America's urban forests: growing concerns

Take a close look at the trees in your neighborhood - they're a good indicator of the overall health of the ecosystem in which you live. Large healthy trees mean that, most likely, soils, wildlife, and other ecological indicators are also in good shape.

Likewise, tree cover can be used to measure the health of a community. Urban areas are expanding rapidly, and it is this expansion that has the biggest impact on our nation's forestlands. As cities sprawl, forested areas decrease and decline. Urban expansion and the resulting lack of tree canopy coverage are the most significant challenges facing public policymakers and ecologists today.

Because of this, AMERICAN FORESTS is embarking on an effort to look at not only individual cities but the state of our urban forests as a whole. What is needed, we believe, are guidelines a community can follow to optimize its canopy cover and the attendant benefits - stormwater management, increased energy efficiency, wildlife habitat, and improved air and water quality. (Canopy coverage is a measurement of how much of an area is covered by the leaves in the crown of its trees.)

While tree cover will vary across a city, AMERICAN FORESTS recommends cities set a canopy cover goal of 40 percent overall - the equivalent of 20 large trees per acre. The core central business district should strive for 15 percent coverage, urban neighborhoods and fringe business areas for 25 percent. Suburbs, which have more growing space for trees, should be able to reach a tree canopy of 50 percent.

Increasing the tree cover in cities to the recommended level would improve the quality of air, water, and soil. In a metropolitan area with a population of around 2 million, planting more trees would mean a savings of more than $2 billion a year in stormwater management costs.

Years of Analyzing Data

Why 40 percent? AMERICAN FORESTS arrived at that figure after analyzing the tree canopy in dozens of cities over the last five years and working closely with the research community. Metro areas most likely to achieve that 40 percent figure would have an ideal mix of central business district, urban residential, and suburban property. Your community's totals may fall slightly above or below that average, depending on land-use, as shown in the examples that follow.

AMERICAN FORESTS has begun surveying individual cities, determining how the tree cover there has changed as the city has grown, and assessing both the value of what remains and the potential that exists if canopy coverage is increased. The findings will be released as the first in a series of State of Our Forests reports, this one focusing on urban areas.

By studying representative sections of the city and some surrounding areas, AMERICAN FORESTS has been able to estimate average tree cover. The results are sobering from the five cities studied so far: Atlanta, Georgia; Austin, Texas; Baltimore, Maryland; Miami/Dade County, Florida; and Milwaukee, Wisconsin. All but one - Austin - fell far short of the recommended goals.

Because they are lacking in tree canopy, communities such as these will experience a direct increase in costs for energy usage and stormwater management. They will also experience deteriorating air quality. For example, a neighborhood near Baltimore with a 40 percent tree cover could reduce stormwater runoff by about 60 percent more than a neighborhood without trees. It's clear that regreening our urban neighborhoods is no longer optional. Trees are a vital preventative measure in protecting the health of our cities.

How Communities Stack Up

Atlanta: Olympic Boost

Tree cover for Atlanta and the surrounding counties averaged only 27 percent. Atlanta's downtown recently received a boost in planting as a result of the Olympic games. More than 12,000 trees were planted downtown; as they grow they'll increase tree canopy in the hottest part of the city by about 3 percent. With a 15 percent target for the central business district, that extra 3 percent will help a great deal.

A low canopy cover is especially alarming in a rapidly growing area like metro Atlanta. Sixty percent of the natural tree cover there has been removed over the last 20 years, with about a third of that loss occurring in a seven year period. This most recent tree loss is equivalent to a $2 billion increase in stormwater management costs.

Austin: Hot but Green

Austin, the top scorer in the survey so far, averaged 34 percent-an impressive score for a city in the arid Southwest. The drier parts of the country should not be expected to have as dense a tree canopy as lusher areas in the East and Northwest. The goal for the Southwest and Plains states should be decreased by 10 percent to 30 percent overall; in other words, their overall tree coverage should be considered good when they achieve a 30 percent canopy

Baltimore: Old City, Old Problems

Tree cover was 25 percent in Baltimore, but add a ring of suburbia and that number averages 31 percent. Baltimore, like many old cities in the East and Midwest, has limited growing space for trees. Row houses are common and, like Milwaukee, heavy industry has occupied a significant portion of the city. This lack of space for trees is a problem Baltimore must work to correct. Urban expansion in the Baltimore/Washington metro region has increased 135 percent since 1950 and continues to grow.