ( i.e. more than churches, historic buildings or homes) (Vigo, 1990).

Researchers report that inner-city girls with greener and more natural views at home had greater self discipline. They were less impulsive and had better concentration. These traits led to better life decisions and better school performance (Taylor, et. al., 2002).

Compared with apartment buildings that had little or no vegetation, buildings with high levels of greenery had 52 percent fewer total crimes, including 48 percent fewer property crimes and 56 percent fewer violent crimes. Even modest amounts of greenery were associated with lower crime rates (Kuo and Sullivan, 2001).

A 1998 study found that the more trees and grass in the common spaces of inner-city neighborhoods, the more those spaces are used by residents. The study also found that, compared to residents living near barren spaces, those closer to green spaces enjoy more social activities, have more visitors, know more of their neighbors, and have stronger feelings of belonging. In other words, relationships between neighbors are made stronger simply through the presence of vegetation (Kuo, et. al., 1998).

Pollution Control

Poplar and cottonwood trees can breakdown carcinogenic groundwater contaminants such as trichloroethylene (TCE) and atrazine into harmless compounds. As a result, trees are now being planted at remediation sites across the county to help reduce pollutants while improving the environment (Black, 1995; EPA, 1996).

A major study of Chicago estimated that trees in that city annually removed 15 metric tons of carbon monoxide, 84 tons of sulfur dioxide, 89 tons of nitrogen dioxide, 191 tons of ozone, and 212 tons of small particulates. The estimated value of this pollution removal was $1 million for trees in the city itself and $9.2 million for the entire Chicago area (Nowak, 1994).

Trees, especially those with large leaf-surface areas, absorb and trap airborne dirt and chemical particles, such as nitrogen oxide, sulfur dioxide, carbon monoxide, and ozone. Trees also help by reducing wind speed so that heavy particles settle out (Nowak 1994; Harris 1992).

Trees and vegetation can form a barrier that partially deadens the sound from traffic, lawn mowers, and loud neighbors Trees also create "background" noise of rustling leaves and wind through the branches that can help muffle other noises (Harris 1992).

Trees can limit soil erosion by helping control storm-water flow. Fibrous root systems hold soil in place so that it is not washed away by rain or flowing water. Erosion can be especially severe at construction sites in urban areas. Research has found that while forested land can lose about 50 tons of soil per square mile per year, developing areas can lose 25,000 to 50,000 tons (Lull and Sopper 1969).

In Milwaukee, where urban trees cover about 16 per cent of the city, trees reduce stormwater flows by 22 per cent. The city saves an estimated $15.4 million by avoiding the construction of additional retention capacity. In Austin, heavy rains make stormwater management a priority issue. Austin's tree canopy, almost twice that of Milwaukee's at approximately 30 per cent, reduced stormwater flow by 28 per cent, providing the city with an estimated $122 million in savings (MacDonald, 1996).

The canopy of a street tree absorbs rain, reducing the amount of water that will fall on pavement and then must be removed by a stormwater drainage system. In one study, an 8-year old Cork Oak intercepted 27 percent of the gross rainfall, while a 9-year old Bradford Pear intercepted 15 percent. Savings are possible since cities can install surface water management systems that handle smaller amounts of runoff. (Xiao, et. al., 2000).

Using the city of Davis, California as a model, existing data on the benefits and costs of municipal trees were applied to the results of a sample inventory of the city’s public and private street trees. Results indicate that Davis maintained nearly 24,000 public street trees that provided $1.2 million in net annual environmental and property value benefits, with a benefit–cost ratio of 3.8:1 (Maco and McPherson, 2003).

Trees reduce storm water through interception and canopy storage of precipitation. Most annual pollutant washoff in urban areas comes during the first flush of storm events. Urban forests have been shown to be most effective at intercepting rainfall from the type of small, short duration storms often responsible for first flush flows (Xiao, et. al., 1998).

Property Value

A study of new housing construction in Atlanta, Georgia found that in many instances, careful preservation of existing trees during construction actually cost less than clearing the land (Seila and Anderson, 1982).

When 250 residents of Detroit were interviewed concerning their preference of trees in urban areas, eight out of ten respondents stated that trees would have an influence on the choice of a place to live. Ninety percent of the respondents believed that trees increase property values in excess of ten percent (Getz, et. al., 1982).

Several studies have shown that the value of homes in neighborhoods with trees are higher than those of comparable neighborhoods without trees. In addition, neighborhood green spaces or greenways typically increase the value of properties located nearby (Thériault et. al., 2002).