By James C. Schwab, AICP
Chicago, IL (March 1, 2009)- Municipal budgets being what they are these days, why not invest in trees? Trees provide most of the green infrastructure needed to manage air and water pollution, saving communities money while helping achieve environmental goals. Trees are good for our health. And they’re nice to look at.
Many cities are putting these ideas to work in urban forestry programs. In fact, identifying best practices in urban forestry was the goal of a three-year study managed by APA’s Research Department that resulted in the new Planning Advisory Service Report, Planning the Urban Forest: Ecology, Economy, and Community Development, published in January 2009. Central to this study was the effective collaboration of planners and tree professionals, including both arborists and urban foresters.
Growing knowledge base
Richard Hedman, in his 1977 book Stop Me Before I Plan Again, included a cartoon showing a client meeting with three consultants. The client says indignantly, “We give you guys $65,000 and 18 months and all you can tell us is that ‘Trees are nice.'” Today, urban forestry researchers are telling us much more, in large part because our tools for monitoring the urban forest are far more sophisticated than those available a generation ago.
Satellite imagery now allows us to measure the degree of canopy cover over an urban area at various times of the year and to detect heat differentials and urban heat islands. It works on a broad scale with Landsat, which has been orbiting since 1972, providing low-resolution imagery that can help document changes in tree cover over time. But newer satellites offering much greater resolution allow planners access to tree-specific detail at the site level – which can help decision making on specific development projects.
Aerial photography also has improved. We know more about the ecological functions performed by the urban forest. This, in turn, allows us to judge more precisely what works and what does not, and to establish reliable baseline data before setting urban tree canopy goals. Those goals are target percentages related to the amount of urban surface covered by the full canopy of all the trees growing in a city.
The U.S. Forest Service and some private forestry organizations have developed analytical software to help communities measure their local urban forest. CITYgreen is a GIS-based software tool-developed by American Forests, a Washington-based nonprofit – used to analyze the ecological and economic benefits of tree canopy and other landscape features. Its strength lies in placing dollar values on the ecosystem services rendered by trees within defined areas.
Ecosystem services is a relatively new term that refers to environmental functions provided by natural landscape features such as absorbing stormwater runoff, removing air and water pollutants, and sequestering and storing carbon. CITYgreen also can track changes over time and develop alternative scenarios. CITYgreen uses datasets derived from aerial and satellite photography, whereas another program, i-Tree, relies on field data. The latter is a suite of tools developed by the Forest Service and available free of charge; it is used to analyze the urban forest to determine tree management needs.
All this, together with the steady evolution of urban forestry as an academic discipline, has provided far more data than was ever available before. Now the question is how to translate this information into local plans and policies that create more livable communities. In many ways, solving that problem relies much more on planners’ political, administrative, and communication skills than on technology.
What trees do
Through photosynthesis, trees absorb atmospheric carbon. Carbon dioxide is the most prevalent, though by unit not the most powerful, greenhouse gas producing global warming. This absorption is known as carbon sequestration. Equally important is the storage of carbon in trees and other plants. The latter is important because the stored carbon is released when forests are cut or burned-the cause of anxiety about development occurring in the Amazon. Cities should look to their urban forests as a means of reducing their carbon footprint. Urban forests may also play a role in carbon trading as that market evolves.
However, trees serve a much more immediate and very measurable function in reducing energy consumption when used properly for shading and wind breaks. Studies conducted over the past 20 years show reductions in air-conditioning costs ranging up to 40 percent when shade trees are located strategically to minimize summer heat load from the sun. Reduced demand for energy from fossil fuels also reduces air pollution.
On a larger scale, the urban forest can also help to mitigate the human discomfort caused by urban heat islands, which result from concentrations of heat-generating buildings and heat-trapping concrete pavement. The shading of parking lots and pavement by trees both cools the microenvironment immediately beneath the canopy and induces light breezes with the heat differential it produces. The temperature differential will vary depending on the size and type of tree and time of day, among other factors, but can easily be several degrees. However, it is important to choose trees that will perform best in particular urban settings because trees, too, can suffer from too much heat.
In addition to helping cut down on carbon emissions globally, trees may aid urban air quality. Forest Service research by David Nowak in 55 U.S. cities examined the capacity of the urban forest to remove five key pollutants: sulfur dioxide, nitrogen dioxide, ozone, particulate matter, and carbon monoxide. Because smog formation is directly related to urban air temperatures, the results become more impressive in warmer cities. (Heat is a factor in the formation of ozone in the lower atmosphere, the key ingredient in smog.)
In Atlanta, for instance, trees annually remove 19 million pounds of pollutants, performing a service worth an estimated $47 million, based on externality costs established by the state utility commission. Even cities farther north get a few million dollars’ worth of benefits when trees mitigate the pollution that accompanies summer heat waves. This filtering service increases the pollution reduction gains resulting from the reduced energy use already described.
Because cities are driven both by infrastructure costs and U.S. Environmental Protection Agency mandates to find better ways to manage stormwater, trees have drawn attention as a particularly efficient way of addressing the problem. The EPA in 2003 issued its Phase II regulations for municipal stormwater under the Clean Water Act, placing cities of 50,000 to 100,000 people under the same tent previously shared by larger cities regulated by Phase I.
Both sets of municipal stormwater permits allow cities to include urban forestry in their best management practices. This lets smaller cities take advantage of new science as well as economic studies documenting the stormwater management benefits of green infrastructure.
Strategically locating segments of the urban forest within watersheds can also reduce runoff significantly, allowing rain to percolate through pervious surfaces to recharge groundwater while preserving surface water quality in streams, rivers, and lakes.
This step alone can reduce water treatment costs. Before precipitation even reaches the ground, however, the leaves, branches, and trunks of trees are absorbing a good deal of water, some of which will evaporate. The amount ranges from 12 to 48 percent, depending largely on climate and which species are involved.
A basic tenet of urban forestry is that several factors are involved in stormwater runoff reduction, including local topography, hydrology, and patterns of development. A study done in 2000 in Dayton, Ohio, found that the existing forest reduced runoff by seven percent, and that a modest increase in canopy cover could increase that reduction to 12 percent.
Down to basics
One other ecosystem service provided by the urban forest has long been understood, whether or not it has always been highly valued – wildlife habitat. For city dwellers, the opportunity to observe and interact with birds, butterflies, squirrels, deer, and other species is an amenity whose value can be hard to measure but is nonetheless very real. Preserving stretches of forest along common migration corridors, such as floodplains, makes the survival of various species in urban settings more likely.
That human hunger for contact with nature relates to a set of psychological and public health benefits that has long been underrated but is now the subject of serious scientific study. It is also the focus of public policy initiatives such as Connecticut’s “No Child Left Inside,” an effort to boost family use of parks and other outdoor recreational opportunities.
Environmental justice is also a factor in discussions of urban forests. It is widely observed that asthma rates tend to be higher in low-income and minority families, and it is common to find their neighborhoods less favored by urban parks and forests. At the same time, University of Illinois researchers Elizabeth Kuo and William Sullivan have studied the social behavior of inner-city public housing residents and found that residents with attractive outdoor settings tend to form safer social ties and experience less violence.
Studies of this sort suggest the need for more effective partnerships between residents of such communities and planners and urban foresters, who can help to deliver fairer solutions to a lingering problem of environmental inequity. In fact, such partnerships have been taking shape in a number of cities, including New York City and Los Angeles. Tree-planting exercises with school children, in particular, have helped to foster new awareness about creating healthy physical surroundings.
Why planning matters
The question that arises with this growing body of scientific knowledge is how best to put it to use through public policy. The purpose of the new PAS Report was to try to answer that very question by examining best practices across the U.S. in integrating urban forest concerns into the planning process. This included the preparation of 13 case studies of communities with successful programs, examining what drove their creation, who is involved, how they are funded, and how they achieve their goals.
Planners and foresters need each other in order to do the job right. Few planners are trained to know the details of which tree species grow best in which locations, and they are not engaged in much of the research that is documenting trees’ environmental and economic benefits. What planners do know well is how to intervene in the decision-making process to ensure that such information gets a fair hearing and that decision makers understand the consequences of choices about managing the urban forest.
Simply instilling in the public mind the idea that trees can be a wise investment-rather than a money sink-if a local program is managed and supported correctly, may be a significant contribution for planners to make.
As part of the project, nine outside experts, a mixture of planning and forestry professionals, joined staffers from APA, the Forest Service, and APA’s two project partners, the International Society of Arboriculture and American Forests, for a symposium in March 2006. One product was a set of guiding principles for the project, which are included in the PAS report. Among the most notable are those described below.
One was the notion that local tree ordinances should be incorporated into the development code. That would force the issue of relating tree protection directly to development activity, so that it is not seen as an isolated activity or ignored by developers and building and zoning officials. Places like Columbus, Georgia, have taken that idea to heart after realizing that earlier, weaker codes simply were not producing adequate enforcement. Columbus provides badges for tree enforcement personnel, who are authorized to visit development sites and halt development activity if they find noncompliance with tree protection requirements. The ordinance spells out specific tree density requirements for various types of development. Arborists are also incorporated into the planning process so that they can discuss tree protection needs directly with city engineers and planners.
The best way to ensure that expert tree advice is part of planning is to give foresters a legally prescribed role in signing off on new development plans. That way, the forester gets the chance to suggest ways of saving some trees while allowing the removal of more vulnerable ones. Only subdivision or other site plans that meet strict tree planting and maintenance requirements would be approved. In Urbana, Illinois, the city arborist automatically reviews development applications because his role is spelled out in the city’s development code.
If plan review is important, so is adequate enforcement during the construction phase of a development project. Someone must not only know what was promised on paper but visit the site to see that proper procedures are being followed to protect designated trees, to see that promised plantings are done correctly, and to ensure that plans are adequate for long-term maintenance.
Of course, the big picture is important as well. Some type of green infrastructure element can be included in the local comprehensive plan, and trees can be mentioned early in the community’s visioning and goal-setting process. The PAS report includes a separate appendix on how to incorporate a green infrastructure element into a comprehensive plan, drafted by Donald Outen, aicp, of Baltimore County’s Department of Environmental Protection and Resource Management.
Future of the urban forest
What does the future hold for this conjunction of urban planning and the urban forest? The answer is probably threefold: technical, human, and strategic.
The technical answer is that our tools are likely to improve, and local planners with access to satellite imagery, aerial photography, and other such means of monitoring the urban forest will be able to integrate those data more fully into their planning models. Those models will increasingly include all scales within the community or region they serve – the overall canopy, the neighborhood, and the trees within the individual development site, allowing planners and allied professionals to track progress both grandly and minutely.
Success, though, will rely on human interaction as planners forge working relationships with tree professionals, both arborists and foresters, as well as the public works officials who often oversee local forestry agencies. In the case studies for the PAS report, it was clear that the best programs established a role for foresters in the development approval process, but also allowed foresters and planners to collaborate on larger green infrastructure problems as well. In this budding relationship, there is great opportunity for each profession to learn from the other.
Finally, as a result of that collaboration and wise use of the latest economic and scientific studies, cities can strategically place urban forestry at the fore, moving it away from its old, tired status as an aesthetic luxury. That old thinking has long guaranteed that forestry serves as a target for budget cuts when the economy is suffering. The latest downturn is one in which trees can provide lasting answers.
Jim Schwab is the manager of APA’s Hazards Planning Research Center, a senior research associate, and co-editor of Zoning Practice. He served as the project manager and general editor for Planning the Urban Forest.
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