Trees remove gaseous pollutants by absorbing them with normal air components through the stomates in the leaf surface. (3)
Some of the major air pollutants and their primary sources are:
Sulfur Dioxide (SO2). Coal burning for electricity/home heating is responsible for about 60 percent of the sulfur dioxide in the air. Refining and combustion of petroleum products produce 21% of the SO2.
Ozone (O3) - is a naturally occurring oxidant, existing in the upper atmosphere. O3 may be brought to earth by turbulence during severe storms, and small amounts are formed by lighting. Most O3 and another oxidant, peroxyacetylnitrate (PAN) - come from the emissions of automobiles and industries, which mix in the air and undergo photochemical reactions in sunlight. High concentrations of O3 and PAN often build up where there are many automobiles.
Nitrogen oxides - Automotive exhaust is probably the largest producer of NOx. Oxides of nitrogen are also formed by combustion at high temperatures in the presence of two natural components of the air; nitrogen and oxygen. Particulates are small (<10 microns) particles emitted in smoke from burning fuel, particular diesel, that enters our lungs and cause respiratory problems. (10)
There is up to a 60% reduction in street level particulates with trees. (1) In one urban park (212 ha.) tree cover was found to remove daily 48lbs. of particulates, 9 lbs of nitrogen dioxide, 6 lbs of sulfur dioxide, and 2 lb of carbon monoxide ($136/day value based upon pollution control technology) and 100 lbs of carbon. (1) One sugar maple (12" DBH) along a roadway removes in one growing season 60mg cadmium, 140 mg chromium, 820 mg nickel, and 5200 mg lead from the environment. (1)
Planting trees and expanding parklands improves the air quality of Los Angeles County. A total of 300 trees can counter balance the amount of pollution one person produces in a lifetime. (10)
Heat from Earth is trapped in the atmosphere due to high levels of carbon dioxide (CO2) and other heat-trapping gases that prohibit it from releasing heat into space -creating a phenomenon known as the "greenhouse effect." Trees remove (sequester) CO2 from the atmosphere during photosynthesis to form carbohydrates that are used in plant structure/function and return oxygen back to the atmosphere as a byproduct. About half of the greenhouse effect is caused by CO2. Trees therefore act as a carbon sink by removing the carbon and storing it as cellulose in their trunk, branches, leaves and roots while releasing oxygen back into the air.
Trees also reduce the greenhouse effect by shading our homes and office buildings. This reduces air conditioning needs up to 30%, thereby reducing the amount of fossil fuels burned to produce electricity. This combination of CO2 removal from the atmosphere, carbon storage in wood, and the cooling effect makes trees a very efficient tool in fighting the greenhouse effect. (11)
One tree that shades your home in the city will also save fossil fuel, cutting CO2 buildup as much as 15 forest trees. (16)
Approximately 800 million tons of carbon is stored in U.S. urban forests with a $22 billion equivalent in control costs. (1)
Planting trees remains one of the cheapest, most effective means of drawing excess CO2 from the atmosphere. (15)
A single mature tree can absorb carbon dioxide at a rate of 48 lbs. /year and release enough oxygen back into the atmosphere to support 2 human beings. (10)
Each person in the U.S. generates approximately 2.3 tons of CO2 each year. A healthy tree stores about 13 pounds of carbon annually or 2.6 tons per acre each year. An acre of trees absorbs enough CO2 over one year to equal the amount produced by driving a car 26,000 miles. An estimate of carbon emitted per vehicle mile is between 0.88 lb. CO2/mi. – 1.06 lb. CO2/mi. (Nowak, 1993). Thus, a car driven 26,000 miles will emit between 22,880 lbs CO2 and 27,647 lbs. CO2. Thus, one acre of tree cover in Brooklyn can compensate for automobile fuel use equivalent to driving a car between 7,200 and 8,700 miles. (8)
If every American family planted just one tree, the amount of CO2 in the atmosphere would be reduced by one billion lbs annually. This is almost 5% of the amount that human activity pumps into the atmosphere each year. (17)
The U.S. Forest Service estimates that all the forests in the United States combined sequestered a net of approximately 309 million tons of carbon per year from 1952 to 1992, offsetting approximately 25% of U.S. human-caused emissions of carbon during that period. Over a 50-year lifetime, a tree generates $31,250 worth of oxygen, provides $62,000 worth of air pollution control, recycles $37,500 worth of water, and controls $31,250 worth of soil erosion. (2)
Trees reduce topsoil erosion, prevent harmful land pollutants contained in the soil from getting into our waterways, slow down water run-off, and ensure that our groundwater supplies are continually being replenished. For every 5% of tree cover added to a community, stormwater runoff is reduced by approximately 2%. (1)
Research by the Unitted States Forest Service shows that in a 1 inch rainstorm over 12 hours, the interception of rain by the canopy of the urban forest in Salt Lake City reduces surface runoff by about 11.3 million gallons, or 17%. These values would increase as the canopy increases. (13) Along with breaking the fall of rainwater, tree roots remove nutrients harmful to water ecology and quality. (13) Trees act as natural pollution filters. Their canopies, trunks, roots, and associated soil and other natural elements of the landscape filter polluted particulate matter out of the flow toward the storm sewers. Reducing the flow of stormwater reduces the amount of pollution that is washed into a drainage area. Trees use nutrients like nitrogen, phosphorus, and potassium--byproducts of urban living--which can pollute streams. (20)
Homeowners that properly place trees in their landscape can realize savings up to 58% on daytime air conditioning and as high as 65% for mobile homes. If applied nationwide to buildings not now benefiting from trees, the shade could reduce our nation’s consumption of oil by 500,000 barrels of oil/day. (12) The maximum potential annual savings from energy conserving landscapes around a typical residence ranged from 13% in Madison up to 38% in Miami. Projections suggest that 100 million additional mature trees in US cities (3 trees for every unshaded single family home) could save over $2 billion in energy costs per year. (10)
Trees lower local air temperatures by transpiring water and shading surfaces. Because they lower air temperatures, shade buildings in the summer, and block winter winds, they can reduce building energy use and cooling costs. (6)
Help to cool cities by reducing heat sinks. Heat sinks are 6-19 degrees F. warmer than their surroundings (Global Releaf GA). A tree can be a natural air conditioner. The evaporation from a single large tree can produce the cooling effect of 10 room size air conditioners operating 24 hours/day. (18)
USFS estimates the annual effect of well-positioned trees on energy use in conventional houses at savings between 20-25% when compared to a house in a wide-open area. (USFS meteorologist Gordon Heisler)(13).
The asphalt paving on streets contain stone aggregate in an oil binder. Without tree shade, the oil heats up and volatizes, leaving the aggregate unprotected.
Vehicles then loosen the aggregate and much like sandpaper, the loose aggregate grinds down the pavement. Streets should be overlaid or slurry sealed every 7-10 years over a 30-40 year period, after which reconstruction is required.
A slurry seal costs approximately $0.27/sq.ft. or $50,000/linear mile. Because the oil does not dry out as fast on a shaded street as it does on a street with no shade trees, this street maintenance can be extended. The slurry seal can be extended from every 10 years to every 20-25 years for older streets with extensive tree canopy cover. (19)
Trees can also enhance traffic calming measures, such as narrower streets, extended curbs, roundabouts, etc. Tall trees give the perception of making a street feel narrower, slowing people down. Closely spaced trees give the perception of speed (they go by very quickly) slowing people down. A treeless street enhances the perception of a street being wide and free of hazard, thereby increasing speeds. Increased speed leads to more accidents. Trees can serve as a buffer between moving vehicles and pedestrians. Street trees also forewarn drivers of upcoming curves. If the driver sees tree trunks curving ahead before seeing the road curve, they will slow down and be more cautious when approaching curves. (16)
The scope and condition of a community's trees and, collectively, its urban forest, is usually the first impression a community projects to its visitors. A community's urban forest is an extension of its pride and community spirit. Studies have shown that: Trees enhance community economic stability by attracting businesses and tourists. People linger and shop longer along tree-lined streets. Apartments and offices in wooded areas rent more quickly and have higher occupancy rates. Businesses leasing office spaces in developments with trees find their workers are more productive and absenteeism is reduced. (11)
Property values increase 5-15% when compared to properties without trees (depends on species, maturity, quantity and location) A 1976 study that evaluated the effects of several different variables on homes in Manchester, Connecticut, found that street trees added about $2686 or 6% to the sale price of a home. (10) A more recent study indicated that trees added $9,500, or more than 18 percent, to the average sale price of a residence in a suburb of Rochester, New York. (8)
Two University of Illinois researchers (Kuo and Sullivan) studied how well residents of the Chicago Robert Taylor Housing Project (the largest public housing development in the world) were doing in their daily lives based upon the amount of contact they had with trees, and came to the following conclusions:
Trees have the potential to reduce social service budgets, decrease police calls for domestic violence, strengthen urban communities, and decrease the incidence of child abuse according to the study. Chicago officials heard that message last year. The city government spent $10 million to plant 20,000 trees, a decision influenced by Kuo’s and Sullivan’s research, according to the Chicago Tribune. Residents who live near trees have significantly better relations with and stronger ties to their neighbors. Researchers found fewer reports of physical violence in homes that had trees outside the buildings. Of the residents interviewed, 14% of residents living in barren conditions have threatened to use a knife or gun against their children versus 3% for the residents living in green conditions. (15) Studies have shown that hospital patients with a view of trees out their windows recover much faster and with fewer complications than similar patients without such views. (13) A Texas A&M study indicates that trees help create relaxation and well being. A U.S. Department of Energy study reports that trees reduce noise pollution by acting as a buffer and absorbing 50% of urban noise.
1. Coder, Dr. Kim D., “Identified
Benefits of Community Trees and Forests", University of Georgia, October, 1996.
2. USDA Forest Service Pamphlet #R1-92-100
3. International Society of Arboriculture Tree Care Bulletin, Benefits of Trees
4. Green Environmental Coalition Website
5. Sierra Club of Canada, “Emission Trends By Country”, October 1997
6. Nowak, David J., “Urban Trees and Air Quality”, November, 1995
7. Martin, Paul, “The Value of Trees”
8. Nowak, David J., “Benefits of Community Trees”, (Brooklyn Trees, USDA Forest
Service General Technical Report,
in review)
9. Galveston-Houston Association for Smog Prevention (GHASP), “Trees and Our
Air”, January, 1999
10. McAliney, Mike. Arguments for Land Conservation: Documentation and
Information Sources for Land Resources
Protection, Trust for Public Land,
Sacramento, CA, December, 1993
11. Michigan State University Extension, Urban Forestry #07269501, “Benefits of
Urban Trees”
12. American Forests, “The Case For Greener Cities”, Autumn 1999
13. American Forests, “How Trees Fight Climate Change”, 1999
14. American Forests, “Why Plant Trees”, 1999
15. Prow, Tina., “The Power of Trees”, Human Environmental Research Laboratory
at University of Illinois
16. National Arbor Day Foundation pamphlet #90980005
17. American Forestry Association Tree Facts: Growing Greener Cities, 1992.
18. USDA Forest Service Pamphlet #FS-363)
19. Tree Guidelines for San Joaquin Valley Communities, March 1999. Published by
the USDA Forest Service's
Western Center for Urban Forest Research and
Education.
20. American Forests Magazine, "Trees Tackle Clean Water Regulations", Summer
2000.
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