Urban forests help to improve our air quality. Heat from the earth is trapped in the atmosphere due to high levels of carbon dioxide (CO2) and other heat-trapping gases that prohibit it from releasing the heat into space. This creates a phenomenon known today as the “greenhouse effect.” Therefore, trees help by removing (sequestering) CO2 from the atmosphere during photosynthesis to form carbohydrates that are used in plant structure/function and return oxygen back into the atmosphere as a byproduct. Roughly half of the greenhouse effect is caused by CO2. Therefore, trees act as carbon sinks, alleviating the greenhouse effect.
On average, one acre of new forest can sequester about 2.5 tons of carbon annually. Young trees absorb CO2 at a rate of 13 pounds per tree each year. Trees reach their most productive stage of carbon storage at about 10 years at which point they are estimated to absorb 48 pounds of CO2 per year. At that rate, they release enough oxygen back into the atmosphere to support two human beings. Planting 100 million trees could reduce an estimated 18 million tons of carbon per year and consequently save American consumers $4 billion each year on utility bills.
Trees also reduce the greenhouse effect by shading houses and office buildings. This reduces the need for air conditioning by up to 30 percent which in turn reduces the amount of fossil fuels burned to produce electricity. The combination of CO2 removal from the atmosphere, carbon storage in wood and the cooling effect makes trees extremely efficient tools in fighting the greenhouse effect. Planting trees remains one of the most cost-effective means of drawing excess CO2 from the atmosphere. If every American family planted one tree, the amount of CO2 in the atmosphere would be reduced by one billion pounds annually. This equates to almost 5 percent of the amount that human activity pumps into the atmosphere each year.
It is estimated by the U.S. Forest Service that all the forests in the United States, combined, sequestered approximately 309 million tons of carbon each year from 1952 – 1992, offsetting approximately 25 percent of human-caused emissions of carbon during that period. Over a 50-year lifespan, a tree generates almost $32,000 worth of oxygen, providing $62,000 worth of air pollution control. This tree would also be responsible for recycling $37,500 worth of water and controlling $31,000 worth of soil erosion.
The Worldwatch Institute, in its Reforesting the Earth paper, estimated that the earth needs at least 321 million acres of trees planted just to restore and maintain the productivity of soil and water resources, annually remove 780 million tons of carbon from the atmosphere and meet industrial and fuel wood needs in the third world. For every ton of new-wood growth, about 1.5 tons of CO2 are removed from the air and 1.07 tons of life-giving oxygen is produced.
Trees also remove other gaseous pollutants through the stomata in the leaf surface by absorbing them with normal air components. Some of the other major air pollutants and their primary sources are:
• Sulfur Dioxide (SO2) – Sixty percent of sulfur dioxide comes from coal burning for electricity and home heating while 21 percent comes from refining and the combustion of petroleum products.
• Ozone (O3) – Ozone is a naturally occurring oxidant that exists in the upper atmosphere. O3 may be brought to Earth by turbulence during severe storms. Also, small amounts are formed by lightning. Automobile emissions and industrial emissions mix in the air and undergo photochemical reactions in sunlight releasing ozone and another oxidant, peroxyacetylnitrate (PAN). Naturally, high concentrations of these two oxidants build up where there are many automobiles.
• Nitrogen Oxides (NOx) – Probably the largest producer of nitrogen oxide is automotive exhaust. These are also formed by high temperature combustion when two natural air components are present; nitrogen and oxygen.
• Particulates – These are small particles emitted in smoke from burning fuel, particularly diesel, which enters our lungs and causes respiratory problems. With trees present, there is up to a 60 percent reduction in street-level particulates.
Studies have shown that in one urban park, tree cover removed 48 pounds of particulates, 9 pounds of nitrogen dioxide, 6 pounds of sulfur dioxide, 0.5 pounds of carbon monoxide and 100 pounds of carbon – daily. It has also been noted that one sugar maple along a roadway removes 60mg cadmium, 140mg chromium, 820mg nickel and 5,200mg lead from the environment in one growing season.
Bianca Bordelon (Urban Forestry Network)
http://urbanforestrynetwork.org/benefits/air%20quality.htm
