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It is a vast and beautiful forest, acres of hillside dotted with oaks and tulip and sugar maple and sassafras, some trees as old as a century peering out onto the sparkling Hudson River. The location of this magnificent woodland is Inwood Hill Park, New York City. You wouldn't know it walking down a crowded avenue, but New York has over 5,000 acres of urban forest - and these make up just one small nook in the more than 70 million acres of American urban forest.
For many, the term urban forest - woodlands amid skyscrapers and concrete sidewalks - seems like an oxymoron, but the field of forestry is as old as agriculture itself, and it makes perfect sense. Our street trees, city parks, public gardens, tree-lined boulevards, town commons, and patches of undeveloped land weave together to form what some call the forest in which we live. This largely man-made forest provides myriad health, economic, ecological, and aesthetic benefits, and its ecosystem includes both nature and humans. Through restoration, protection, and management, urban foresters work to maintain a balance between the two and are making our cities better, one tree at a time.
The Urban Forest and History
The field of urban forestry came to prominence in the United States in the 1960s, jumpstarted in part by the ubiquity of Dutch elm disease and President Lyndon Johnson's urban beautification program, part of the 1965 Housing Act. That same year, Professor Erik Jorgensen at the University of Toronto first coined the phrase "urban forestry." Defined as "the cultivation and management of trees for their present and potential contributions to the physiological, sociological, and economic well-being of urban society," urban forestry is the art and science of caring for a city's forest ecosystem.
Humans have used trees as religious symbols, for firewood and timber, medicinal properties, and in landscape design, but their relationship to and studies of the urban forest are different.
Because of the actions taken by some city dwellers in response to overdevelopment in the 19th century, in the Northeast in particular, many of the trees in urban forests are older than trees in the outlying areas. Land may have been set aside for gardens or yards as development began, whereas areas surrounding cities might have been cleared for fuel and lumber, or deforested and made into farms.
And in most cities, there are pockets of land that somehow evaded the mighty hand of progress. "Most urban areas have places that were never developed," says Tim Wenskus, senior forester for the Natural Resources Group, a division of the City of New York Parks & Recreation Department. "Some of our best urban forests were left intentionally (as woodlots) or they were too steep or inaccessible to be used."
Because humans have inhabited urban areas for hundreds of years, urban forests allow us to study the effects of humans on the environment over time. "Looking at urban forests can help us understand how humans have affected them in the past, and how we can positively affect them in the future," says Steven Windhager, Ph.D., director of landscape restoration at the Lady Bird Johnson Wildflower Center.
Today's Urban Forest
Forty years after the phrase was coined, with over 80 percent of our population squeezed into cities, urban forestry is increasingly important. Most cities plant only one tree for every four or five removed each year, and more than 2,400 acres of land are converted from rural to urban use each day. The non-profit group American Forests says tree canopy in urban areas has decreased by 21 percent in the last ten years.
Urban sprawl or decentralization - development for low-density, automobile-dependent populations that expand and surpass the city limits - has annexed a lot of green space, pushing urban forestry issues to a head. And some cities are even suffering from the effects of what scientists call "the urban heat island effect," a rise in temperature due to the loss of green space which can make temperatures as much as 10 degrees higher in cities than in surrounding regions.
Urban forests not only reduce urban heat island effect, they can help purify city air, reduce heating bills through windbreak trees, absorb storm water runoff, lower noise pollution, and create oxygen. One study reports that an acre of trees can absorb 26,000 miles worth of a car's carbon dioxide emissions. "Trees are the lungs of our city," says Wenskus. "They filter polluted air and turn it into clean air, removing particulates, ozone, and sulfur dioxide. There are a lot of things in the air that trees filter out, and it's just part of doing their everyday business." Trees also help clean water, so surrounding a city's water supply with greenery can greatly reduce purifying costs.
The International Society of Arboriculture reports that trees increase property values between five and 20 percent, and a study of the Robert Taylor Homes housing project in Chicago showed that trees improved living conditions, increased a feeling of safety and promoted neighbor relations. Studies have indicated that trees can reduce violence and shorten hospital stays, draw shoppers to business districts, and increase occupancy rates in office parks.
Restoring the Urban Forest
Despite the urban forest's increasing notoriety, urban forestry remains a difficult and delicate science. It's a multidisciplinary undertaking, combining the fields of botany, biology, anthropology, landscape architecture, urban planning, and even sociology. The practice requires cooperation on the part of many groups, and it takes a long time both to implement projects and measure their success, especially where restoration is concerned.
"Urban foresters increasingly are people who understand not just arboriculture, but who understand the whole ecological system - the trees and the soil and the understory, the whole system that supports those trees," says Alice Ewen Walker, executive director of the National Alliance for Community Trees.
Restoration is the process of returning an ecosystem to its natural state - or as close as foresters can get it - and can mean replicating certain natural processes that have halted. In Seattle, for instance, the parks and recreation department draws up individual vegetation management plans for each park to help restore native vegetation and wildlife and manage growth and development: many parks are overwhelmed with invasive blackberry and English ivy, endangering native hazelnuts and Indian plum trees and the animals that feed on them.
The Chicago Wilderness Coalition does more than thin, remove, and prune invasive trees; they also burn between 10 and 20 percent of their more than 250,000 acre nature preserve every year. "We implement prescribed burning to set back invasive woody species that moved into communities when fire was stopped," says Jim Anderson, natural resource manager for the Lake County Forest Preserves, part of the coalition. It's what they call restoring "nature's house cleaning." Restoration mimics or reinvents dormant or extinct ecological functions or processes that help remove invasive plants and promote propagation of native flora and fauna. This might mean prescribing fire, or allowing areas to flood, or uprooting aggressive non-natives.