Roughly 75 percent of the inhabitants of developed nations live in an urban environment. The impervious surfaces created by roads, sidewalks, parking lots, driveways and buildings accumulate contaminants like oil and pesticides. When a rain event occurs, urban drainage systems such as sewers and storm drains accelerate the flow of water through communities and into drainage areas and waterways.
By returning some of that footprint to natural landscaping such as trees, the leaves, branches and trunk intercept the rainfall and temporarily store it, slowing the stormwater runoff down and reducing the pollution that washes into storm sewers. For every 5 percent of tree cover added to a community, stormwater runoff is reduced by approximately 2 percent according to a study by the University of Georgia.
Tacoma’s Urban Forest Policy aims for “30 by 30†or a 30 percent citywide tree canopy by 2030. Canopy coverage is the combined total area of tree crowns when viewed from above. Ramie Pierce, Tacoma’s Urban Forester said that this area is generally measured using satellite or other aerial imagery and sometimes used in combination with LIDAR (Light Detection and Ranging), which Pierce said, is a way to measure the heights of things to differentiate, in this case, between lawn or shrubs and trees. The most recent analysis is from 2009 data that puts Tacoma’s tree canopy at around 19 percent.
To get a feel for how this increased canopy might look, one has only to travel to North Tacoma. Large tree-lined avenues bisect many of North Tacoma’s streets, muting traffic and neighborhood noises and providing welcome shade in summer and protection from the elements in winter.
Several things happen beneath those mammoth trees that might go unnoticed however. If you’ve ever been caught in a sudden rain shower, you know that standing beneath a large tree provides shelter from much of the rain. Trees help to retain the water on site either permanently or by temporarily slowing the flow to waterways. Their roots prevent soil erosion and filter out nutrients such as nitrogen, phosphorus and potassium, which pollute streams.
Research from American Forests, a nonprofit conservation organization, suggests that urban tree coverage translates into dollars by reducing the size and costs of storm water infrastructures.
Congress passed the Clean Water Act in 1972. In 1990, the Environment Protection Agency (EPA) reported that one-third of U.S. waterways were impaired by stormwater runoff, directly impacting water quality. Locally, cities can meet the EPA’s regulations by using trees to help clean water naturally and tying urban greening to property cost-saving incentives.
Some cities in the United States have begun to link stormwater fees to the amount of impervious surface on a homeowner’s property. By increasing the number of trees and other vegetation, homeowner’s can reduce their stormwater utility fee. The U.S. Forest Service estimates the lifetime value of a large tree to a community at $4,440 and the benefits extend to homeowners as well through the added real estate value of 5 percent to 7 percent for residential lots, to the ability to sell the property faster. In addition, urban trees near buildings intercept the sun’s energy during summer months drastically reducing air conditioning costs and reducing the consumption of oil. According to the U.S. Department of Agriculture, “The net cooling effect of a young, healthy tree is equivalent to 10 room-size air conditioners operating 20 hours a day.â€