We all know that April showers bring May flowers. And we know that water—aided by the sun, soil and air—makes plants grow. But some of the less-known connections between plants and the environment should be better appreciated.
For example, community trees help maintain the ecological integrity of our urban and suburban landscapes.
Rainwater washes quickly across concrete and asphalt, leaving little to soak into the ground. More runoff means more pollutants and sediments entering waterways.
Trees and shrubs intercept rain, slowing its arrival to the ground, where low-growing plants, roots and organic matter help it to infiltrate the soil. There it gets filtered and then passes into aquifers, from where it may be extracted. Excess groundwater, now purified, seeps slowly and more constantly into surface water.
In many cities, stormwater and sewage systems are often connected, outdated and inadequate to handle peak flows. Heavy downpours and prolonged rains can result in flooding and raw sewage discharges into waterways.
Trees can help minimize these problems. Research led by American Forests found that tree clearing in approximately 11.4 million acres of the southeastern Chesapeake watershed from 1973 to 1997 resulted in a 19 percent increase in runoff during heavy rains. The cost to build retention facilities to handle the additional water could exceed $1 billion.
Trees also improve air quality by absorbing gaseous pollutants, trapping particulates and lowering summer temperatures.
This is good news, because medical science has recently put a death toll on air pollution. Like cigarette smoke in the past, the link was long known, but not quantified.
We now know that up to 7 percent of cardiopulmonary deaths in the United States are tied to particulate air pollution, according to the National Resource Defense Council. The Environmental Health Coalition of San Diego estimates that the number for that city is close to 12 percent.
Dr. Murray Mittleman, of Boston’s Beth Israel Deaconess Medical Center, has observed that heart attack rates spike on days with elevated pollution levels.
The World Health Organization considers air pollution one of the leading health concerns in Europe, and found daily mortality rates may increase 20 percent during severe pollution episodes in the most-polluted cities.
Although emissions certainly need to be reduced, more trees would help.
A U.S. Forest Service study, for example, found that one large tree annually removes more than a pound of ozone and two pounds of sulfur dioxide, nitrogen dioxide and other airborne pollutants.
Chicago’s 4 million trees collectively remove 15 metric tons of carbon monoxide, 89 metric tons of nitrogen dioxide, 191 metric tons of ozone, 84 metric tons of sulfur dioxide and 212 metric tons of particulates a year.
American Forests estimated the trees removed from the Chesapeake Watershed would have cleansed about 9.3 million pounds of pollutants from the atmosphere annually.
Additionally, by producing shade, trees cool cities in the summer, reducing the amount of pollutants that enter the air via power plants and the evaporation of toxic compounds associated with industry and automobiles.
Significant economic savings accompany these health benefits. The U.S. Forest Service estimates that trees annually save New York City $10 million and Chicago $9 million in air pollution services.
American Forests estimates that the Bay watershed’s lost trees cost the region $24 million annually in pollution services.
In addition, forests in general produce the most and best water in the United States.
Although forests cover only a third of the land, two-thirds of the runoff in the contiguous 48 states flows from them. Woodland vegetation and soils combine to produce high-quality water, minimize floods and maximize streamflow during droughts.
Cities near forests can benefit from these services. New York City, for example, has some of the world’s best water because it maintains healthy, forested watersheds.
From a combined area of 2,000 square miles, New York’s three watersheds supply remarkably clean water to more than 10 million people. New York is one of the few major U.S. cities not required to use expensive purification plants.
But New York’s “champagne of water” title could be lost. The EPA recently warned the city that a $6 billion purification facility—with annual maintenance costs of $4 million—will be needed if recent water quality declines continue.
The message should be clear: Urban area forests are efficient at producing high-quality water.
American Forests recommends that 40 percent of urbanized areas should be under tree canopies, twice the current cover. The National Arbor Day Foundation estimates that 200 million spaces to plant trees exist just along city streets.
Ultimately, tree-planting campaigns are more than feel-good activities—they have real health, resource and economic consequences, and should be greatly expanded.