FO: NAFC/2000/8(a)



Item 6(a) of the Provisional Agenda


St. Andrews, New Brunswick, Canada,
12-16 June 2000




CASE STUDY: The Ice Storm of 1998 and its Effect on Urban Forests in Canada

In early January of 1998, an extensive area of southeastern Canada and the northeastern United States experienced an ice storm of unprecedented impact and duration. The storm produced freezing rain over a period of six days, resulting in ice accumulations 11 cm thick in severely affected areas. In addition to seriously impacting public infrastructure, the event had a devastating effect on managed and urban forests, botanical gardens and specimen trees. Damage to trees in urban areas largely depended on tree location, age and crown architecture. Poor species selection and planting methods, especially the installation of trees with broad crowns and limited load bearing capacity in close proximity to utility lines, greatly increased the extent of the storm's damage.

Two years after the event, forest recovery in urban and rural areas is ongoing. Many municipalities affected by the storm have developed street tree replacement programs, with emphasis given to selection of species and cultivars that exhibit morphological characteristics of limited height, upright branching pattern and high load bearing capacity. A positive effect of the storm has been an increase in public awareness of the functional and aesthetic value of urban forests.

CASE STUDY: Management of Green Areas in Mexico City

The public green areas in Mexico City are an important civic resource and heavily impacted by users. In the past little thought was given to the type of species planted, funding for long-term maintenance, irrigation design, and visual quality of the landscape. As a result, management was costly. To rectify this situation a plan was created to improve the condition of these green areas and increase their financial self-sufficiency. Three aspects of the plan are Planning, New Technologies, and Maintenance.

Planning. Prior to the current administration Mexico City had 4 m2 of greenspace per inhabitant. Recently, creation of new parks has increased greenspace to 7 m2 per capita, but this amount still falls short of the recommended 9 m2 to 12 m2. The government has two programs to improve the situation: the rehabilitation of existing greenspace that has fallen into disrepair and the creation of new parks.

New Technologies. Several new concepts are being applied to the design and management of green areas in Mexico City. To avoid overcrowding all new trees are planted a minimum of 8 m apart. Soils that have been altered by addition of earthquake rubble are improved with compost. Plantings are mulched with chipped material from pruned plants and slow release fertilizers are added at the time of planting.

Subsidence of land in the old lake bottom of the Valley of Mexico has been compounded by increased impervious surfaces that reduce infiltration of rainwater. Pervious paving materials are now being used that have the same structural characteristics as regular asphalt and concrete, but allow runoff to infiltrate.

Another important innovation is "Naturization," the establishment of green areas where there is no space to plant trees because development is so dense. Rooftop gardens are the primary means of achieving Naturization. Drought tolerant species such as succulents native to the region are planted after the roof has been structural reenforced, waterproofed, and a drainage layer constructed into which the planter boxes are placed. These rooftop gardens keep the city cooler in the summer and trap air pollutants.

Maintenance. One of the greatest challenges in Mexico City is funding the maintenance of green areas. Although contractors maintain new plantings for one year, funds for subsequent care are often lacking. To create a more sustainable source of funding trust funds are now being established to support park maintenance. A portion of the funds generated from park concessions and advertising is used for maintenance and improvements. Local businesses make tax deductible contributions to the trusts and local councils determine how the funds are spent. Also, private businesses are directly participating in financing and maintenance by "Adopting a Green Area" that borders their land.

CASE STUDY: Using the Best to Make it Better: Applying the Best Practices of Urban and Community Forestry to Make Cities Livable and Sustainable

Urban and community forestry holds the key to saving our cities in ways that we could not have envisioned twenty-five years ago when TreePeople first got started. What began as simple tree planting has now grown into a project that extends to urban infrastructure management. T.R.E.E.S. (Trans-Agency Resources for Environmental and Economic Sustainability) was created to achieve an integrated approach to managing the urban ecosystem as an urban/forest watershed through multi-agency partnerships and an educated, empowered citizenry. This unified, systemic approach represents a new paradigm. It also requires profound new levels of education about how to live in a wholesome relationship with nature. It casts individuals and families in the role of stewards-or "urban ecosystem managers," and it allows agencies to serve as educators, facilitators, and monitors rather than as enforcers.

The T.R.E.ES. project has four main elements and goals: 1) re-design urban sites to function as mini urban-forest watersheds; 2) demonstrate that the designs actually work; 3) create a cost-benefit analysis and computer model that quantifies the environmental and social gains-or losses that would come from employing these designs on a large scale basis-i.e. managing urban infrastructure from an integrated watershed approach; and 4) bring the key agencies and stakeholders together to devise an implementation plan for financing and carrying out a large-scale retrofit of the watershed.

Fulfilling the vision of a sustainable city will not take any new money but only a different way of spending what is already being planned. Funds are being spent every day on both new projects and redevelopment projects that could instead be made available for watershed improvement. For instance, the Los Angeles area anticipates an investment of up to $20 billion over the next 10 years in water supply, flood control and stormwater pollution facilities.

The major differences between the kind of fixes spelled out in T.R.E.E.S. and the huge engineered fixes of the past are that this urban forest watershed approach requires more time for implementation and a high degree of public awareness and participation. However, the multiple benefits of safety, pollution prevention, economic development, and beauty resulting from this approach far outweigh the benefits of single purpose projects. The truly good news is that stakeholder agencies are investing in these approaches. For detailed descriptions of every phase of the T.R.E.E.S. project, see the web site at


Urban forests are green spaces within communities that provide services vital to enriching quality of life. As North American cities continue to grow, increasing numbers of people will choose to live, work, and play in urban forests. For this reason it is critical to understand how these forests can be planned and managed to provide the full range of benefits that are possible.

Urban forestry is the planning and management of trees, forests, and related vegetation within communities to create or add value. Urban forests add value to local communities because they are integral to: land use planning, mitigating water and energy shortages, improving air quality, protecting global climate, enhancing public health programs, increasing land values and local tax bases, providing job training and employment opportunities, reducing costs of city services, and increasing public safety. In North America the urban and community forestry paradigm is shifting from focus on beautification to one that encompasses all of the environmental, conservation, economic, and social benefits of community trees (Rowntree, 1995). This shift has been accompanied by increasing levels of local participation and new partnerships that link professionals, non-governmental organizations, industry, and government agencies.

This paper describes ways that urban forestry can address important environmental and social issues associated with urbanization. It identifies some of the ecological, economic, and political constraints to urban forestry that must be overcome before the benefits of urban forestry programs can be fully realized. Opportunities for the North American Forest Commission (NAFC) to promote more sustainable urban ecosystems, assist in development of regional urban forest plans, and promote collaborative watershed restoration projects are discussed.


In 1995 North America's urban population was 294 million or 74% of its total population (Roberts, 1996). The number of people residing in urban areas is expected to increase to 430 million or 85% in 2025. Although the pace of urban growth will be slowing in North America as a whole, rapid urban growth will continue in the poorest regions and those undergoing the greatest economic change. In other areas, populations will shift away from vast sprawling metropolitan regions to small or intermediate-sized cities. The benefits of urbanization include higher incomes and literacy rates, as well as increased health. However, with urbanization has come a host of environmental and social problems. Some of these problems are associated with urban poverty (i.e., sanitation, disease, access to clean water, food, and fuel supplies) and others with economic growth or affluence (i.e., air and water pollution, congestion, loss of biodiversity). The problems of pollution, poverty, and environmental hazards facing North American cities are similar to but less extreme than those faced by much of the world's population in developing world cities.


Urban forests are important because of their geographic extent, their impact on local economies, and their proximity to people. Collectively, urban trees in the United States account for nearly one-quarter of the nation's total tree canopy cover - forest products some 74.4 billion trees (Dwyer et al., In Press). The annual total impact of urban forestry on sales in California was $3.8 billion, while the state's commercial forest products industry had sales of $12.5 billion (Templeton and Goldman, 1996). Because of their proximity to people, urban forests can provide substantial environmental and recreational benefits to urban dwellers. Trees are a solar-powered technology that can help restore balance to dysfunctional urban ecosystems. Also, urban forests are strands in the urban fabric that connect people to nature and to each other. These strands weave through communities, and have the power to profoundly influence how people live at the scale of a family's yard and garden, the neighborhood, town, watershed, and region. Urban forestry is not a panacea for all the problems cities face, but it can make cities more livable.

Air Quality and Climate Protection - Although urban air quality has improved over the past two decades in most North American cities, worldwide more than 1.1 billion people live in urban areas with polluted air. Rising motor vehicle use, reflecting the increasingly sprawling form of many cities, poses the greatest threat to air quality. Urban centers, where large quantities of energy are consumed, are important sources of greenhouse gases that pose a threat to the stability of global climate.

Urban forests have a positive impact on air quality through deposition of pollutants to the vegetation canopy, sequestration of atmospheric CO2 in woody biomass, and reduction of summertime air temperatures and associated ozone formation. Urban trees can, however, reduce air quality by reducing the dispersion of pollutants within the urban canopy layer, emitting biogenic volatile organic compounds (BVOCs) that are involved in ozone formation, and indirectly increasing emissions of pollutants associated with tree care activities (e.g., chain saws, chippers, trucks). Although the net effect of urban forests on air quality has not been fully determined, results from modeling studies indicate that benefits can be substantial (Nowak, 1994; Taha, 1996; Scott et al., 1998). Moreover, urban forests are one of the most cost-effective means of mitigating urban heat islands and associated expenditures for air conditioning (Akbari et al., 1992; Simpson, 1998). In Mexico City, where greenspace is at a premium, rooftop gardens are turning heat traps into natural oases that cool the city and clean the air.

There are an increasing number of private and public sector investments in urban forestry for air quality improvement and energy conservation. Mexico City is reforesting the city's perimeter with 175 million trees to improve air quality and create a natural barrier to further urban expansion. Electric utilities in the United States and Canada have invested in shade tree programs to reduce peak demand, offset emissions of carbon dioxide, and better market their goods and services (McPherson and Simpson, 1999; Nowak, In Press).

Urban Poverty: Urban poverty is a serious problem in the largest North American cities because the poorest groups face the greatest exposure to biophysical threats (e.g., inadequate water, housing, sanitation), biological hazards (e.g., infectious and parasitic diseases), and social ills (e.g., violence, substance abuse, unemployment) and have the least access to protective services (Roberts, 1996). Extremely hot and cold weather conditions can threaten large numbers of people who live in buildings without adequate heating and cooling or live outdoors. Greenspace resources are often inadequate in areas where they are needed most as a source of shelter, food, fuel, building supplies, and income-generating activities. In some cities urban gardening contributes significantly to the food supply (Kuchelmeister, 1998).

Water Resources: Cities produce wastewater that requires treatment and polluted runoff that threatens human health, as well as the functioning of freshwater and coastal ecosystems. Although water is not scarce in North America, many large cities face water shortages. For example, Mexico City's rapid growth and dependence on groundwater has led to aquifer depletion, ground subsidence, and implementation of water conservation measures. In parks and other areas, pervious paving materials are used to increase infiltration of rainfall.

Trees intercept and store rainfall on leaves and branch surfaces, thereby reducing runoff volumes and delaying the onset of peak flows. Root growth and decomposition increase the capacity and rate of soil infiltration by rainfall and reduce overland flow. Urban forest canopy cover reduces soil erosion by diminishing the impact of raindrops on barren surfaces. By reducing runoff from small storms, which are responsible for most annual pollutant washoff, trees protect water quality (Xiao et al., 1998).

Recycling urban wastewater into greenspace areas can be an economical means of treatment and disposal, while at the same time providing other environmental benefits. For example, irrigated tree plantations or nurseries can be a safe and productive means of wastewater disposal. Reused wastewater can recharge aquifers, reduce stormwater treatment loads, and create income through sales of nursery or wood products. These hydrological services are attracting the attention of cities to forests outside municipal boundaries. Some cities own peri-urban forests that provide municipal drinking water, and others, such as New York City, have invested in conservation easements to protect watersheds instead of investing in new treatment facilities. Seattle owns a large tract of urban forest that it uses for land treatment of sewage wastes.

Solid Waste: Approximately 20% of the urban solid waste stream is yard (organic material from lawns, shrubs, and trees) and wood waste (Plumb et al., 1999). Recycling of this green waste can reduce the environmental and economic costs associated with landfill disposal. Returning green waste to the soil can benefit plant growth and conserve water. Portable mills are being used in pilot programs to create lumber that is used for products such as picnic tables, park benches, flooring, and veneer. In Mexico City, smaller woody material is chipped and applied as mulch. Non-woody material is composted and used as soil amendments and mulch.

Mental Health and Well-Being: City life is stressful. However, visiting green areas in cities can counteract stress, renew vital energy, and speed healing processes. The urban forest is where most people experience and learn about forests. Involvement in tree planting, management, and restoration of urban forests is, in itself, an important form of outdoor recreation with significant individual and community benefits. Humans derive substantial pleasure from trees, whether it be feelings of relaxation, connection to nature, or religious joy (Dwyer et al. 1992).

Loss of Biodiversity: Urban areas affect biodiversity through conversion of land to urban uses. Although the amount of land converted to urban uses is relatively small in North America, a trend is emerging: cities are rapidly expanding outward, consuming large quantities of land, increasing infrastructure costs, energy use, and air pollution. Expanding urban forests can serve as reservoirs for alien plant species that threaten native forest flora. For example, Norway maple (Acer platanoides), Scotch broom (Cytisus scoparius), and buckthorn (Rhamnus spp.) are invaders that pose significant barriers to restoration of native plant communities.


Although there is potential for urban forests to mitigate a variety of impacts associated with development, there are also a number of obstacles to overcome before significant urban forest benefits can be realized. These constraints run the gamut, from loss of planting space to lack of funding for tree programs (Nilsson et al., 2000). Resolving these limitations will require coordinated efforts among cities, regions, and countries.

Loss of Greenspace: Available growing space is limited in city centers, and this problem is compounded by pressure to convert greenspace, parks, and vacant lots into building sites. In the early 1990s approximately 3.7% of the remaining greenspace was lost each year to development in Mexico City (Chacalo et al. 1994). Recently, creation of new parks has increased greenspace from 4 m2 to 7 m2 per capita. However, this amount still falls short of the recommended 9 m2 to 12 m2. Studies by American Forests and others indicate that as temperate climate cities sprawl outward there is loss of tree canopy cover. Land around Puget Sound, Washington was once heavily forested and now has less than 20% tree cover (Glickman, 1999). This de-greening has resulted in loss of critical natural areas and the ecological services they provide.

Planning and Management: Constraints to planning and managing healthy urban forests have been described extensively in the literature (Meza, 1992; Tschanz and Sacamano, 1994; Kenney, 1996; Konijnendijk, 1999; Nilsson et al., 2000). They include:


Urbanization is occurring on a global scale, making urban forestry an increasingly relevant aspect of the forestry profession. Urban forests provide benefits that go well beyond their beauty (McPherson et al. 1999). They represent an increasingly precious form of "natural capital" and the ecological services they provide are vital to sustaining quality of life in our growing communities. Many of these services address issues that are regional and global in scope, such as air quality/climate change, water resource management, and impacts of urban sprawl on natural resources. At the same time, the essence of urban forestry is the people-to-land and people-to-people connections that can transform the way we live. Those connections are made at the grassroots level and they have profound implications for how people care for the land that nourishes their bodies and spirits. There are an imposing number of constraints that need to be overcome before the potential benefits of urban forestry can be realized. Perhaps most acute is the failure of society to fully value the services that judiciously planned and managed urban forests provide. Increasingly livable communities are economically powerful communities; places where a high quality of life attracts the best-educated and trained workers and entrepreneurs, where good schools and strong families fuel creativity and a sense of community. Urban forests are vital components of communities that are striving to be more than plots of bulldozed land, networks of roads, and collections of buildings. Initiatives are needed to demonstrate the potential for urban forestry to enhance quality of life in North American cities.

Sustainable Urban Ecosystems

For the last 50 years the establishment and care of urban forests has relied on the use of increasingly sophisticated machines, chemical formulations, and technologies with the goal of maximizing plant growth and appearance. Recently, increased environmental awareness has brought attention to some adverse impacts associated with this approach, such as detection of pesticide residues in ground water, sound and air pollution from small engines, and excessive green waste and water use. As the green infrastructure expands it is necessary to face the challenge of designing and managing landscapes as functioning ecosystems rather than "pictures." The concept of sustainable urban ecosystems recognizes the interconnection of natural resources, human resources, site design, building design, energy management, water supply, waste prevention, and facility maintenance and operation. Sustainable urban ecosystems are landscapes designed and managed to minimize impact on the environment and maximize the value received for the dollars expended in the long term. In principle, they are economically beneficial because the full life cycle of processes and products is evaluated and optimized.

Regional Urban Forest Plans

Leadership and vision that soars above jurisdictional boundaries is needed to realize the many environmental, social, and economic benefits urban forests can provide. Implementation of regional urban forest plans can foster multifunctional regional greenspace systems with connecting corridors and easy access. More efficient delivery of tree care services can result from greater collaboration among agencies. Multiple sets of policies, ordinances, standards, and specifications can be merged. Developing a shared awareness of the benefits healthy trees can produce among the business, utility, and public works communities can generate support for coordinated regional urban forest inventory, maintenance, and health monitoring programs.

Watershed Restoration Project in North America

One way to implement concepts outlined above is through parallel and collaborative watershed restoration projects. Three "sister" watersheds could be studied, one each in Canada, Mexico, and the United States.

Watersheds or catchments provide a definable organizing structure for study of a region's ecosystem. A central question that this work could address is "How does the quality of water, air, soil, vegetation, and wildlife habitat change as one travels from the headwaters of rivers to their confluence with downstream water bodies?" Answering this question requires understanding the individual and cumulative effects of urbanization and land management practices on land, air, and water resources (e.g., watershed health) along the urban-rural gradient. A second question is "What are the Best Management Practices (BMPs) for sustaining healthy watersheds in urban, suburban, and rural lands?" The last question is "How can international and national resources be best applied to facilitate local efforts to create landscapes for sustainable living?"

To address these questions a number of steps can be undertaken:

  1. Select 3 to 5 catchments along an urban-rural gradient for the demonstration in each region.
  2. Assist local collaborators to identify environmental indicators to assess the condition of water, air, forest, and wildlife resources.
  3. Develop volunteer-based inventory and monitoring protocols, as well as necessary training materials.
  4. Conduct training programs, collect data, and assess baseline conditions for each catchment. Center some plots on school sites and implement educational programs.
  5. Using GIS technologies already developed by American Forests and T.R.E.E.S., create maps that display ecosystem structure and environmental conditions for each catchment.
  6. Develop an Internet web site to display this information so that residents of each catchment can better understand changing conditions along the urban-rural gradient and differences among "sister" watersheds in each country.
  7. Model and display impacts of current ecosystem structure on fluxes of energy, water and materials. Assess extent to which vegetation and other natural resources regulate/mitigate byproducts of human consumption and quantify the value of ecological services provided by the urban forest.
  8. Identify Best Management Practices to improve environmental quality and develop computer-based tools (science education) that students and residents can use to assess impacts and cost-effectiveness of BMPs.
  9. Work with local citizenry to implement BMPs and develop "Demonstration Forests" within each catchment to highlight ecosystem management practices. Promote information exchange among sister watersheds.
  10. Develop media relations and fund-raising programs to promote reinvestment in management of urban and rural catchments.
  11. Evaluate program successes and failures and assess the transferability of this approach to other urban areas in developed and developing countries.

This initiative will develop, implement, and initially evaluate a community-based watershed restoration process in three highly urbanized regions. It will foster new partnerships among NAFC entities, within national forestry organizations (e.g., in the United States that is the National Forest System, State & Private Forestry, Research), as well as with a variety of urban constituencies. It will advance urban ecosystem science, develop shared goals for creating more sustainable urban ecosystems, prompt development of regional urban forest plans, and leverage new resources for management of our urban and rural forest lands.


International organizations such as the North American Forest Commission can be instrumental in providing support for initiatives that will demonstrate the role and potential of urban forestry. First and foremost is the need for coordinated efforts between countries, research and academic entities, public and private sectors, volunteer groups and individual citizens (Carter, Undated). Implementation of initiatives will improve the knowledge base, strengthen institutional capabilities, encourage local participation, and promote more integrated urban forest planning and management.


The author, Mr. E. Gregory McPherson, received very helpful comments on earlier versions of this paper from Drs. Ole Hendrickson (Scientific Advisor - Biodiversity, Canadian Forest Service), Rowan Rowntree (Senior Scientist Emeritus, U.S. Forest Service), Thomas Randrup (Senior Consultant, Danish Forest and Landscape Research Institute), Ed Dickerhoof (Urban Forest Research Liaison, US Forest Service), and Ing. Rubén Lazos Valencia (Executive Director of Special Projects, Natural Resources Commission, Mexico City). Simon Wilkins (Integrated Pest Management, City of Calgary) provided timely information on urban forestry issues in Canada.


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