The state of our urban forest.
Concerns over global warming and' air pollution are now beginning to focus interest-locally and nationally on urban forests. The American Forestry Association has led the effort to put city trees on the national agenda and has worked with hundreds of community groups around the country to raise local consciousness. AFA has been gathering data by doing everything from straining past research for relevant information to begging state forestry agencies to help us gather new data from the field. Now the rewards are coming in.
For the first time, researchers with AFA, the U.S. Forest Service, state forestry agencies, and the National Urban Forest Council have conducted a study employing a random sample of street trees in some 300 cities and towns. Even as I write this article, the results are coming in and undergoing evaluation, adding significantly to the rather skimpy body of knowledge accumulated to date. What is most alarming, the survey is showing that urban forests are starving for trees.
In a book published in 1978, Grey and Deneke estimated the size of the urban forests in the United States at 69 million acres. Urban areas have expanded greatly during the past decade, and the Soil Conservation Service in its 1982 natural resources inventory suggested that the urban forest has grown to 93 million acres. Obviously, not all developers have a heavy hand with their bulldozers.
Still, land classifications vary considerably from one locality to another, so that gathering reliable data from local agencies remains a challenge. At this point, we simply do not know the extent of the nation's urban forests. One thing we do know, from the current survey, is that the streets in American cities have about half as many trees as they could have. For every tree you see, another one could be planted. The current survey is also showing that in eight of nine categories of cities with populations under one million, the potential planting space is even greater. In those cities, nearly 60 percent of the roadsides are bare of trees.
Looking beyond the streets, the story is much the same. Potential growing areas in cities include parks, greenways, and the yards of homes, as well as street curbs. Most of the information about the condition of urban trees focuses on the trees growing between the sidewalks and the curbs because that is the area that is publicly owned and managed.
Dr. James Kielbaso of Michigan State University, one of the lead researchers in the National Urban Forest Council survey, estimated in a 1988 study on the management of urban forests that U.S. municipalities contain 61 million street trees. A forest that size is equivalent to an area approximately five times the size of Rocky Mountain National Park, if all the street trees were grouped together in one forest stand and spaced 30 feet apart, Kielbaso suggests.
Sixty-one million street trees may be enough to give us the feeling of a forest, but from the standpoint of tree cover, street trees make up only about 10 percent of the total forest canopy in cities. For every tree on the street, there are at least 10 more in parks and on private lots. (There are even more if small trees in the understory are counted.) That means a forest of 610 million city trees. Grouped together in one stand, they would shade an area the size of West Virginia.
Although our urban areas have enough trees per acre to qualify as a forest, the room exists in our cities for many more trees. The urban forest is too thin and needs to put some meat on its bones. When it comes to urban trees, we need to ask, "Where's the beef?" By "beef" I mean the leaf area. The environmental benefits of trees are directly related to the volume of their leafy crowns.
The beef can be calculated by measuring the canopy cover off aerial photographs of our communities. The percentage of tree cover is determined by taking into consideration other ground features such as buildings, concrete, and grass.
Rowan Rowntree, a researcher with the U.S. Forest Service, and his colleagues have studied the urban forest canopy in 11 cities. They found the average tree cover to be 30 percent.
The average city could accommodate a doubling of that cover to about 60 percent, which would triple environmental benefits. Since tree canopies have heights, widths, and depths, measurements of canopies are measurements of volume. Doubling the tree canopy therefore triples the leaf area and hence the amount of pollutants absorbed.
When it rains, the leaves and branches slow the movement of stormwater by 14 percent and can thus help alleviate many of the problems caused by heavy rains. These problems range from soil erosion to over-loaded sewer systems.
In studies published in Urban Ecology and Landscape and Urban Planning, Rowntree, McPherson, Stevens, and others estimated that the roadside growing space in American cities is 38 percent full. Kielbaso's figures suggest that these spaces are 45 percent full, and the American Forestry Association's street-tree quotient-a measure of potential street-planting space-is showing a 50-percent average from measurements in 15 cities.
Bean-counting aside, the estimates reveal a public roadside starving for trees.
In 1987 Kielbaso and I estimated the value of street trees at $25 billion. This figure was based on a formula, established by the Council of Tree and Landscape Appraisers, in which tree type, size, condition, and location are used to determine value. The estimates of tree cover employed in these formula calculations are conservative.
Using the commonly accepted multiplier effect of 10, the total value of the urban forest weighs in at $250 billion -a large figure but one that I expect is actually very much below the mark.
In recent years, estimates of the value of urban forests have explored various methods of calculation based on ecological and environmental factors. These estimates suggest an even higher measurement. As our knowledge of ecology increases, so do the estimates.
Take energy, for example. In the early 80s, a number of researchers estimated the cooling value of trees and based their estimates largely on the shading effects on a single building. But research by Akbari, Rosenfeld, and Parker-published in 1988-indicates that the cooling effect is much higher than expected when the figures are extended beyond a single home to an entire city.
Increased city temperatures-the so-called urban heat island (see AMERICAN FORESTS , September/October 1989)-are major contributors to global warming. Each increase in urban temperatures nationally costs $1 million per hour/per degree of increase. This was a figure well beyond what we as urban-forest researchers were conceiving just four years ago. Add other environmental factors such as stormwater management, sediment and erosion control, air pollution, and wildlife, and the value of the nation's urban forests far exceeds our past estimates.
While measurements of value climb, the quality of the urban forest does not. Air pollution, scorching sun reflecting off concrete and glass, oil, salt, and other pollutants from sidewalks and streets all add varying levels of stress.
So does the space available. Limited planting space, especially underground space, is probably the major factor in premature death. A too-small pit in a downtown sidewalk will usually put a stranglehold on the roots that will kill a tree slowly, but effectively.
Trees suffer severe damage also when metro buses participate in "unscheduled trimming" or when major surgery is performed to branches because building and utility lines are competing with the trees for the same space.
A noticeable improvement in tree health occurs progressively from downtown to the suburbs. Average lifespans increase from about seven years for a downtown tree to an average of 32 years for a suburban tree. (See Figure 1 on page 62.) Suburban lawns contain more room for growth, temperature extremes are more moderate, the air is less polluted, and runoff from streets and sidewalks is cleaner.
Differences in tree health also correspond to the size of communities. The larger the city, the less healthy is its forest. In 1986 the National Urban Forest Council surveyed 20 cities, a study that resulted in the shocking finding that four city trees are dying or have died for every one tree planted. (See AMERICAN FORESTS, june 1986.) The largest cities surveyed, which included New York and Chicago, were losing twice that many, while medium-sized cities were doing much better.
The 1988 study by Kielbaso indicates that the medium-sized cities care for their trees better and have a much better planting-to-removal ratio than the larger cities. The survye of over 300 cities now underway should dramatically improve our understanding of these conditions.
James Urban, a landscape architect from Annapolis, Maryland, is conducting a study of trees in downtown planting pits for the National Endowment for the Arts. So far, he has unearthed the roots of 1,200 trees, exposing them temporarily for examination, and measured the growth rates of many other trees.
Urban has determined that tree health and growth rates are very unstable when the amount of soil in the planting pit is less than 100 cubic foot. Growth rates improve markedly as the growing area is increased from 100 to 200 cubic feet. After that, the rate is less dramatic.
Since most downtown planting pits are somewhere in the 100-cubic-foot range or less, the short lifespans measured in past surveys are not surprising. Urban's studies also show that trees in small pits in downtown areas grow barely one-tenth of an inch in diameter per year and trees in the largest pits reach five-tenths of an inch per year. In contrast, the trees on my rural lot in Maryland are growing at 10 times that rate, or one inch a year, with no supernatural help from their owner. I attribute the difference to the quality and quantity of soil available.
While knowledge of the urban forest slowly increases, the outer edges of this forest are expanding at a lightning pace and challenging our ability to monitor the rate of conversion-let alone maintain the health of the resource in transition. Official measurements of the total area affected are lacking, but my 1987 estimates suggest 1.3 million acres are affected each year.
The most dramatic increases occur in the suburbs around major cities. In Anne Arundel County, Maryland, home of the state's capital and the U.S. Naval Academy in Annapolis, the 175,000 acres of trees that covered the rolling Maryland landscape in 1950 dwindled to about 110,000 acres by 1985. During the same period, Fairfax County, Virginia, on the outskirts of Washington, lost about 80,000 acres of trees.
In the Atlanta area, forestland and cropland in transition to urban uses was estimated at 50 acres per day in 1988. In Fulton County, just outside Atlanta, forest cover has decreased by almost 50,000 acres since 1972, and the rate of loss is increasing, according to the U.S. Forest Service.
Many counties in the West are experiencing the same trend. In King County, which borders Seattle, the Douglas-fir and hemlock forests that once covered the rolling countryside have largely been replaced with homes and business. Building permits increased from 864 in 1987 to 1,956 in 1989, and the forest cover decreased substantially, with 42 percent of the loss on the outer fringe of existing development.
New suburbs offer the best opportunity to improve the overall condition and health of urban forests. When suburbs are built on former forestland, the attributes of those forests can be incorporated into the new urban forest.
So far, measurements of our attempts to incorporate former forests successfully in suburbs have met with mixed success.
Efforts to save trees during development are clearly increasing. Communities all over the country have already passed or are considering tree ordinances requiring tree cover to be saved during development.
Major problems exist, however. The trend is to build larger developments, for one thing. For another, developers are tending to purchase and use larger earth-moving equipment, which limits their ability to retain the soil and topography critical to tree survival.
Right now a race is on between our desire to have more trees and our ability to maintain healthy populations for tomorrow's urban forest. A large percentage of the trees undergoing the transition from forestland to suburbia are damaged, leaving them open to insect and disease attack.
The gypsy moth, for example, is today one of the most feared urban forest pests in the nation. It has been around for over a century, having spread from a jar on a porch in Massachusetts to over half the states in the U.S. The gypsy is a devastating pest in the urban forest because the trees in cities are already stressed by pollution and high temperatures. If unchecked, defoliation by the gypsy moth larva adds the straw that kills.
Research is showing the value of trees in our communities, and it is revealing that our tree cover is much too low. The World Future Society predicts that the cities of tomorrow will be filled with trees and thus more pleasant and friendly to humans. AFA's Global ReLeaf program has set a goal for dramatically increasing the number of trees planted in cities and towns. We are calling for 100 million new trees by 1992, which will represent a 15- to 20-percent increase.
This goal has caught the attention and fired the hopes of many Americans. That was our first objective. But we here at the AFA have another goal as well, and that is to change the ecology of our cities by building bigger and healthier urban forests. Not only do we need more trees in our cities, we need better trees.
In a community with a tree cover of only 30 percent, a third of the total land area is green. Even so, the leaf and branch area is four times the surface of the buildings and sidewalks.
We need more tree cover shading our cities. The larger the trees, the more water they evaporate and thus the more they cool our cities through a kind of air-conditioning effect. Also, the bigger they are, the more carbon dioxide they absorb, and the more environmental good they do in general.
If we want larger trees, we will need bigger and better planting spaces and healthier trees. Although 100-percent cover is not desirable or practical, we expect the ideal tree cover would be about 60 percent, or twice what we currently have. The measure of our success tomorrow will be revealed in how thick the canopy of our forest is and how large the trees become.
What Do 100 Million Trees Mean?
Some foresters familiar with the fact that the forest-products industry plants a billion seedlings a year may wonder why the AFA's Global ReLeaf goal is to plant only 100 million trees around homes and buildings by 1992. The number sounds rather trifling until you stop and think that the AFA is talking about planting relatively large young trees rather than seedlings.
Moreover, the forest-products industry expects only a small percentage of their seedlings to survive and become canopy trees. Unlike rural trees, every tree planted in a city or town is likely to become a crown tree, simply because it has the space to spread out. Those large canopy trees are exactly the ones needed to combat global warming, since those are the ones that provide the greatest amount of cooling shade and tile air-conditioning effect of evaporation.
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|Date:||Nov 1, 1989|
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