Abstract
We present a model for the development of sustainable urban forests. The model applies general principles of sustainability to urban trees and forests. The central tenet of the model is that sustainable urban forests require a healthy tree and forest resource, community-wide support and a comprehensive management approach. For each of these components, we present criteria and indicators for assessing their status at a given point in time. The most significant outcome of a sustainable urban forest is to maintain a maximum level of net environmental, ecological, social, and economic benefits over time.
Creation and management of urban forests to achieve sustainability is the long-term goal of urban foresters. The notion of sustainability in urban forests is poorly defined in both scope and application. Indeed, the question of how to define sustainability, and even whether it can be defined, is an open one (9, 12). At a simple level, “a sustainable system is one which survives or persists” (5). In the context of urban forests, such a system would have continuity over time in a way that provides maximum benefits from the functioning of that forest.
Since there is no defined end point for sustainability, we assess sustainability by looking backwards, in a comparative manner (5). In urban forests, we measure the number of trees removed against those replanted or regenerated naturally. In so doing, we assess progress towards a system that “survives or persists.” Therefore, our ideas of sustainability are “really predictions about the future or about systems … (5).”
This paper presents a working model of sustainability for urban forests. We describe specific criteria that can be used to evaluate sustainability, as well as measurable indicators that allow assessment of those criteria. In so doing, we accept sustainability as a process rather than a goal. As suggested by Kaufmann and Cleveland (12) and Goodland (5), we consider social and economic factors as well as natural science. Goodland believed that “general sustainability will come to be based on all three aspects” (social, economic and environmental). Maser (14) described sustainability as the “overlap between what is ecologically possible and what is societally desired by the current generation”, recognizing that both will change over time.
Therefore, our approach integrates the resource (forests and their component trees) with the people who benefit from them. In so doing, we acknowledge the complexity of both the resource itself and the management programs that influence it. We also recognize that communities will vary in both the ecological possibilities and societal desires.
Defining Sustainability
In developing a model of sustainable urban forests, we first examined how other sustainable systems were defined and described. Although we have concentrated on forest systems, other examples were considered. While some principles of sustainable systems were directly applicable to urban forests, others require modification or were in conflict with the nature of urban forests and forestry.
The Brundtland Commission Report (21) has generally served as the starting point for discussion about sustainable systems. It defined sustainable forestry as:
“Sustainable forestry means managing our forests to meet the needs of the present without compromising the ability of future generations to meet their own needs by practicing a land stewardship ethic which integrates the growing, nurturing and harvesting of trees for useful products with the conservation of soil, air, and water quality, and wildlife and fish habitat.”
Both Webster (22) and Wiersum (23) examined this definition from the perspective of forest management. They recognized that issues of what is to be sustained and how sustainability is to be implemented are unresolved. Wiersum ( 23) acknowledged the historical focus on sustaining yield and its recent broadening to sustainable management. Webster (22) suggested a need for focus on the issue of scale: the size of the area or space to be included.
Further refinements in the Brundtland Commission’s definition of sustainability were made by Salwasser (16) and Sample (17). Salwasser (16) described sustainability as:
“Sustainability means the ability to produce and/ or maintain a desired set of conditions or things for some time into the future, not necessarily forever.”
Salwasser (16) included environmental, economic and community based components, acknowledging that sustainability is not simply a resource matter. He also stressed that the goals and objectives for forest management cannot exceed the biological capacity of the resource, now and into the future.
Sample (17) focused more closely on forest management, emphasizing the need for shared vision among diverse property owners. In a workshop on ecosystem management, Sample described sustainable forestry as:
“Management and practices which are simultaneously environmentally sound, economically viable and socially responsible.”
Some definitions of sustainable forests are not directly applicable to urban settings. For example, the description presented at the conference on Sustainable Forestry (18) included comments about capacity for self-renewal. Since regeneration of urban forests must occur in a directed, locationspecific manner, use of such a definition is inappropriate.
Other definitions consider the goal of sustainable forests in a manner inconsistent with our concept of urban forests. Thompson et al. (20) described sustainability as “programs that yield desired environmental and economic benefits without wasteful, inefficient design and practices.” While these authors were interested in urban settings, their approach was limited to municipal forestry programs rather than city-wide processes or results. Dehgi et al. (6) focused on California’s native Monterey pine forest and restricted their definition of sustainability to that system. Moreover, their interest was limited to sustaining the “natural dynamic genetic process.” In another approach, the American Forest and Paper Association’s Sustainable Forestry Initiative (1) is largely aimed at industrial forest practice and products. This focus on industrial forestry seems largely incompatible with urban environments.
Given the examples noted above, the role of humans in sustainable systems (including forests) is generally accepted. However, Botkin and Talbot (2) (as criticized by Webster) argued that sustainable development of tropical forests requires non-disturbance by humans. Again, this idea is incompatible with urban forests.
Applying Concepts of Sustainable Forests to Urban Forests
In moving the concepts of sustainable development of forests towards implementation and practice, Webster (22) raised several significant questions. We have considered these questions from the urban forest perspective:
What objects, conditions, and values are to be sustained?
In urban areas, we focus on sustaining net benefits of trees and forests at the broadest level. We are sustaining environmental quality, resource conservation, economic development, psychological health, wildlife habitat, and social well-being.
What is the range of forest activities that contribute to sustainable development?
Simply put, urban forests require a broad set of activities, from management of both single trees and large stands to education of the community about urban forests and development of comprehensive management plans.
What is the geographic scale at which sustainable development can be most usefully applied?
Political borders do not respect biology (and vice versa). Principles of ecosystem management argue for a scale based on ecological boundaries such as watersheds. However, cities form discrete political, economic and social units. We must respect the reality that political borders may be more significant to management than ecological boundaries. Urban forestry programs work within this geographical framework.
For this project and model, we have chosen to focus on the city and its geographic limits. While this approach may violate some of the biological realities of forest stands, it logically reflects the jurisdictional boundaries and typical management units found in cities. The more common alternative approach, working with ecosystems, is not without problems of definition and scale (7).
What is the relationship of sustainable development for (urban forests) to new technology, effectively applied research and investment in forest management?
Urban forests stand to benefit tremendously from new technology, information and investment. Not only will the ability to select and grow trees in cities be enhanced, but the ability to quantify the benefits accrued by their presence will expand.
Wiersum (23) provided an in-depth look at sustainability in forest systems, noting the long history of the concept in forest practice. Many would argue that the concept of sustained yield is not equivalent to sustainable development. Gatto (9) discusses this fact at length. However, Wiersum (23) observed the evolution of forest sustainability towards multiple use, biological diversity, mitigating climate change and socioeconomic dimensions. Wiersum summarized four concepts involved with sustainable forest management as maintenance or sustenance of:
forest ecological characteristics
yields of useful forest products and services for human benefit
human institutions that are forest-dependent
human institutions that ensure forests are protected against negative external institutions.
A similar perspective on sustainable forest management (13) described the measurable criteria as:
desired future condition (the vision of the forest in the future)
sustained yield
ecosystem maintenance
community (city) stability
Keene (13) also noted that these principles can be practiced in traditional forest management. Products derived from forests in which sustainable forest management is practiced may receive a third-party certification as such, in a manner similar to certification of organically-grown produce.
Maser, (14), Wiersum (23) and Charles (4) all argued that a sustainable forest would include biological, social and economic issues. For example, from the perspective of a fishery resource, sustainability is the simultaneous pursuit of ecological, socioeconomic, community and institutional goals (4). In Maser’s view of ecological sustainability, the goals and needs of society must reflect the potential of the resource to meet them. This idea may be universal for sustainable development and must certainly be for urban forests.
This approach can be directly applied to cities, for we want urban forests to contribute to environmental, economic and social well-being. We need not sacrifice one goal in pursuit of another. Trees reduce atmospheric contaminants at the same time that they enhance community well-being. While there may be conflicts in specific situations (eg. planting trees under utility lines or using invasive species), in general, all of the broad goals for urban forest sustainability are compatible with the others. In this sense, when we focus on appropriate management of trees and urban forests, where management activities take place with community-supported goals and objectives, we focus on sustaining a broad range of values.
We also concur with Charles’ (4) conclusion that sustainability can only be achieved when:
Control is local (for fisheries, community and region-wide)
Management is adaptive, recognizing the dynamic resource and its complexity
Property rights are respected
In summary, a wide range of definitions for sustainable development have been derived from the original concept of the Brundtland Commission. No universally accepted derivation has arisen for forestry. Despite this problem, progress has been made in identifying criteria and markers for success.
Characteristics of Urban Forest Sustainability
Given the general characteristics of sustainable systems and the specific nature of urban forests, we identified 4 principles to which any model of sustainability must adhere.
1. Sustainability is a broad, general goal
While we may be able to describe the desired functions of a sustainable urban forest, we cannot yet design the forest to optimize them. Although we know that urban forests act to reduce atmospheric contaminants, we do not yet know how to design those forests to maximize that function. However, we accept that existing urban forests provide these functions to some degree. Trees in cities serve to improve community wellbeing, reduce the urban heat island, eliminate contaminants from the atmosphere, etc. While there are costs involved in planting, maintaining and removing trees in cities, in a sustainable urban forest the net benefits provided by these functions are greater than the costs associated with caring for the forest. A sustainable urban forest provides continuity of these net benefits over time and through space. We therefore have decided to recognize the general character of sustainable systems and develop steps that form such a system in urban areas.
2. Urban forests primarily provide services rather than goods
Descriptions of sustainable systems usually focus on the goods that system provides, i.e. sustained yield. Forests provide fuel and fiber, agronomic systems provide food and fiber, fisheries provide food, etc. In such examples, goods are the primary output.
In contrast, goods comprise a rather limited output of the urban forests. The most important outputs are services, such as reducing environmental contamination (from removing atmospheric gases to moderating storm water runoff), improving water quality, reducing energy consumption, providing social and psychological well-being, providing for wildlife habitat, etc. These services, or benefits, are provided in two ways: 1) direct (shading an individual home, raising the value of a residential property) and 2) indirect (enhancing the well-being of community residents).
In planting and maintaining sustainable urban forests, we should strive for a balance among all benefits and not maximize the output of one service at the expense of all others. For example, one of the benefits that urban forests provide is wildlife habitat. Maintaining the largest wildlife habitat possible could conflict with other services, such as limiting economic development from property development or creating conflicts with humans.
3. Sustainable urban forests require human intervention
One of the wonderful characteristics of natural systems is their capacity for self-maintenance. Sustainable forests, farms and fisheries take advantage of this fact by harvesting some limited segment of the resource, often with a period of rest to allow renewal and replacement. The Brundtland Commission Report (21), Maser (14) and Charles (4) emphasized this critical aspect of the resource to be sustained. For example, Goodland (10) defined environmental sustainability as “maintenance of natural capital.” Maser noted that a biologically sustainable forest is the foundation for all other aspects of a sustainable system. In forestry, there can be no sustainable yield, sustainable industry, sustainable community or sustainable society without a biologically sustainable resource. As Charles put it (for fisheries), “If the resource goes extinct, nothing else matters.”
Many (but not all) urban forests are a mosaic of native forest remnants and planted trees. The native remnants may have some capacity for self-renewal and maintenance, particularly in greenbelts and other intact stands. However, the planted trees have essentially no ability to regenerate in place. Therefore, we must accept, acknowledge and act on the fact that urban forests (particularly in the United States) may have a limited ability to retain or replace biological capital (to use Maser’s term). This is particularly the case when we desire that regeneration occur in a manner appropriate for human benefits. Indeed, unwanted tree reproduction may actually have a net cost for control and eradication programs.
Sustainable urban forests cannot be separated from the activities of humans. Such activity can be both positive and negative. In the latter case, creation and maintenance of urban infrastructure can be extremely destructive and disruptive. In essence, we superimpose cities atop forests. The greater the imposition, the less natural the forests appear and function (D. Nowak, personal communication).
The adverse impacts of humans can be mitigated by positive actions such as planning, planting, and management; all occurring with common commitment and shared vision. We cannot separate sustainable urban forests from the people who live in and around them. In fact, we want to meld the two as much as possible.
The implications of this principle are far-reaching. First, urban forests require active, consistent, continuing management. The accrual of net benefits can only occur when adequate and reasonable care is provided. Second, tree managers (both public and private) must involve the surrounding community in decisions and actions regarding urban forests. We do not suggest abdicating responsibility on the part of tree managers; we advocate sharing it.
4. Trees growing on private lands compose the majority of urban forests
While publicly - owned trees (primarily in parks and along streets and other rights-of-way) have been the longstanding focus of urban forestry, they comprise only a portion of the urban forest. An estimated 60 - 90% of the trees in urban forests in the United States are found on privately owned land (see 19; also G. McPherson, pers. communication). Therefore, sustainable urban forests depend to a large degree on sustainable private forests.
If we consider further that trees probably are not evenly distributed among all private landholders, then we may also conclude that a small number of land owners and managers may be responsible for a large fraction of urban trees. For example, universities, business parks, corporate campuses, commercial real estate, autonomous semi-public agencies, utilities, etc. may manage large numbers of trees. The success of any effort at sustainability must include their participation and commitment.
However, small private landholdings, particularly residential properties, may also constitute a significant fraction of community trees. Their contribution to the urban forest must be considered in any effort towards sustainability.
Defining Sustainable Urban Forests
Applying these 4 principles leads to the following definition of a sustainable urban forest:
“The naturally occurring and planted trees in cities which are managed to provide the inhabitants with a continuing level of economic, social, environmental and ecological benefits today and into the future.”
Applying this definition in urban areas requires accepting 3 ideas:
Communities must acknowledge that city trees provide a wide range of net benefits. Planting, preserving and maintaining trees is neither simply a good thing nor an exercise. Rather, urban forests are essential to the current and future health of cities and their inhabitants.
Given the goal of maintaining net benefits over time, the regeneration of urban forests requires intervention and management by humans. To quote David Nowak, “people want and need to direct the renewal process because natural regeneration does not meet most urban needs.” Therefore, urban forests cannot be sustained by nature, but by people.
Sustainable urban forests exist within defined geographic and political boundaries: those of cities. Moreover, sustainable urban forests are composed of all trees in the community, regardless of ownership.
A Model of Urban Forest Sustainability
Given the 3 premises listed above, we developed a model of urban forest sustainability which is founded on three components: 1) vegetation resource, 2) a strong community framework and 3) appropriate management of the resource. Within each component are a number of specific criteria for sustainability (see Tables 1, 2 and 3).
Criteria of urban forest sustainability for the Vegetation Resource.
Criteria of urban forest sustainability for the Community Framework.
Criteria of urban forest sustainability for Resource Management.
1. Vegetation resource
The vegetation resource is the engine that drives urban forests. Its composition, extent, distribution, and health define the limit of benefits provided and costs accrued. As dynamic organisms, urban forests (and the trees that form them) change over time as they grow, mature and die. Therefore, sustainable urban forests must possess a mix of species, sizes and ages that allows for continuity of benefits while trees are planted and removed (Table 1).
The vegetation resource of a sustainable urban forest is one that provides a continuous high level of net benefits including energy conservation, reduction of atmospheric contaminants, enhanced property values, reduction in storm water run-off, and social well-being.
There are costs associated with the accrual of these benefits. Dead, dying and defective trees may fail and injure citizens or damage property. Some species may pose a health risk from allergenic responses. Others may compete with native vegetation and limit the function of naturally occurring fragments and systems.
2. Community framework
A sustainable urban forest is one in which the all parts of the community share a vision for their forest and act to realize that vision through specific goals and objectives (Table 2). It is based in neighborhoods, public spaces and private lands.
At one level, this requires that a community agree on the benefits of trees and act to maximize them. On another level, this cooperation requires that private landowners acknowledge the key role of their trees to community health. Finally, in an era of reduced government service, cooperation means sharing the financial burden of caring for the urban landscape.
3. Resource management
In many ways, this component is not simply management of the resource but the philosophy of management as well (Table 3). On one hand, specific policy vehicles to protect existing trees, manage species selection, train staff and apply standards of care focus on the tree resource itself. In contrast, acceptance of a comprehensive management plan and funding program by city government and its constituents allows shared vision to develop.
Cities must recognize that management approaches will vary as a function of the resource and its extent. A goal of maintaining native wildlife habitat may best be achieved where there is a strong native forest resource. For some cities, this is simply not attainable. Similarly, management of the urban forest must exist in connection to the larger landscape (such as adjacent forests). For example, maintenance of intact riparian corridors requires the cooperation of the managing agency of the stream.
Achieving Sustainable Urban Forests
A sustainable urban forest is founded upon community cooperation, quality care, continued funding and personal involvement. It is created and maintained through shared vision and cooperation with an ever-present focus on maximizing benefits and minimizing costs. Taken together, they acknowledge the need for shared vision and responsibility, for direct intervention with the resource and for programs of care that are on-going and responsive. The implementation of a model for urban forest sustainability would further redirect the traditional orientation of urban forest management away from municipal trees to the mix of public and private trees.
Achieving sustainability for urban forests involves meeting each of these criteria. To assist in this task, we have described indicators of success for each criteria (Tables 4, 5, and 6). A city that meets the highest level of each indicator for each criteria would have the best tools and resources to achieve sustainability.
Criteria and performance indicators for the Vegetation Resource.
Criteria and performance indicators for the Community Framework.
Criteria and performance indicators for Resource Management.
Our approach of developing criteria and indicators is patterned after that found in the Santiago Agreement (11) which suggested criteria and indicators for the conservation and sustainability of temperate and boreal forests. It recognized that both quantitative and qualitative (descriptive) indicators were needed, for not all criteria could be accurately measured.
Conclusions
Maser suggested that ecological sustainability encompasses 4 ideals:
Providing a long-term balance between society and the resource, today and in the future.
Seeking to increase the overlap between societal desires and ecological possibilities.
Developing assessment tools for both the resource and its outputs (benefits, services).
Restoring ecosystems.
Our model for urban forest sustainability adheres to these 4 ideals, placing them in an urban context. It recognizes the nature of society in cities and encourages participation at the broadest level. The model also acknowledges the need to foster regeneration, to provide for the continuity of the resource. Management of a sustainable urban forest is based upon a shared vision for the resource, in which goals and needs are balanced. Since sustainability is a general goal, we must be able to assess our progress relative to defined standards. Finally, we recognize that our actions, through such activities as development, will damage forests and their function. We accept the responsibility of restoration.
Urban trees and forests are considered integral to the sustainability of cities as a whole (3,8). Yet, sustainable urban forests are not born, they are made. They do not arise at random, but result from a community-wide commitment to their creation and management.
Obtaining the commitment of a broad community, of numerous constituencies, cannot be dictated or legislated. It must arise out of compromise and respect. While policy vehicles such as ordinances play a role in managing the urban forest, developing commitment is probably more a function of education, awareness and positive incentives. This may represent our most significant challenge: to provide information that creates commitment and guides action.
This is not to ignore the budgetary requirements for sustainable urban forests. It has long been our belief that if education were adequate, funding would soon follow. Despite the current state of funding, we must hold to this perspective.
Finally, sustainable urban forests also require a viable resource base. While urban foresters and arborists have long felt confident in their ability to sustain the resource, we must acknowledge our limitations as well as our strengths. The optimal structure of urban forests, i.e. the arrangement of trees in a city, remains the subject of research. Our industry must strive to resolve conflicts such as quality of nursery stock, appropriate cultural practices and the match between site considerations and species selection.
Acknowledgments
Thanks to Greg McPherson, Dave Nowak, Richard Rideout, Paul Ries, Ed Macie, and Ray Tretheway for their comments and suggestions. Funding for this project was provided by a grant from the National Urban and Community Forestry Advisory Council through the U.S.D.A. Forest Service Urban and Community Forestry Challenge Cost-share Program (No. G-5-94-20-095).
- © 1997, International Society of Arboriculture. All rights reserved.
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