Global problems, local solutions: measuring the value of the urban forest.

Farsighted and chronically green people have always known that the urban ecosystem provides multiple benefits to denizens of the world's teeming population centers. But to engineers, developers, and urban leaders, natural features often appeared to be impediments to progress. When trees were added back to the urban landscape - after development - they were seen as a soft benefit and an expense.

It has taken 20 years of research and the very recent application of high-speed computer mapping technology to begin to provide accurate and quantifiable information on the hard economic and ecological benefits of trees and forests in urban areas.

However, the impact of these new scientific findings would not be nearly as significant without the even more recent explosion of creative ways that ecologists, foresters, planners, and others are combining aerial photography with satellite images, ecological research, and computer technology for easy-to-use, cost-effective local applications. The coming together of the science and its popular application means that a whole new database of urban ecological and economic information is coming on-line. Using this information will be like opening an important new window on the urban environment.

In the forefront of the application of computer mapping and analysis to community environments is a system developed by AMERICAN FORESTS to demonstrate urban forests' hard-dollar value to their communities. At the heart of the system is a software package called CITYgreen, which uses Geographic Information Systems (GIS) software and sophisticated computerized mapping techniques to measure, map, and analyze urban ecosystems. Cutting-edge scientific findings about the functional values of natural resources are then applied to calculate the financial contribution of the urban forest and natural resources. The software provides the framework to convert natural resource values into public policy concerns by analyzing three key areas: energy conservation, stormwater, and air pollution abatement. The resulting Urban Ecological Analysis looks at the entire ecosystem, making it part of the planning process without pitting development against the environment. According to AMERICAN FORESTS' vice president Gary Moll, "When urban forests are viewed for their role in larger ecosystems, a wide range of values and benefits can be connected to them."

Population growth may not be this year's sexy environmental issue, but the increasing concentration of humanity in urban centers around the globe is creating many of the major crises facing us, and that includes crises in how we manage our natural resources to comply with federal, state, and local environmental laws. Urban populations are growing two and a half times faster than their rural counterpart. Today, almost 50 percent of the world's population lives in cities; by the year 2025, more than two-thirds will inhabit cities.

The 1992 United Nations Conference on Environment and Development, held in Rio de Janeiro, set a goal of sustaining economic growth while maintaining the essential integrity of the earth's ecosystems. The Conference recognized that while world leaders and national governments could support sustainable development, implementation would necessarily occur at the local and regional levels. Rene DeBois' philosophy of "Think globally; Act locally," a popular bumpersticker, says it all.

William E. Rees of the University of British Columbia has questioned whether the remaining stocks of natural capital are adequate to sustain the anticipated lead of the human economy into the next century. In the book Population and Environment, he defines the "ecological footprint" as "the corresponding area of productive land and aquatic cosystems required to produce the resources used, and to assimilate the wastes produced, by a defined population at a specified material standard of living, wherever on Earth that land may be located."

According to Rees, "Most economies are running massive ecological deficits with the rest of the planet..." using far more than their fair share of natural resources. The pat answer that technology and trade will solve these problems has not been demonstrated, Rees alleges. And as urban centers continue their rampant growth, these inequities will increase. When people change the landscape dramatically, the environment changes as well. Air and water pollution, flooding, erosion, rising urban temperatures, and increased rates of asthma and respiratory illness are but a few of the crises our urban environments have created or exacerbated.

Solving these problems seems like a big task for local government. But it's a task that can only be accomplished locally. Because ecosystems vary so much from region to region and from country to country, a one-size-fits-all approach won't work. What works in Florida is wrong for the Northwest. Sometimes the answers to complex problems are elegantly simple, but we fail to see them right in front of us. Such is the case with the urban forest: Sometimes, we can't see the forest for the trees.

In Milwaukee, Wisconsin, and Austin, Texas, AMERICAN FORESTS used CITY green to perform an Urban Ecological Analysis. The UEA used satellite imagery, low-level aerial photography, and ground truthing together with the software to produce a report that measured the work performed by the local ecosystem. Results were made available to both cities over the summer.

The UEA for Milwaukee was completed in June, and city and state officials are excited about its potential to create change. "The Urban Ecological Analysis quantifies what we knew intuitively," says Preston Cole, manager of city forestry services. "It's an excellent management tool because it moves us away from single tree management. Now we are looking at the total urban forest."

The forestry division of Milwaukee's Department of Public Works has an efficient approach to planting, caring for, and maintaining publicly owned trees that is highly regarded nationwide. In the 1960s, Dutch elm disease decimated the urban canopy; nearly 50,000 trees were removed. According to Cole, although the public forest has recovered remarkably well, only 20 percent of the urban forest is on public land. And private property owners have not reinvested in trees after removal.

"If we'd had numbers like these, a report like this," asserts Cole, "we might have been able to convince planners and policymakers that we needed to plan for the replacement of the totality of the city's trees, not just the public canopy."

The analysis found Milwaukee has an existing tree canopy of about 16 percent, with a wide disparity in the amount of cover across the city - from 1 percent to 42 percent. Nonetheless, the canopy was found to reduce stormwater flows by up to 22 percent, and the city saves an estimated $15.4 million by not having to build additional stormwater retention capacity.

Heavy rains last spring almost filled the city's retention facility, Deep Tunnel. Reducing stormwater flows by planting trees is an attractive supplement to the heavy capital investment required to increase physical retention capacity.

Poor air quality is another key issue in Milwaukee. Under the Environmental Protection Agency's standard for complying with the Clean Air Act, the city is classified as a non-attainment zone, requiring it to reduce volatile organic compound emissions, which include ozone. Steve Henniker, Milwaukee's environmental policy coordinator, is particularly excited about the potential to use trees as an ozone control strategy - something new for the city.

According to Henniker, Milwaukee experiences heat island effects similar to those documented by studies in Atlanta, where urban temperatures were shown to have increased 6 to 9 degrees since 1972 (see "Atlanta's Changing Ecosystem," Spring 1996). The increases corresponded to a severe decline - by 65 percent - in heavily forested areas in the region at the same time there was a dramatic increase in the built environment. These urban heat islands have a deleterious effect on air quality in several ways. They not only trap pollution but result in the increased use of air conditioners, which causes more carbon to be released into the air. Trees improve air quality in several ways. They reduce heat in general and thereby reduce the use of air conditioners, which emit carbon. At the same time, trees absorb and store carbon and remove numerous other particulates from the air. They also cool the air by evaporating water, much like an air conditioner.

The Urban Ecological Analysis determined the carbon sequestration provided by Milwaukee's urban forest amounts to 1,677 tons annually. By increasing the cover across the city to match existing well-canopied sites, this could be increased to about 4,800 tons per year.

The current direct summer energy savings from Milwaukee's existing urban forest was valued at $650,000 per year. It was estimated that by adding one mature tree in the right location at each home (west or east side and shading an air conditioner), the energy savings citywide would jump to more than $1.5 million a year. Peak-load demand would decrease as well. That's an important point in cities like Milwaukee, where when existing electricity demands cannot be met, the city must consider building a new facility, a huge expense.

One of the major contributions of the UEA is its focus on the total urban ecosystem. "The urban forest doesn't stop at an easement line," says Dick Rideout, state urban forestry coordinator. Wisconsin helped fund the UEA because it is looking for a viable demonstration of how neighborhoods and small communities can connect to address large problems together. "Besides giving us the language to speak to engineers, the UEA provides physical pictures of what's happening on a broad scale. When people can actually see it, it's easier to understand," Rideout says.

Down south, Austin, Texas - one of America's fastest-growing cities - is equally enthusiastic about the Urban Ecological Analysis it received from AMERICAN FORESTS in mid-August. Within two days of receiving the report, City Forester John Giedraitis presented the findings to the city's Resource Management Commission, which oversees citywide energy and water conservation programs.

Conserving energy is a high priority in a city that usually experiences more than 100 days of above-90-degree heat in a year. Austin has been one of AMERICAN FORESTS' Cool Communities since 1992. Designed to reduce energy consumption by increasing strategic tree planting and light-colored surfacing, the Cool Communities program spawned a new local program - NeighborWoods - that targets tree planting. According Jarr Fulkerson, an arborist who coordinates both programs, NeighborWoods brings together the resources of private industry, which pays for the trees, and Austin's residents, who plant and water them.

By the end of 1996, after just three years, NeighborWoods will have planted 10,000 trees toward its overall goal of 50,000. Fulkerson is hopeful the UEA can be used to persuade city officials to expand her organization's scope to include work with the private side of the urban forest.

The UEA estimated current direct summer energy savings from trees in Austin is valued at $6.3 million. Adding one mature tree in the right location at each home would increase savings citywide to more than $8.2 million and lower peak-load demand. Until recently, Austin's city-owned utility company offered a rebate program providing incentives for citizens to plant trees in energy efficient positions. It was discontinued because the cost-benefit calculation with the young trees didn't provide an immediate payout. Now, Austin has the numbers to demonstrate the long-term benefits of tree planting, and the Resource Management Commission has asked the program's director to consider reinstating the rebates.

Austin City Council member Gus Garcia recently attended an international conference on global warming, which he followed up with a town hall meeting attended by close to 200 citizens. Soon thereafter, the City Council requested a report on the sources and quantity of carbon pollution in the Austin area. Timing is everything. Austin's UEA has already documented that Austin's average existing tree canopy (approximately 30 percent) sequesters 5,728 tons of carbon annually. The report further indicates that by increasing the urban tree cover to match existing well-canopied sites, the total carbon sequestered annually could reach 10,000 tons.

Stormwater management is a huge issue for Austin, and the city is now developing a comprehensive master plan to guide future decisions on watershed improvements. With the UEA report showing the existing tree canopy reducing stormwater flow by up to 28 percent and providing the city with $122 million in benefits, Giedraitis is hopeful that some of the amount currently charged on utility bills to manage stormwater runoff might be earmarked for improving the urban forest.

Meanwhile, he's busy taking the report to every agency, commission, and public official he thinks will listen. "We have new arrows in the quiver and I'm ready to aim them."

Glee Ingram, chair of the Resource Management Commission, was thrilled with the report, which she said evens the playing field and will help to change policy. "Those of us who favor tree planting won't be relegated to 'tree hugger' status anymore." Ingram hopes to use the report to integrate tree planting, preservation, and maintenance into Austin's Green Builder program. This program educates and certifies builders in constructing energy and water-efficient homes with sustainable materials.

Of particular note to all those who have received the report are the overlapping benefits. "It allows us to tie into overarching social problems, to prove that trees are a serious part of the solution, and that there's a dollar value to the benefits they provide," says Giedraitis.

Cities that have used CITYgreen and benefited from the Urban Ecological Analysis have really just begun their work. But with this sharp-edged tool, trees and the role they play in the total ecosystem cannot fail to be part of the solution. Planners now have a monetary reason to conserve, plant, and maintain trees. The entire ecosystem has become part of the planning landscape. And putting a dollar value on our assets seems to make us treasure them even more.

RELATED ARTICLE: The Benefits of Community Trees

Oxygen Replenishment - During photosynthesis, trees convert carbon dioxide and water into carbohydrates and oxygen.

Carbon Dioxide Sequestration - To photosynthesize and release oxygen, trees absorb carbon dioxide from the atmosphere.

Groundwater Filtration - Trees' hair-like root fibers help filter groundwater by trapping contaminates such as nutrients and pollutants.

Pollution Control - Tree leaves and roots act as natural filters for air and for rainwater and groundwater, removing particulate matter.

Aesthetics - Trees beautify such urban and community areas as parks, streets, and schoolyards.

Education - Forested areas offer numerous resources as outdoor classrooms.

Mental Health - Trees provide soothing green scenery that has been shown to speed up patient recovery in hospitals.

Floodwater Control - Tree root systems hold in place soil that, if washed away by heavy rains, would flow into streams and rivers, making them shallower and allowing floodwaters to overflow protective banks.

Mineral and Nutrients, Cycling and Retention - Through growth, transpiration, and death, trees tie up minerals and nutrients from the air, water, and soil.

Climate Control - Trees work as natural barriers to wind, snow, rain, and solar rays, controlling climate in micro-areas.

Habitat for Wildlife - Trees and forests provide homes for many different species of animals and birds.

Physical Health and Recreation - The forest makes a great exercise area for hikers, hunters, and skiiers, and provides natural areas for those who like to birdwatch.

Natural Source of Medicines - Trees provide substances that have medicinal value, such as the active ingredients used in asthma medications and cough remedies.

Economy - The urban forest provides jobs for city foresters, park managers, and the arboricultural industry. Studies also show that trees create a welcoming environment that attracts shoppers to downtown business districts.

Soil Retention and Rejuvenation - Tree roots hold soil in place so it cannot easily be washed away by wind or water; the decaying of dead tree parts returns nutrients to the soil.

Lynn MacDonald is a freelance writer living in Berkeley, California.