Many thanks to The Providential Gardener, who had fielded some questions from curious readers who wanted to know more about the relationship between trees and air quality. She posed the question–how do trees affect air quality?–and posted some links on her blog a couple of weeks ago.
This post made me think about how trees benefit the environment in general, and inspired the following article that I wrote for my weekly column in the Kent County Daily Times.
Scientists have learned a lot about how trees improve air and water quality. Here’s a look at how trees help us have a healthier environment.
Trees have a tremendous effect on air quality. Through the pores of leaf surfaces, trees absorb harmful pollutants produced by humans, including nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), carbon monoxide (CO). Small particulate matter—such as pollen, dust, smoke and ash—are trapped and filtered by leaves and branches.
Some estimates conclude that 100 trees remove up to five tons of CO2, 400 pounds of ozone, and 300 pounds of small particulate matter. A study by the U.S. Department of Agriculture (USDA) Forest Service determined that trees in New York City annually removed more than 1,800 metric tons of air pollution from that city’s atmosphere, and estimated that the value of this service to society was approximately $9.5 million.
Trees use energy from the sun to fuel photosynthesis—think of it as a tree’s food production process. Photosynthesis converts water and C02 absorbed from the atmosphere into carbohydrates that are used by the tree for nutrients. The by-product of photosynthesis is oxygen, which is released by tree into the atmosphere. So by absorbing CO2, trees rid the environment of an excess pollutant and in return, they give us oxygen to breathe.
In fact, some governments are starting to consider planting trees to address air quality and global warming issues. The Kyoto Protocol, the international treaty whose goal is to reduce global warming by decreasing the emission of greenhouse gasses, allows countries that have signed the treaty to meet their carbon emission targets by cutting emissions—or by planting forests.
Trees keep us cooler, which means that less energy is used, fewer emissions are created, and pollutant levels are reduced. Shaded areas are naturally cooler, and trees also keep us cool through a process called transpiration that releases moisture into the air. In the transpiration process, water is absorbed by tree roots and moves through its trunk, branches, and leaves, where it evaporates into the atmosphere.
Urban areas experience a phenomenon that scientists call the urban heat island—the temperatures of urban areas are 2-10 degrees (F) warmer than surrounding areas. This is due to the prevalence of concrete, asphalt and tall buildings, and the absence of vegetation, including trees. The goal of many urban tree plantings is to reduce the urban heat island effect.
Trees also do a lot of heavy lifting when it comes to keeping water quality high. Water runoff in forested areas is much less than in developed areas. That’s because tree roots help keep soil in place, preventing the erosion of soil and hills in severe storms. And trees slow water runoff by holding rainfall on their leaves and branches and storing water in their roots. This helps replenish the groundwater supply and decreases flood potential. One hundred mature trees catch approximately 250,000 gallons of rainwater each year.
In addition, trees actually clean water as it passes through their roots on its way into the groundwater supply. And shady lawns conserve water because they need less irrigation, which also helps homeowners save money.
It’s interesing to think about how many different “jobs” that trees have. Everything in nature is inter-related in this way–all the parts of an ecosystem serve multiple purposes that aren’t always immediately visible to humans. That’s why so many problems–chain reactions, really–happen when humans alter ecosystems.
Photo of the Davie Poplar, a 300+ year old tree on the campus of UNC-Chapel Hill, courtesy of Wikipedia.
Earth Friendly Gardening 