America's urban forests: growing concerns.

AS CITIES SPRAWL, TREE COVER DIMINISHES AND WHAT'S LEFT DECLINES. SOME GUIDELINES FOR HEALTHIER COMMUNITIES.

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.

Milwaukee: Good Program, Tough Challenges

Milwaukee presents a particularly interesting picture. With one of the country's most progressive tree-management programs, Milwaukee has rated at the top in past surveys of street trees. But measure the entire canopy within the city limits including both public and private lands - and you get a figure of only 11 percent. Adding a suburban buffer to city measurements increases canopy cover to 18 percent.

There are several reasons for this low number, including the city's gradual recovery from the loss of its elms to Dutch elm disease. Also, as mentioned above, much of the land within city limits is zoned business and industrial. A detailed analysis most likely will show Milwaukee should set its sights on increasing tree cover within city limits from 11 percent to about 20.

Miami/Dade County: Andrew's, Legacy

The effects of Hurricane Andrew continue to be felt in southern Dade County, Florida. The devastating 1992 hurricane left Miami and metro Dade with a tree cover of just 10 percent, but it made residents there realize just how valuable tree cover is in their tropical climate.

A landscape ordinance passed in 1995 specified the need to plant trees where they will be most beneficial for energy conservation. Replacement of those lost trees has proceeded rapidly, and the trees that have been planted are in optimum locations, enabling residents to realize the full value of those trees when they reach maturity.

Making Better Use of Trees

AMERICAN FORESTS' goal is to develop standards and techniques for a new kind of assessment of the urban environment, a "State of Our Urban Forests" report to help decisionmakers in our cities better use trees and take advantage of the natural capital Mother Nature can provide. Scientific research has documented the benefits that come from natural systems and the folly of removing them from the urban landscape. Sustainable cities need to incorporate nature's design; our tree canopy guidelines are a logical means of beginning that process.

Over the past 10 years AMERICAN FORESTS has issued three reports on the state of urban forests. The focus historically was on street trees - typically 10 percent of the community's tree canopy - because those were the only ones for which data was kept. But thanks to new computer technology, AMERICAN FORESTS has now devised a way to map, measure and analyze an entire urban ecosystem - all the trees and many of the associated resources - resulting in an accurate and graphic description of the landscape. This is especially important to community leaders because it provides them with valuable information about the dollar benefits produced by existing natural systems.

The new technique uses satellite images, aerial photography, and on-the-ground surveys to gather and analyze information about local ecology. We can measure the work a tree does for its community by identifying various ecological structures using Geographic Information Systems technology and then calculating a dollar value for each component.

While coordinating a research project for the U.S. Forest Service in 1993, it became clear to AMERICAN FORESTS that the status of an urban forest should be as a whole, rather than just the health of street trees. To address this need we developed an Urban Ecosystem Analysis technique that not only measured local ecology but provided information to communities in land-use-planning language so that natural resource information could be incorporated into the decisionmaking process. While the technique was successful, the hardware and software were expensive, putting it out of reach financially for many communities and local organizations.

That obstacle was overcome with the development of desktop GIS software by a California company, Environmental Systems Research Institute (ESRI). AMERICAN FORESTS created an application for that software, CITY green, which we use - and which communities can use themselves - to analyze the value of urban ecosystems. We will continue to release our findings in a series of reports on the state of America's urban forests.

Thousands of city dwellers long for communities that are greener, more beautiful places to live. Striving for increased canopy cover is one way to help make the places where we live healthier and more sustainable, and in the process begin to create cities by nature's design.

Gary Moll is vice president of AMERICAN FORESTS' Urban Forest Center.