In communities across the country that are striving to become more sustainable, the discussion revolves around ecosystem services.
Ecosystem services are all the critical services that nature performs, free of charge, that keep earth functioning. Examples of ecosystem services include pollination of plants, production of oxygen, removal of carbon dioxide from the atmosphere and the cycling and recycling of nutrients such as nitrogen.
An ecosystem that provides important ecosystem services for humans is the urban forest. Urban forests have been studied for years in an attempt to quantify, in language that can be understood by the general public, the many benefits they provide to their communities. The U.S. Fish and Wildlife Service has done multiple studies on these ecosystem services and has found that urban forests:
• Make cities healthier places;
• Improve air and water quality;
• Result in energy savings, noise abatement and improved soils;
• Provide billions of dollars of value in these services per year.
The urban forest of Hilton Head Island consists of an ecosystem dominated by live, laurel and water oak trees; loblolly and slash pine; red maples, sweet gums and black gums. This forest was recently analyzed by town staff to estimate the benefits it provides, using software developed by U.S. Department of Agriculture Forest Service scientists, called iTree Canopy.
This program allows analysis of random points chosen from aerial photography of the island for the type of ground cover present.
The analysis of 3,000 points on Hilton Head Island showed about 42 percent of the points surveyed were tree cover; another approximately 12 percent were other types of plants, and impervious surfaces (concrete, asphalt, roof tops) accounted for about 10 percent of the points surveyed.
Based on these surveyed points, it is estimated that the island’s forest removes about 388 tons of carbon monoxide, nitrogen dioxide, ground-level ozone and sulfur dioxide a year from our atmosphere. Carbon monoxide, nitrogen dioxide and sulfur dioxide all contribute to the formation of photochemical smog. Ground-level ozone is a particularly corrosive chemical that is harmful to human lungs and is known to worsen lung and heart disease.
Our forest also removes about 126 tons of particulate matter per year. Particulate matter consists of very small pieces of carbon that enter the atmosphere when fossil fuels are burned. These pieces can enter the lungs and can cause or worsen many lung conditions. Particulate matter has also been classified as a cancer-causing substance by the International Agency for Research on Cancer (IARC), a specialized cancer agency of the World Health Organization.
The iTree Canopy program also estimates the dollar value of these ecosystem services. For example, the estimated amount of carbon dioxide captured and stored by the island’s tree cover is almost 67,000 tons per year at a dollar value of about $1.3 million; the total amount of carbon dioxide permanently stored by the surveyed tree cover is 1.68 million tons, at a dollar value of $32.6 million.
It is also estimated that the island’s urban forest removes about 328 tons of ozone per year (dollar value of about $46,000 per year) and 126 tons of particulate matter per year (about $128,000 a year).
It is noteworthy that these estimates of forest benefits are based on a portion of the island and not the entire island, and therefore are likely underestimated.
Different tree species vary in their ability to remove air pollutants. Since leaves are responsible for uptake of pollutants such as carbon dioxide, evergreen species that have leaves year-round capture more air pollutants than deciduous trees that are leafless for part of the year.
Multiple studies have shown that the effectiveness of removal of particulate matter can be increased by using tree species with fine, complex leaf structure, such as conifers. So the pine trees of Hilton Head Island provide year-round air quality services for our residents and guests.
The island’s urban forest provides humans with so many benefits other than the air quality benefits mentioned above. The forest provides soil erosion protection, flood control, air cooling (and thus reduction of the production of secondary pollutants in the atmosphere), neighborhood aesthetics and stress relief. It is one of the island’s features that attract travelers to come and visit, and return again. It also provides food and shelter to all of the non-human species that share this island with us.
Our urban forest deserves our continued respect, protection and care as a partner in our community, our economy and our environment.
Sally Krebs is the sustainable practices coordinator with the town of Hilton Head Island.