FIGURE 1.1 An urban forest in Etobicoke, Toronto. Photo by Sam Javanrouh.
For most people, the concept of an “ecosystem” brings to mind rural or wilderness areas. And indeed, most ecological research of the past several decades has focused on these remote settings. But today, the frontiers of ecological research can often be found in our cities-the places where most people live, work, and play, and where our everyday decisions (e.g., about housing, transport, consumption) can have profound effects on the environment.
One important element of “urban ecology” is the role of trees in providing a wide variety of environmental benefits, such as
- sequestering of carbon, thus contributing directly to climate change mitigation;
- reduction of air pollution through direct deposition of pollutants and through cooling effects that reduce the formation of ozone;
- shading of buildings, which can lower energy demand for air conditioning;
- reduction of urban heat island (UHI) effects;
- interception of water runoff, thus buffering local waterways from pollution and helping to control stormwater overflow problems;
- provision of vital habitat for wildlife; and
- access to nature.
Many of these benefits, especially those related to air pollution, water pollution, and local cooling effects, have direct, significant impacts on physical human health. There is also growing recognition that exposure to trees and green spaces provides many important
socioeconomic and mental health benefits, including enhanced social cohesion, increase in real estate values, improved health and recreational opportunities, and cultural and spiritual values. In fact, one distinction between rural/wildland ecosystems and urban ecosystems in the services they provide is that in urban areas, there is potential daily contact by thousands of people with any single nature element (trees, parks, green space), resulting in a range of possible psychosocial and health benefits.
On Feb 25-26, 2013, the National Academy of Sciences (NAS) held a workshop that brought together over 100 people with a wide diversity of interests in urban forestry research to share information and perspectives, to foster communication across specific areas of ecosystem service research, and to consider integrated approaches that cut across these different realms. The other specific goals of the workshop were to examine the following (see Appendix E for full statement of task):
- current capabilities to characterize and quantify the benefits (“ecosystem services”) provided by trees and forest canopy cover within a metropolitan area, which may include benefits to public health and well-being;
- key gaps in our understanding and our ability to model, measure, and monitor such services, and improvements that may be needed to allow tree planting to be sanctioned as a “creditable” strategy in official regulatory control programs (i.e., for air quality, water quality, and climate change response);
- current capabilities for assigning quantitative economic value to these services, and strategies for improving these capabilities (for instance, to allow for rigorous cost/benefit analyses, and for policies that compensate land owners for good forestry conservation and planting practices);
- the challenges of planning and managing urban forests in a manner that optimizes multiple ecosystem services simultaneously (e.g., synergies, tradeoffs in selecting tree species, and determining planting locations); and
- opportunities for enhancing collaboration and coordination among federal agencies, academic researchers, and other stakeholders.
In his introductory remarks, Gary Allen (Executive Director of the Center for Chesapeake Communities and Chair of the workshop planning committee), noted that urbanization can result in most of the available land being utilized for building and “hard” infrastructure development. As cities grow, trees and green spaces are often lost, and with them valuable ecological services, as well as all the benefits stemming from those services. Such concerns are closely related to public health issues and economic and social inequities—all of which, if addressed together, could make cities more sustainable.
While most urban areas in the United States have been losing green space over time, there has been a significant growth in the number of cities declaring ambitious goals for expanding their tree canopy, along with a growing recognition that urban green space is critical to sustaining environmental quality and human well-being. A few regions are now even attempting to include large-scale tree-planting as an official measure in air and water quality control plans, as well as climate change action plans. This represents a potential major step forward in how the ecosystem services provided by trees are valued. But it also entails substantial new requirements to rigorously quantify these ecosystem services.
Our ability to do this sort of quantitative analysis is improving due to a growing base of scientific research, the development of new modeling tools, and advances in remote sensing, Geographic Information Systems (GIS), and other mapping and monitoring technologies. But many uncertainties and challenges remain in developing standard, widely-accepted methods for making such estimates. One challenge, for example, is linking the different types of models needed for these analyses (e.g., forestry and vegetation models, air chemistry and meteorology models, hydrological models, human health impact
models) which differ greatly in structure and operate on a wide range of spatial scales. Another challenge is the difficulty in collecting empirical data available to evaluate the effectiveness of specific urban forestry projects in comparison to the modeled estimates of these impacts.
The growing body of (mostly discipline-specific) research aimed at better characterizing the ecosystem services listed above has been accompanied by growing interdisciplinary research on how trees fit into the broader context of urban sustainability (e.g., Dobbs et al., 2011; Chiesura, 2004; Pataki et al., 2011). This research involves not just advancing scientific understanding of the physical, chemical, and ecological processes of urban forestry, but also advancing our social science understanding of public values and attitudes regarding land use decisions, access to nature, and the role of regular citizens as stewards of urban green spaces. Mr. Allen urged the workshop participants to consider such questions in the context of complex governance issues because the land in and around metropolitan areas is often owned and managed by a broad patchwork of federal, state, local government, businesses, and private individuals, all with differing interests and priorities, governance structures, and capacity for forest conservation and stewardship efforts.
Thus there is growing interest in an important, multifaceted area for research that reaches across many disciplines of physical, biological, and social sciences. A major goal of this research is to be able to provide clear, compelling scientific guidance that can help cities grow and sustain forest canopy cover in a way that maximizes and sustains benefits and minimizes costs and potential unintended consequences (such as increased pollen load, risk of fire and storm damages, and greater requirements for water resources). “Smart strategies” for urban forestry include, for instance, selecting the right tree species (e.g., those with low volatile organic compounds [VOC] emissions and high pollution-absorbing capacity, that do not contribute to invasive species problems, or that will have a high survival rate), and choosing strategic planting locations (e.g., should planting strategies focus on maximizing interception of water runoff, on maximizing interception of air pollution plumes, on maximizing cooling of “hotspots,” on maximizing social benefits?).
There are a wide array of stakeholders with interests in such issues who can help advance our scientific understanding and technical capabilities. This includes numerous federal agency programs—for instance, the U.S. Forest Service’s (USFS) urban forestry programs, the National Science Foundation’s (NSF) ecological, social, and geophysical research programs, the Environmental Protection Agency’s (EPA) air and water quality research activities, the National Aeronautics and Space Administration’s (NASA) remote sensing programs, the public health programs of the Centers for Disease Control and Prevention (CDC), and the Department of Energy’s (DOE) energy efficiency programs. It also includes a wide array of state and local forestry and land-management organizations, along with private foundations, non-governmental organizations, and academic researchers.
Mr. Allen closed his remarks by noting that nearly 80 percent of the U.S. population lives in cities, and as these cities continue to grow and develop, there will be both challenges and opportunities for designing more sustainable development pathways. To aid in the design of sustainable cities, urban forest research programs should recognize urban areas as systems. Many of the complex human-environment interactions taking place at the urban scale are not yet well understood. A central challenge for the future is to develop strategies for “sustainable stewardship” of urban ecosystems that can support a healthy tree canopy and healthy, safe, diverse environments for the people living in cities and their surrounding metropolitan areas.
WORKSHOP SETTING AND GOALS
A National Research Council (NRC) ad hoc committee of six volunteers, chosen to provide expertise in different elements of urban forestry research, was tasked to plan a workshop that addresses the questions listed in the Statement of Task (Appendix E). The workshop focus was deliberately limited to a particular scope of questions within the boarder realm of urban ecosystem services and sustainability, which center around how to quantify and characterize the biophysical and human health services provided by urban trees. The workshop did not explore questions such as possible alternative strategies for providing such services (e.g. using mechanical structures rather than trees for shading and cooling benefits), or issues such as “cultural ecosystem services” provided by green infrastructure.
Using the Statement of Task as a guide, the planning committee identified the workshop’s organizational structure, invited speakers and other participants, and helped facilitate sessions at the workshop itself. The committee organized the workshop around four main themes: (i) urban forestry in the greater urban ecosystem, (ii) biophysical services of the urban forest, (iii) tools for ecosystem service evaluation, and (iv) managing the urban forest. In addition to having a variety of expert speakers on each of these topics, the workshop included substantial time for interactive discussion among all of the participants in four breakout groups. For each of the themes above, the breakout group participants discussed: (a) what are the key remaining questions and challenges, and (b) what is needed to address these questions and challenges? Finally the breakout groups were asked to consider what research activities they themselves would pick as high priorities if in a position to support urban forestry-related research.
This report summarizes the presentations and discussions that took place in all of the various workshop sessions.1 This effort was designed as a “convening activity” rather than a “consensus study,” and thus there was no attempt to reach consensus on specific findings or recommendations. Rather, this report simply presents the full diversity of ideas and suggestions that arose in the workshop discussions.
The committee hopes that this report will provide a useful resource to the wide array of urban forestry stakeholders (e.g., researchers and program managers in agencies such as the USFS and the EPA, academic researchers, and foundations and non-governmental organizations that support community forestry issues), in particular to help shape their support for future research. More generally, this report might help inform some decisions made by urban-level policymakers, planners, and managers regarding investments in large-scale tree planting efforts and other elements of green infrastructure.
Ultimately, the hope is that the field of urban forestry research, in all of its dimensions, will be advanced by the personal interactions and connections that took place at the event and by the summary outcomes presented here.
The workshop featured a range of presentations by scientists, stakeholders, and policymakers as well as time spent in breakout groups to allow for interactive discussion. This is reflected in the three chapters of this report:
1 This report has been prepared by the workshop rapporteurs as a factual summary of what occurred at the workshop. The planning committee’s role was limited to the planning and convening of the workshop. The views contained in this report are those of individual workshop participants and do not necessarily represent the views of all workshop participants, the planning committee, or the National Research Council.
- Chapter 1 (this chapter) provides the context for this study and introductory material from the workshop.
- Chapter 2 summarizes the presentations from the four panels: Urban Forestry within the Greater Urban Ecosystem, Biophysical Services of the Urban Forest, Tools for Ecosystem Service Evaluation; and Managing the Urban Forest. Key points from the discussion sessions following each panel are also included in this chapter.
- Chapter 3 presents a brief overview of the issues discussed by the workshop breakout groups (key remaining questions and challenges of urban forestry, strategies to address these challenges, and priorities for future research). The detailed summary of those breakout discussions are presented in Appendix A.
A definition of some key terms used throughout the report can be found in Box 1.1.
Definition of Terms
Biophysical services: Ecosystem services provided by the physical environment (water, soil, air, etc.) and the biological activity within it (plants, animals, etc.).
Cultural ecosystem services: Nonmaterial benefits people obtain from ecosystems, such as cultural diversity, spiritual and religious values, knowledge systems, educational values, inspiration, aesthetic values, social relations, sense of place, cultural heritage values, recreation and ecotourism.
Disservices: Negative or unintended consequences.
Ecosystem services: Life-sustaining benefits humans receive from nature, such as clean air and water, fertile soil, pollination, and flood control.
Gray infrastructure: Refers to traditional practices for stormwater management and wastewater treatment, such as pipes and sewers.
Green infrastructure: A variety of natural elements (trees, grasses, gardens) designed and landscaped to manage water naturally.
Hyperfunctional or hyperfunctionality (referring to systems of managed landscapes, infrastructure): Since cities can only afford to allocate limited space to infrastructure and land, each unit needs to be hyperefficient to achieve its goal (e.g., reductions in pollution, runoff, temperature, etc.).
Street tree: Trees located on a strip of land between a roadway and a sidewalk.
Urban forestry: The care and management of urban forests.
Urban foresta: A collection of trees (including any woody plants) that grow within a city, town or a suburb.
Urban heat island: A phenomenon where air temperatures in urban areas are 2-10 F hotter than surrounding rural areas due to the high concentrations of buildings and pavement in urban areas.
Urban metabolism: Quantification of the total resource inputs, outputs, and transformations in a city stemming from urban socioeconomic activities and regional and global biogeochemical processes.
a There is no commonly accepted definition of the term “urban forest.” Although there are trees in the urban environment, and their density, or canopy cover, varies in different cities, at what point does it constitute an urban forest? Trees in a city, chosen by residents over time from different ecotones and planted together may descriptively be a forest (i.e., a grouping of co-located trees), but functionally it may not.