In the past few decades, Washington, D.C. has lost half its tree cover, San Diego, Calif.’s is off about a quarter. The cover in cities in Michigan, North Carolina and Florida has fallen to about 27 percent of what it once was. Chicago and Philadelphia have just 16 percent of their former cover.
“Urban deforestation,” says Ed Macie, an urban specialist with the U.S. Forest Service in Atlanta, “compares with what’s going on in the world’s rain forests. Some regions have been urbanizing at a pace of over 50 forested acres a day, 365 days a year for over 20 years. That’s a pretty big deal.”
Every tree that's subtracted from a city's ecosystem means some particulate pollution that should have been filtered out remains. In Washington, that amounts to 540 extra tons each year.
Urban canopy helps absorb carbon dioxide, pull particulate matter from the air, prevent floods and keep temperatures at livable levels. How much tree cover a city needs depends on local climate, but in the U.S., the guidelines divide roughly along the Mississippi River, with cities to the east needing a 40 percent cover and cities to the west a less-leafy 25 percent.
All this hits the environment hard, starting with air quality. Every tree that’s subtracted from a city’s ecosystem means some particulate pollution that should have been filtered out remains. In Washington, that amounts to 540 extra tons each year. Simply replanting does not suffice because small, young trees require decades to grow to full size.
“A big tree does 60 to 70 times the pollution removal of a small tree,” says David Nowak, a project leader with the U.S. Forest Service’s Northern Research Station in Syracuse, N.Y. The crown of a large tree is also a freestanding antiflood reservoir, in some cases intercepting so much rainfall that more than 1,500 gallons a year evaporates instead of hitting the ground. Chop down the tree, and you increase the volume of storm water a city must manage—something that affects older cities with aging drainage systems especially severely.
In 2006, Los Angeles volunteers kicked off a campaign to plant 11 million trees over the next 30 years. Tree advocates are calling for similar efforts across the country.
Photo: City of Los Angeles
Heat Island Increases
In Atlanta, where developers bulldozed 380,000 acres from 1973 to 1999 — much of it heavily forested — temperatures have climbed 5 degrees to 8 degrees higher than in the surrounding countryside, according to NASA, which studies global hydrology and climate. Scientists fear the heavily developed corridor between Boston and Washington could be the next big hot zone.
Keep the Trees You Have
Local governments are finally responding to the problem. More than 2,000 big and small cities have launched long-term planting and preservation programs. For now, the most immediate answer is less the planting strategy than the preservation one, something that can best be achieved by curbing sprawl and downsizing our taste for too-big homes.
Source: Time magazine