Environmental effects of urban trees and vegetation

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Introduction An urban forest in Boston, MA Trees and other vegetation can mitigate the urban heat island effect (Environmental effects of urban trees and vegetation) because they shade buildings, intercept solar radiation, and cool the air by evapotranspiration By cooling, trees reduce evaporative emissions from vehicles and other fuel storage, and by cooling homes and offices, trees reduce power generation emissions. General cooling also reduces the speed of chemical reactions that lead to the formation of ozone and particulate matter. Trees and other vegetation also can improve air quality (Air quality index) as well as provide other amenity and aestheic benefits such as shade and beauty.

Scientists also have known for nearly two decades that the biogenic emissions from certain trees can actually increase the levels of some pollutants, particularly ozone, in urban air. Realizing the potential benefits requires careful selection of the right species of tree for local conditions.

Mitigation of heat islands effects

Trees and vegetation cool the air by providing shade and through evapotranspiration (the evaporation of water from leaves). Shade reduces the amount of solar radiation transmitted to underlying surfaces, keeping them cool. Shaded walls may be 9 to 36°F (5° to 20°C) cooler than the peak surface temperatures of unshaded surfaces. These cooler walls decrease the quantity of heat transmitted to buildings, thus lowering air conditioning cooling costs. Cooler surfaces also lessen the heat island effect by reducing heat transfer to the surrounding air.

A key factor detemining a trees cooling effect is its transmittance: the fraction of radiant energy that, having entered the trees canopy, reaches the ground. For example, when sunlight reaches a tree's canopy, some amount of light is absorbed by the leaves and used for photosynthesis, some amount is reflected back into the atmosphere, and some amount is transmitted to the grass or ground below. Transmittance varies by tree or vegetation type, but for deciduous species – which shed their leaves in winter – transmittance ranges from 6 to 30% in the summer and 10 to 80% in the winter.

Another way trees and vegetation cool the air is by absorbing water through their roots and evaporating it through leaf pores. This process uses heat from the air to convert water contained in the vegetation into water vapor. A mature tree with a 30-foot crown transpires approximately 40 gallons of water per day. Evapotranspiration alone can result in peak summer temperature reductions of 2 to 9°F (1° to 5°C). While this process reduces air temperatures, it does add moisture to the air. The positive cooling effect of vegetation usually outweighs any undesirable gains in humidity.

The U.S. Department of Agriculture Forest Service estimates that every 1% increase in canopy cover results in maximum mid-day air temperature reductions of 0.07 to 0.36°F (0.04° to 0.2°C). However, trees and vegetation are one factor among many that affect prevailing weather conditions.

Removal of air pollutants

Trees remove gaseous air pollution (Air pollution emissions) primarily by uptake via leaf stomata, though some gases are removed by the plant surface. Once inside the leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner leaf surfaces

Trees also remove pollution by intercepting airborne particles. Some particles can be absorbed into the tree, though most particles that are intercepted are retained on the plant surface. The intercepted particle often is resuspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig fall. Consequently, vegetation is only a temporary retention site for many atmospheric particles.

In 1994, trees in New York City removed an estimated 1,821 metric tons of air pollution at an estimated value to society of $9.5 million. Air pollution removal by urban forests in New York was greater than in Atlanta (1,196 t; $6.5 million) and Baltimore (499 t; $2.7 million), but pollution removal per square meter (m2) of canopy cover was fairly similar among these cities (New York: 13.7 g/m2/yr; Baltimore: 12.2 g/m2/yr; Atlanta: 10.6 g/m2/yr). These standardized pollution removal rates differ among cities according to the amount of air pollution, length of in-leaf season, precipitation, and other meteorological variables. Large healthy trees greater than 77 cm in diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small healthy trees less than 8 cm in diameter (0.02 kg/yr).

Air quality (Air quality index) improvement in New York City due to pollution removal by trees during daytime of the in-leaf season averaged 0.47% for particulate matter, 0.45% for ozone, 0.43% for sulfur dioxide, 0.30% for nitrogen dioxide, and 0.002% for carbon monoxide. Air quality improves with increased percent tree cover and decreased mixing-layer heights. In urban areas with 100% tree cover (i.e., contiguous forest stands), short-term improvements in air quality (one hour) from pollution removal by trees were as high as 15% for ozone, 14% for sulfur dioxide, 13% for particulate matter, 8% for nitrogen dioxide, and 0.05% for carbon monoxide.

Shade trees planted in parking lots reduce evaporative emissions of volatile organic compounds (VOCs) from parked cars. Scientists suggest that increasing tree cover from 8% to 50% in Sacramento parking lots may reduce evaporative emissions by 2%.

Reduction in energy use

Researchers in a joint study by the Department of Energy's Lawrence Berkeley National Laboratory (LBNL) and the Sacramento Municipal Utility District (SMUD) placed varying numbers of trees in containers around homes to shade windows and walls. Cooling energy savings ranged between 7% and 40% and was greatest when trees were placed to the west and southwest of buildings.

Another LBNL study modeled the effects of shading homes with vegetation in seven U.S. cities. By providing 20% tree canopy – the equivalent of planting one tree to the west and another to the south of a home – buildings could achieve annual cooling savings of 8% to 18% and annual heating savings of 2% to 8%.

The effectiveness with which trees provide shade and save energy depends on their tree density, shape, and placement. The dimensions of the shaded building, the position of the sun in the sky, and whether a tree keeps its leaves year-round also determine overall energy savings.

The Tree Benefit Estimator on American Public Power Association's (APPA) Web site presents a simple method for estimating the energy savings and other benefits of tree planting. It was developed by Sacramento Municipal Utility District based on experience with their successful Shade Tree program.

CO2 removal

Community trees reduce atmospheric carbon dioxide (CO2) by storing it or by reducing demand for heating and cooling. On the other hand, vehicles, chain saws, chippers, and other equipment release CO2 during the process of planting and maintaining trees. And eventually, all trees die and most of the CO2 that has accumulated in their woody biomass is released into the atmosphere through decomposition. A comprehensive study of these “opposing” effects was conducted in Sacramento County, California by The Center for Urban Forest Research of the U.S. Forest Service. Sacramento's 6 million trees contribute to an annual net reduction of CO2 by about 335,000 tons. Of that total, 262,300 tons of CO2 remain sequestered in the trees. But an additional 83,300 tons — nearly 25% of the reduction — is attributable to tree shade on homes, buildings, and other structures. The CO2 released due to tree planting, maintenance, and other program-related activities is only about 2 – 8 percent of annual CO2 reductions and the release of CO2 through decomposition accounts for only another 1 percent. So, the total CO2 released in Sacramento County is less than 10,600 tons per year.

Mitigation of stormwater runoff

During rain events, the ground can become saturated and turn excess rainfall into runoff. Stormwater runoff problems, such as flooding and polluting of open water bodies, are worsened by the large amounts of water-resistant surfaces in urban areas. Trees and vegetation can help reduce the runoff problem by decreasing the volume of runoff. Studies indicate that evergreens, conifers, and trees in full leaf can intercept up to 36% of the rainfall that hits them.

Quality of Life

Trees and vegetation can help reduce noise, which may be highly valued in urban areas. They also provide shade from harmful ultraviolet radiation, particularly in playgrounds, schoolyards, and picnic areas. In addition, trees and vegetation may increase property values, as several studies have shown that home values are higher on tree-lined streets. Lastly, community gardens and neighborhood parks can help reduce physiological stress, aesthetically improve an area, and provide an urban habitat for birds, animals, and insects.

Effects on volatile organic compounds

Scientists have known for years that trees and other vegetation produce certain hydrocarbon compounds, such as monoterpenes and isoprene. These volatile organic compounds (VOCs), the source of the appealing scents associated with pine needles and cut grass, are used by plants to attract pollinating insects or to repel leaf-eating ones. Research shows that VOCs play a significant role in the formation of one of the most damaging pollutants, ground-level ozone, which is the major component of what is more commonly known as smog. The ozone forms in the presence of sunlight when volatile organic compounds react with nitrogen oxides emitted by cars and industrial plants. It is important to note that VOC plant emissions are harmless in the absence of the human-generated nitrogen oxides.

Emissions of VOCs by trees, particularly the isoprene emitted by deciduous trees, which shed their leaves annually, have been shown to increase levels of ground-level ozone in some urban areas. However, in atmospheres with low nitrogen oxide concentrations (e.g., some rural environments), VOCs may actually remove ozone. Because VOC emissions are temperature-dependent and trees generally lower air temperatures, increased tree cover can lower overall VOC emissions and, consequently, ozone levels in urban areas.

VOC emission rates also vary by species. Low VOC emitting species include certain types of pine and maple trees. High VOC-emitting trees include eucalyptus, sycamore, willow, and certain oak varieties. Thus, maximizing the net benefits of trees and other vegetation requires careful selection of species for each individual location.

While it is important to be aware of VOC contributions from trees and vegetation, the air quality (Air quality index) improvements gained from direct pollutant removal, reduced energy use and power plant emissions, slower rates of ground-level ozone formation from lower air temperatures, and other benefits generally outweigh the negative impact of biogenic emissions.

Indirect energy use

Because urban trees often receive relatively large inputs of energy, primarily from fossil fuels, to maintain vegetation structure, the emissions from these maintenance activities need to be considered in determining the ultimate net effect of urban forests on air quality. Various types of equipment are used to plant, maintain, and remove vegetation in cities. These equipment include various vehicles for transport or maintenance, chain saws, back hoes, leaf blowers, chippers, and shredders. The use and combustion of fossil fuels to power this equipment leads to the emission of carbon dioxide and other chemicals such as volatile organic compounds (VOCs), carbon monoxide, nitrogen and sulfur oxides, and particulate matter.

Further reading

Choosing the right trees to improve urban air, Environmental Science and Technology online.

Effects of Urban Forests and their Management on Human Health and Environmental Quality, USDA Forest Service

McPherson, E.G. 1998. Atmospheric carbon dioxide reduction by Sacramento's urban forest. Journal of Arboriculture. 24(4): 215-223.

Disclaimer: This article is taken wholly from, or contains information that was originally published by, the Environmental Protection Agency. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the Environmental Protection Agency should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

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(2012). Environmental effects of urban trees and vegetation. Retrieved from http://editors.eol.org/eoearth/wiki/Environmental_effects_of_urban_trees_and_vegetation
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