SETTING GOALS AND DEVELOPING STRATEGIES IN URBAN FORESTRY
Ann Bartuska, U.S. Department of Agriculture
There has been an increased emphasis on sustainable cities. One component of a sustainable city is the inclusion of trees as part of the greater urban ecosystem. This shift toward the concept of socioecology will require a deliberate integration of social and biophysical sciences, breaking down silos in governance and management, market-based solutions, and valuing green infrastructure.
A significant challenge in urban forestry is fostering a sense of environmental stewardship. How do you engage all the needed stakeholders and provide them with useful tools and information? Environmental stewardship requires various groups to conserve, manage, monitor, advocate for, and educate their friends, neighbors, and representatives about their local environments. Everyone deserves access to green space, which ties into the idea of environmental justice.
Tools developed by the U.S. Department of Agriculture (USDA) are now focusing on an integrated ecological system, rather than simply trees, and are being developed to help foster environmental stewardship. For example, the Stewardship Mapping and Assessment Project (STEW-MAP) is a geospatial tool utilized by several cities, including New York City, to understand the intersections of green space and social space. These maps quantify stewardship networks and linkages by indicating where particular types of organizations are working together and where improvements can be made to encourage more cooperation among these organizations. These networks allow communities to share the skills that they have learned in developing green space in urban areas. STEW-MAP highlights existing stewardship gaps and overlaps to strengthen organizational capacities, enhance citizen monitoring, promote broader public engagement with on-the-ground environmental work, and build effective partnerships between stakeholders involved in urban sustainability.
This shift toward an integrated ecological system is impacting the types of R&D being conducted at USDA. For example, USDA conducts urban research in forest inventory and management, ecosystem services, health and wellbeing, urban sustainability, green infrastructure, water and watersheds, and urban long-term research. Urban agriculture challenges USDA to think about how more traditional aspects of agriculture can contribute to more sustainable urban ecosystems.
USDA is just one of several agencies that study urban issues. In the spirit of environmental stewardship, how can we bring these agencies together with the common goal of sustainable cities? The NSF Long-term Ecological Research (LTER) Program consists of 26 sites with over 1800 scientists and students studying ecological processes over extended temporal and spatial scales. This valuable effort highlights the importance of long-term observations in an interdisciplinary setting. Including urban systems into LTER networks (e.g., Baltimore and Phoenix) has been an important step forward.
The 2010 NRC report Pathways to Urban Sustainability: Research and Development on Urban Systems explores the landscape of urban sustainability research programs in the
United States and provides useful advice that could be used by many agencies that work on urban forestry. The report explores how urban sustainability can move beyond analyses devoted to single disciplines and sectors to systems-level thinking and effective interagency and intergovernmental cooperation. It concludes that it is critical to better integrate science, technology, and research into catalyzing and supporting sustainability initiatives; find commonalities, strengths, and gaps among rating systems; and incorporate critical systems needed for sustainable development in metropolitan areas.
Dr. Bartuska was asked how USDA is defining “sustainability” in the context of an increase in population, economy, and agriculture. She said there is a balance of three factors in the context of sustainability: people, planet, and profit. USDA does have a sustainability office and they must continue to be aware of what constitutes sustainability and sustainability practices. For example, USDA’s Beginning Farmers and Ranchers Program ensures that participants address water and air issues, as well as biodiversity issues and then incorporate these into practice. Dr. Bartuska also noted that the USDA Agricultural Research Service has a project in small and organic farms in urban areas.
URBAN FORESTRY WITHIN THE GREATER URBAN ECOSYSTEM
Moderator: Marina Alberti
Urbanizing regions pose enormous challenges to ecosystem’s capacity to deliver important ecological services (Alberti, 2010). At current rates of urban growth, global urban land cover will increase by 1.2 million km2 by 2030, nearly tripling the global urban land area of 2000, with considerable loss of habitats in key biodiversity hotspots (Seto et al., 2012).
Scientists have made significant progress during the last few decades in studying the role of urban forests in both mitigating urbanization’s impact and providing a variety of ecosystem services. Yet scientific understanding of key mechanisms governing ecosystem functions across multiple scales is incomplete. There are important tradeoffs across scale and between functions. There is also great variability across metropolitan areas and biophysical regions.
The goals of this panel were to (1) explore the role of trees within the greater urban ecosystem and the ecosystem services they provide, and (2) review current understanding of the ecosystem services provided by urban forests, and identify research needs.
Urban Ecosystems and their Potential to Provide Ecosystem Services
Richard Pouyat, United States Forest Service (USFS)
The environmental changes and landscape alterations typical of urban areas make it difficult to be “green.” Urban areas have highly modified environments, sealed surfaces, and species introductions that are human-caused and thus represent novel habitats made up of novel assemblages of plants and animals. From an evolutionary perspective, these assemblages are relatively new, since cities have been around for only 5,000 or so years. As a result, urban landscapes are typically thought of as artificial, harsh environments where cultivated plants grow outside their native habitats, and where animals introduced as pets (such as domesticated cats) wreak havoc on prey species such as native song birds.
Despite these alterations, urban ecologists are finding high levels of biological activity and biodiversity in urban areas (Gregg et al., 2003; Ziska et al., 2004). Measurements thus far suggest there are high flux rates, large sinks for carbon and nitrogen, and high resource
availability (e.g., cities emit large amounts of carbon dioxide which are utilized by plants). Therefore, urban ecosystems possess the potential to provide ecosystem services. However, our ecological knowledge of these systems is lacking because ecologists in North America have only relatively recently begun to study them in a comprehensive way.
Because of the novelty of urban ecosystems, urban landscapes represent a “new heterogeneity” for ecologists to quantify and understand. This term is used because, depending on the scale of observation, urban landscapes are not necessarily more complex. In fact, in some cases urban landscapes may be less heterogeneous since they have been more “homogenized” due to management activities, scales of disturbance, human preferences, and the parcelization of the landscape into management units. Since the level of heterogeneity largely depends on the scale of observation, four dimensions should be considered: longitudinal and lateral spatial dimensions, the vertical dimension (e.g., vertical air column, soil column), and the time dimension (e.g., hydro-curve for an urban stream). One of the biggest challenges for ecologists is accounting for human behavior and decision making, because humans may make irrational decisions, and human culture and value systems vary spatially. It is also difficult to quantify intrinsic and monetary values from an ecosystem services perspective.
Another key point related to ecosystem services is that all life on earth is limited by available energy. Therefore, there are tradeoffs in between ecosystem services and costs. For example, there is no organism that can do everything well—allocating resources for one function takes away resources from another function. The same can be said for ecosystem services.
As mentioned earlier, ecological science is a relatively young science (about 100 years) compared to the physical sciences, and urban ecological science is even younger (less than 50 years). Therefore, there is a steep learning curve. Moreover, an ecological definition of “urban” has yet to be developed (Ellis and Ramankutty, 2008). One possible definition is the threshold in human population density at which the population cannot be sustained with the resources available locally and must depend on resources brought in from outside the local area The importation of resources can cause disservices in areas at great distances from cities (Newman, 1999). Moreover, if imported resources are not used efficiently, there is a waste stream which can impact ecosystems at great distances (another potential disservice). With this definition, one may think that cities are bad; however, densely populated areas such as cities are part of the solution, since the distribution of people from cities across rural landscapes would arguably cause even greater environmental disservices than concentrating people into cities (Brown et al., 2009).
Whatever the case, there are also tradeoffs of ecosystem services occurring within cities. A higher human population density will diminish ecosystem services and resources locally. For instance, cities have many polluting sources, fragmented habitats, built structures, and impervious surfaces, which lead to disrupted nutrient cycles and a loss of native biodiversity. The field of civil engineering was developed to design “gray infrastructure” to overcome some of these disservices. Civil engineers have had many more centuries of experience in developing gray infrastructure than ecologists have had with their new concept of green infrastructure. Good examples of gray infrastructure exist in ancient Rome and more modern “sanitary” cities rising from the industrial revolution such as New York City. However, there are detrimental side effects in the use of gray infrastructure that can lead to disservices. For example, gray infrastructure interrupts natural flow paths such that urban streams can become prone to flash flooding causing stream erosion downstream. Moreover, gray infrastructure degrades with time (Kaushal and Belt, 2012).
Land use change has impacts on ecosystem services, which has been a major concern for converting natural to agricultural systems. Natural systems typically provide multiple ecosystem services, but in converting these systems to agricultural production systems, these services are greatly diminished. To address this issue, efforts are underway to design agricultural production systems so that they provide multiple services along with producing food (Foley et al, 2005; Figure 2.1). In the case of urban land use conversions, much less space (or pervious area) is available to provide ecosystem functions. Therefore, not only do we need to design urban landscapes that provide multiple functions, but those that include hyper-functioning systems as well.
In urban areas, the integration of green (vegetation), brown (soils), and blue (streams) infrastructure is one way to develop a multifunctional landscape. It is best to design these infrastructures in parallel, linking one to another—for example, a green roof that is linked to a rain garden, which is then linked to a retention pond system, so that storm size events are moderated. Advantages of integrating these types of infrastructures include: avoiding side effects (e.g., high peak flows), utilizing biological processes to self-maintain, and preserving the function of pre-existing ecosystems.
Unintended effects, risk, infrastructure performance, system longevity, and the possibility of disservices occurring at great distances all need to be considered when designing green infrastructures and locating those infrastructures in urban landscapes. Natural experiments can be conducted to examine the tradeoffs that occur as landscapes are urbanized. For example, when comparing forest fragments in an urban context to a rural one, roughly half the natural sink for methane, a greenhouse gas (GHG), is lost. When a forest is converted to turfgrass, the entire methane sink is lost (Pouyat et al., 2009). These kinds of unintended effects should be considered, and decision tools are needed that will optimize multiple factors simultaneously, because making a poor decision in designing or locating green infrastructures in urban landscapes may be worse than not doing anything.
Pouyat summarized by stating that (1) a basic understanding of urban ecosystems should be developed, which can be accomplished by utilizing the urban mosaic to conduct “natural experiments,” conducting cross-system comparisons (local, regional, global), and developing integrated models that spatially and temporally quantify the “new heterogeneity” represented by urban landscapes; (2) urban observations should be expanded into networks (e.g., a network of urban LTER sites or existing environmental monitoring networks such as the National Atmospheric Deposition Program); (3) decision tools need to be developed that can optimize across factors (e.g., species selection, management) while considering tradeoffs and providing a decision space (e.g., uncertainty, risk); and (4) multifunctional and hyperfunctional infrastructures need to be designed and developed.
Services and Regional Tradeoffs: Resolving the Desert Forest Paradox
Diane Pataki, University of Utah
Urban forests in desert areas are an extreme example of novel ecosystems. Salt Lake City, Utah, for example, is naturally a shrubland, yet the city has an extensive urban tree canopy (Figure 2.2.). Virtually all of these trees are planted and irrigated, making this an extreme example of a human-created and managed forest.
Given that ecosystem services is a concept intended to quantify the value of natural rather than designed ecosystems, urban ecosystems originally were assumed to have negligible monetary value on a global scale. What happens when we are designing ecosystems to have intended values? How do we cope with the costs of designing and managing novel ecosystems that require resource inputs?
FIGURE 2.1 Conceptual framework for comparing land use and tradeoffs of ecosystem services. The natural ecosystems (left) are able to support many ecosystem services at high levels, except for food production. The intensively managed cropland (middle) is able to produce food in abundance (at least in the short term), but loses other ecosystem services. However, a cropland that is explicitly managed to maintain other ecosystem services may be able to support a broader portfolio of ecosystem services (right). This framework could be applied to urban land use conversions, albeit on a smaller scale since there is less land available in the urban landscape. SOURCE: Foley et al., 2005.
Novel and non-native ecosystems often have significant monetary and environmental costs, but this is not necessarily a bad thing. For example, urban forests in arid and semi-arid cities use a lot of water. These designed ecosystems will often have significant costs, which may be acceptable if benefits outweigh the costs. However our research increasingly shows that the most important benefits of novel urban forests are cultural and thus are very difficult to quantify with existing tools. We need a new set of tools that extends beyond the standard ecosystem services framework to capture the complex relationship between urban residents and the novel urban environment.
One tool we can bring to an expanded toolbox for planning and managing urban forests is urban metabolism (Kennedy et al., 2012). This concept has been used by several different disciplines for decades and has been variously defined, but it generally involves quantifying the total resource inputs, outputs, and transformations in cities. Although there are some data constraints in quantifying urban metabolism, this concept is critical for quantifying the role of urban forests in the functioning of the city as a whole.
Urban metabolism can be used as a tool to help us characterize the benefits of trees in a larger context. Urban forests are often thought of as a tool for mitigating climate change; however, carbon sequestration by urban trees does not have a significant impact in offsetting fossil fuel emissions (Pataki et al., 2006; 2011). Trees do, however, have a significant cooling effect (through evapotranspiration and shade), which may impact GHG emissions indirectly (Franco and Sanstad, 2008). For example, a city can save on energy costs by requiring less air conditioning. It is important to understand these mechanisms because urban forests designed for carbon sequestration may look quite different than forest canopy designed to maximize cooling.
FIGURE 2.2: The left image is a picture of Salt Lake City, UT. Notice the natural shrubland in the foreground and the novel (planted) trees in the city. The right image shows what Salt Lake City would like in its natural state. SOURCE: Barry Howe/Corbis (left image); Diane Pataki (right image).
There are other useful tools for designing and planning urban tree populations that originate in engineering. There is currently a great deal of discussion about substituting green infrastructure for “gray” infrastructure. However, utilizing trees as urban infrastructure requires monitoring and validation to ensure that urban forests meet design targets. For example, to consider pollution removal by trees as urban infrastructure, we need measurements and monitoring of the specific and local impacts of trees on pollutant concentrations. This regularly occurs in gray infrastructure projects; sewage treatment plants, for example, are routinely monitored to ensure that effluent meets water quality standards. It is not necessary to quantify the ecosystem services provided by sewage treatment plants—they are engineered to meet specific regulatory requirements. The scientific methodology necessary to make similar measurements for green infrastructure, such as urban trees, currently exists as shown by the other workshop speakers, and needs to be more commonly implemented along with tree planting programs.
Other tools for designing and planning urban forests are available from the disciplines of architecture, planning, and design. Existing tools can also be used for stakeholder engagement, which can help determine local values. “Envision Utah”2” was a well-known program that used a participatory process to develop a set of common, shared scenarios for future urban growth. It is possible and necessary to develop similar planning and visioning processes for urban trees and green space. The beginnings of such programs are underway; “Envision Tomorrow+” is a planning tool being developed to include environmental outcomes and some initial estimates of ecosystem services.
In conclusion, tools for characterizing the net services of urban trees should be place-specific and spatially explicit, have visualization components, include community values and visioning, incorporate urban metabolism stock and flows, and capture measurable performance-based metrics. These tools can also be utilized by people from different disciplines. This approach extends the tools and vision for urban forests beyond the ecosystem services concept, to capture the larger role of urban forests in the functioning of cities.
Challenges for Green Infrastructure at the Interface of Science, Practice, and Policy
There are many challenges in reaping ecosystem services within a city, including competing agendas (e.g., many goals, many languages, and many metrics), immature science and technology, need for hyperfunctional design, and unanticipated findings in case studies in air pollution. As one example of failing to meet expectations, Bernhardt et al. (2005) found that many stream restoration projects did not accomplish their goals.
Several as-yet-unpublished air quality case studies from the New York City area found that air quality was poorer downwind of trees. In Case Study 1, it was hypothesized that greener surroundings (e.g., trees, shrubs, etc.) in an urban environment leads to cleaner air because leaves filter out pollution. The study found that particulate matter (PM2.5) concentrations were higher ten meters from the curb and downwind of two rows of mature trees than at five meters, suggesting that trees impede dispersion, creating zones of increased pollution. Fifty meters of separation were needed to disconnect a location in the landscape from events occurring on the street (Figure 2.3). In Case Study 2, researchers monitored two transects downwind of Van Wyck Parkway in New York City and found that PM2.5 concentration decayed more rapidly along an open transect than a vegetated transect.
In Case Study 3, measurements were taken at a rural site to test the influence of tree canopy on background concentration. Researchers discovered that air quality was worse more than 90 percent of the time in a stand of either spruce or deciduous trees compared to an open field. In Case Study 4, the extinction of particle plumes was monitored in a wind tunnel containing varying amounts of leaf surface. Leaf area had no effect on the decay rate of the plumes. In Case Study 5, human health implications were studied using cytokines3 as biomarkers for inflammation. Cell cultures challenged with airborne particulates collected from parks showed higher cytokine induction than samples near streets or rooftops.
In all of these cases, findings ran counter to expectation, indicating that we need a more sophisticated understanding of the mechanisms influencing particulate behavior if we hope to design effective pollution mitigation using green infrastructure.
Another challenge for green infrastructure is to move from multi-functionality to intentional hyperfunctionality. That is, if cities can only afford to allocate limited space to green infrastructure, each unit of green needs to be hyperefficient if we intend to achieve meaningful reductions in pollution, runoff and temperature; green space needs to be deliberately designed to enhance its benefits.
In conclusion, we should move beyond the simple notion that “more green is better.” Designing hyperfunctional green infrastructure requires an adaptive management approach involving experiments, modeling, ground truthing, and comparative studies in order to promulgate useful policy and effective practices.
Urban Nature: an Artifact of the Industrial City4
Stephanie Pincetl, University of California, Los Angeles
We are living in a new age: the Anthropocene5. Humans are now an urban species and shape many of Earth processes. This raises questions about what it is to be human in an
3 Substances that are secreted by specific cells of the immune system and are used extensively in cellular communication.
4 Dr. Pincetl was unable to attend the workshop, but provided her PowerPoint presentation to all workshop participants.
urban age, how cities are built and grow, as well as our “need for nature.” Cities are nature—inert minerals transformed by humans into infrastructure. However, where does living nature fit in?
Until the industrial revolution, cities were essentially devoid of living nature, except for elite gardens. There was a hierarchical order of civilization out toward the wilderness— cities were surrounded by agriculture and the countryside, which were surrounded by wilderness. In fact, nature was feared and powerful. The wilderness had wolves, bears, and other predators. Agriculture was a struggle against weather, weeds, animals, soils, water supply, and trees.
The harnessing of fossil energy enabled industrialization and changed humans’ relationship with the planet. This led to a dramatic transformation of nature, enormous increases in manufacturing productivity, and the concentration of humans in urban centers as never before. The Industrial City was polluted, crowded, and insalubrious.
During the early years of the industrial revolution, living conditions in cities were abysmal. Tree-lined streets and parks were seen as agents of change to make cities more livable. Frederick Law Olmsted’s Central Park was seen as the lungs of the city for the working class: “A park is a work of art, designed to produce certain effects on the mind of men (Olmsted, 1868).” This led to the rise of landscape architecture and interest in the exotic, including plants that were non-native. This interest reflected the new cosmopolitanism, reaching far beyond the local.
Human views of trees began to change. George Perkins Marsh6 showed the importance of trees for watershed function, which led to preservation of forests that were still in the public domain. This coincided with the rise of the preservation movement and the idealization of nature.
Eventually there was a tree-planting movement in cities. The urban expansion across the American west into the treeless plains provoked deliberate urban tree planting, starting in the 1870s in Nebraska with the founding of Arbor Day, as lands west of the 100th Meridian were arid and treeless. Citizen-based urban tree planting spread in mostly affluent areas. Tree planting became a civic obsession; there was an association of virtue with trees. In the United States, emphasis was placed on neighborhood trees (planted by individuals along streets). Gifford Pinchot, the first director of the USFS, actively promoted tree planting in cities.
In the 20th century, parks and open space became normalized as part of urban planning and design. Urban trees were seen as part of the health of residents and a sign of a well-tended neighborhood. Postwar prosperity led to urban expansion.
In the mid-20th century, concerns were raised about the preservation of nature and the environment. Rachel Carson (1962) sounded the alarm on chemical impacts, which led to the modern environmental movement. In the 1970s there was formal federal Forest Service assistance for urban tree planting. Eventually Tree City USA was initiated by the National Arbor Day Foundation in cooperation with the U.S. Conference of Mayors, the National League of Cities, the National Association of State Foresters, and the USFS7.
5 An informal geologic chronological term for the present geological epoch (from the time of the Industrial Revolution onwards), during which humanity has begun to have a significant impact on the environment.
FIGURE 2.3: PM2.5 concentration taken from various distances from a curb and downwind of 2 rows of mature trees plotted against time. Unexpectedly, the air 10m from the mature trees is dirtier than the air 5m from the mature trees. SOURCE: Thomas Whitlow.
Urban sustainability has been part of the public focus since the 1980s. Cities are now seen as sites of their own pollution and impacts remediation. An instrumental urban nature can be developed to help in this endeavor, as it can provide provisioning, regulating, cultural, and possibly supporting services. Trees have become emblematic of urban ecosystem services in cities across the country, and million tree planting programs have become popular.
But what is sustainable for whom and where? Do alleged services add up? Some parts of the country are naturally treeless and water-restricted; yet planting trees requires water resources. Maintaining trees also requires long-term funding and specialized knowledge. This is problematic if residents have neither. It also should be acknowledged that not all people like trees. Some ecosystem service structures such as bioswales, water infiltration, and trenches are also costly and require fundamental changes in urban morphology.
How do we implement the right urban ecosystem services for each place? This will require new forms of public administration and different rules to create new agendas, sharing of budgets, and co-management of new infrastructure (e.g., water and sanitation with street services). New sources of funding are also needed, as well as new skills to maintain “living infrastructure.” Each region will have different climatic tolerances, and ecosystem services will have to be appropriate to the conditions. Success will depend on public acceptance of a different-looking city, and willingness to lend their individual private property to the effort. This will require a deep shift involving public stewardship,
and new ideas of property rights and obligations. Finally, the sanitary city8 of the 20th century needs to be retrofitted so natural processes can work to help mitigate urban impacts and to develop the sustainable city of the twenty-first century.
Urban ecosystems have costs and benefits, and quantifying the benefits is difficult. Trees perform differently across different ecosystems and in different urban locations. Does their performance translate to the benefits claimed such as reducing the use of air conditioning or sequestering GHG emission? Trees that are brutally pruned will see their ecosystem services severely curtailed. These kinds of factors should be taken into account.
What is the value of ecosystem services? This is still largely unknown and represents the instrumentalization of nature. Humans have transitioned from fear of and vulnerability to nature’s impacts and processes, to domination and pricing of its functions, with meager quantification compared to the complexity of what is being proposed. There has been minimal effort to address the public administration and land management changes that are necessary to implement the changes proposed. The issues of beauty and wellbeing are also unaddressed. Yet humans are now urban dwellers and our relationship to nature has changed. Do we need nature to feel happy?
Some points raised in the open discussion that followed this panel’s presentations:
- An important goal for improving urban forestry models is to link hyperfunctional ecosystem services to regulatory requirements.
- Optimizing hyperfunctionality across many outcomes while focusing on the factors that the local community most values, would take into account people’s widely differing values and priorities.
- The urban environment brings together many different types of plant and animal species that have no history of co-evolving. The mechanisms of how these unique ecosystems function is therefore largely unknown.
- National-level support could help capture knowledge and foster collaborative learning across cities.
BIOPHYSICAL SERVICES OF THE URBAN FOREST
Moderators: Kenneth Potter, University of Wisconsin; ST Rao, North Carolina State University
As discussed in the previous session, urban forests provide a variety of functions including climate mitigation, carbon sequestration, mitigation of stormwater runoff, and regulation of nutrient cycling, as well as habitats for many species of wildlife. This session was a continuation of the previous session and focused on the biophysical services of trees with respect to air, water, climate, wildlife, and health. Panelists were asked to discuss the current state of the science in their respective disciplines on the biophysical services provided by urban forests. They were also asked to discuss the remaining challenges and open questions surrounding the science and the additional research, data, and observations that are needed to resolve these questions.
8 An urban form developed to correct the ills and hazards of the industrial city.
FIGURE 2.4: A schematic of a subsurface gravel wetland. SOURCE: University of New Hampshire Stormwater Center.
Trees Incorporated into Urban Stormwater Management
Tom Ballestero, University of New Hampshire
Sewage treatment utilizes very sophisticated systems, whereas stormwater management is relatively low tech. Many types of processes are utilized in stormwater management, including hydraulic control, storage, sedimentation, filtration, infiltration, sorption, biodegradation (microbial, rhizospheric, plant), and chemical. Systems that perform filtration yield higher water quality effluent than other systems. Common filtration systems can include constructed systems (e.g., permeable pavements and sand filters) and biological systems (e.g., subsurface gravel wetland, tree filter, and bioretention systems).
Green infrastructure can be designed to perform better at stormwater management than pre-development ecosystems. Often, aside from filtration, these designs incorporate infiltration as part of the stormwater management.
A tree box filter is a mini-bioretention system. A bioretention system consists of a high permeability, manufactured organic soil bed planted with suitable, preferably native vegetation. Vegetation in the soil planting bed assists in removing pollutants from stormwater runoff.
Subsurface gravel wetlands, an example of a biological mechanism for filtration, are an innovative variation on the traditional stormwater wetland (Figure 2.4). Subsurface gravel wetlands have high efficiencies for removing sediments, nutrients, and other pollutants commonly found in runoff. The stormwater is filtered as it flows underground, horizontally through the wetland. Because the primary flowpath is subsurface, the system runs anaerobically, which supports denitrification. However, an aerobic zone needs to be placed in front of the subsurface gravel wetland to convert most of the dissolved nitrogen forms to nitrate. As stormwater moves from the aerobic zone through the subsurface gravel, it becomes denitrified. This type of system requires a significant amount of land, but it does allow for more diversity in the types of vegetation that can be planted over it (e.g., native wetland grasses, reeds, herbaceous plants, and shrubs).
There are various metrics that can be used to measure the social benefits of the use of green infrastructure for stormwater management. One example is cost. Conventional technologies (e.g. gray infrastructure) are typically the cheapest initially, however, more advanced methods (e.g., low impact development) have the lowest maintenance costs overall. A normalizing method of comparing costs considers dollars per pound of pollutant
removed per watershed area treated. There are hidden costs to gray infrastructure: water quality degradation due to poor removal efficiencies, lost recreational values, watershed impairments, property value loss, uncontrolled contaminants (temperature, energy), and sustainability (water supply, low flow).
It is important to determine the objective of the infrastructure and then match technologies to that objective. Green infrastructure designs should not be considered too generically. There are also low-hanging fruit. For example, a substantial reduction in pollutant loading could be achieved by modifying some of the areas with relatively low land cover but high loading and imperviousness. This includes both commercial and industrial sites (building sites, parking lots, etc.).
There are several barriers to the implementation of green infrastructure. These include: maintenance misperceptions, initial cost, ease of permitting acceptance, designer/regulator unfamiliarity, turf wars in administrative management, and the “impossible challenges” thrown at new technology compared to the general acceptance of conventional technologies (ponds, swales, curb, gutter). The science exists, but implementation remains slow. Green stormwater management is not yet part of the DNA of urban planning and design. Ultimately, in the absence of green infrastructure, everyone will have to continue to subsidize the cultural and ecosystem consequences resulting from conventional land development, whether new development or redevelopment.
Urban Forest Effects on Meteorology and Air Quality9
Jonathan Pleim, EPA
In recent years, EPA has been pushing towards integrated, transdiscplinary research where air quality is considered along with climate change and meteorology. Coupled modeling systems are important tools for this research, but the models can become so complex that they are difficult to run and interpret.
There are several key questions related to the effects that urban characteristics and urban forests have on meteorology and air quality. For example, do we have the data and models that can adequately capture and assess these effects? What are the gaps in our understanding and modeling capabilities? How should we consider changes in air quality along with other effects of increased urban tree coverage?
The UHI effect is a well understood phenomenon that leads to hotter daytime and nighttime temperatures in urban areas, compared to surrounding rural areas. Hotter daytime temperatures in cities are a result of widespread dark impervious surfaces and less vegetation, which leads to reduced evapotranspiration and thus greater sensible heat flux. Solar radiation is trapped in the urban street “canyons,” adding to surface heating. Warmer nighttime temperatures are caused by the high heat capacity of building materials, which store more daytime heat and release it at night. There are also the effects of limited sky view, which reduces radiational cooling (i.e., buildings in urban areas partially block upwelling long wave radiation from the ground). Anthropogenic energy use from cooling, heating, industrial processes, and vehicular traffic also adds heat during both the day and night.
Trees mitigate the UHI by increasing evapotranspiration, reducing the sensible heat flux and providing shade over high heat capacity surfaces. However, studies have also found that trees impact pollutant dispersion by reducing convective turbulent mixing, boundary layer depth (the zone through which pollutants are well mixed), and ventilation. These three factors all lead to higher pollutant concentrations.
9 Dr. Pleim was unable to attend the workshop. His presentation was given by S.T. Rao.
Trees also have direct and indirect impacts on air chemistry. They enhance the removal of air pollutants and the emission of volatile organic compounds. The cooler temperatures that can result from trees lead to reduced evaporative anthropogenic emissions, slower photochemistry, and reduced energy use in the summer.
There has been a significant amount of research on trees’ impact on pollutant removal. An increased number of trees provides greater leaf surface area for dry deposition of both gas and particulate pollution. Dry deposition of gases occurs via two pathways: onto leaf surfaces and through leaf stomata. Particulate deposition occurs by impaction, interception, and diffusion at leaf surfaces. The efficiency of aerosol uptake depends on the type of tree (i.e. needle leaves are more efficient than broad leaves). Also, reducing the air temperature by a couple of degrees will lower energy [cooling] demand, which in turn reduces pollutant emissions from power generation. These types of feedbacks have not yet been fully taken into account in studies of the effects of urban trees on air quality. The net impacts could be that air pollution levels are lowered by trees, but this is not necessarily the case in all situations.
The extent of tree cover varies widely across cities. For example, Salt Lake City has more than twice the tree cover of Chicago (EPA, 2008). The greatest effect of urban trees is on the surface energy budget, because cooling results from the latent heat of evapotranspiration. Observations across many cities show that the fraction of surface energy converted to latent heat increases proportionally to vegetation coverage, with the greatest cooling benefits in higher density urban areas.
Urban land surface modeling varies widely in complexity. Models with greater complexity require specifications of a large number of parameters that are difficult to obtain or to specify. There are tradeoffs between complexity and computational requirements, with more complex models generally requiring more computational resources. Also, evaluation studies suggest that increased complexity does not necessarily result in improved performance (Grimmond et al., 2011). Determining the appropriate complexity depends on the scale and application of the model. Accurate specification and modeling of vegetation is crucial for accurate simulation of the surface fluxes. Vegetation data and land surface modeling are especially important for assessing the impacts of urban forests.
Based on model runs, urban trees generally mitigate the UHI effect by partitioning surface energy more into latent heat and less into sensible heat. The cooling benefits of additional tree coverage are greatest in medium- and high-density urban areas. The effects of trees on air quality are complex with opposing tendencies. Trees tend to increase pollutant concentrations by reducing dispersion and increasing biogenic volatile organic compound emissions. Trees decrease air pollutant concentrations through enhanced deposition and cooler photochemistry. Primary pollutants may increase while secondary pollutants (e.g., ozone) may decrease.
Urban canopy models are needed that balance complexity with data requirements and realistic response to changing tree cover and land use. There is also a critical need for accurate high-resolution site-specific land use, impervious, canopy, and vegetation data. Land use and vegetation data need to be harmonized with parameterizations across various scales and all meteorological and chemical processes (e.g., land surface models, dry deposition and bidirectional fluxes, biogenic emissions). Modeling techniques are needed that distinguish trees from other vegetation. Accurate high-resolution emission data are also required in addition to high-resolution, fully coupled meteorology-chemistry models. A comprehensive evaluation of meteorology and air quality in urban areas should also be performed.
FIGURE 2.5: New York City’s historical urban heat island (UHI). Top chart: Central Park’s annually averaged temperature from 1900 to the present (upper line) compared to the average of 23 surrounding rural and suburban stations far from the city (lower line; Rosenzweig and Solecki 2001). The UHI is indicated by the vertical offset between the two lines. Bottom chart: The annually average strength of New York City’s UHI calculated from the difference between the two historical records shown in the top chart. The blue arrow highlights the city’s strong UHI in the early 1900s. SOURCE: Gaffin et al., 2008.
Urban Climate and Urban Forests: A View from New York
Stuart Gaffin, Columbia University
The UHI effect is a significant environmental issue for New York City and will exacerbate local climate change. Two temperature variables are often used to measure the UHI: surface temperature and air temperature. Controlling surface temperature (i.e., what a person feels if they place their hand directly on a surface) is important for mitigating the heat island, whereas controlling air temperature (i.e., what a person feels walking around a city) is more important for determining energy demands. The first priority is to try to reduce surface temperatures, thereby mitigating air temperatures.
New York’s UHI has been strong (over 2 degrees Celsius) at least since 1900 (Gaffin et al., 2008; Figure 2.5). The UHI effect is much more pronounced at night than during the daytime. New York’s heat burden is increasing due to climate change and an increased UHI effect.
Dr. Gaffin has conducted several studies aimed at using urban trees to help mitigate the UHI in New York. In one study, a LANDSAT map at 60m resolution was used to find hotspots and to assess street-tree cooling benefits. Two streets in the Bronx were compared in the field. Tree-lined streets had lower temperatures, but it is important to note that many other factors (such as building type, etc.) can dominate the causes for differences in temperature. Measuring the temperature of these streets is also a challenge because there is no standard protocol for how to collect these kinds of observations. The lack of a standard data collection protocol needs to be addressed.
Using projections of how the heat burden will change over time, Dr. Gaffin is finding that the temperature extremes are changing rapidly. This is a difficult and important phenomenon to study, and taking representative measurements is a challenge. For instance, a weather station in a forested area of Central Park may not be the best representation for temperature conditions on the street where people live and work and children play.
Another key question is: Are there different levels of urban warming? The projections of future extremes may be greatly underestimated if we are not looking at different microenvironments. There is a broad spectrum of environments that may impact the temperature within a city (e.g., parks, well greened streets, poorly greened streets, poorly greened buildings, etc.).
In conclusion, UHIs are generally well documented on large space and time scales. Urban green infrastructure and albedo strategies are clearly understood as UHI mitigation methods. However, better tools, methods, and strategies are needed to understand small-scale microclimates and benefits of urban green infrastructure. Better modeling capabilities are needed to allow scientists to study large-scale greening and albedo strategies to determine overall and long term benefits vis a vis global warming. More research is needed to understand the potential biases of urban weather stations located in parks and airports and how these may be affecting statistics for extreme heat and precipitation events at the street level, where people work and reside.
The Role of Urban Forests in Biodiversity Restoration
Doug Tallamy, University of Delaware
The planet is losing biodiversity. This is important because the relation between the number of species and ecosystem function is linear (MacArthur, 1955; Maestre et al., 2012; Naeem et al., 2012; Reich et al., 2012). 950 million acres of virginforests in the eastern United States have been converted to tiny patches of secondary-growth woodlots. Most of these habitat fragments are too small to sustain biodiversity. Creating corridors between the fragments allows species to travel from habitat fragment to habitat fragment. This connectedness is one solution to increasing biodiversity. However, this connectedness is typically divided by houses, highways, and other areas where people live and work. Landscapes have been built only from an aesthetic perspective, not from the perspective of managing ecosystems.
It is very difficult for species to survive in parks and land preserves because as habitats shrink, so do the populations. Small populations are more vulnerable to local extinction (Pimm and Redfearn, 1988). Species extinction should be considered on the local level, not just the global level. Our natural areas are not large enough to support the needed biodiversity.
Plants play a significant role in animal biodiversity because they are the first trophic level and the primary producers of energy. Managed landscapes are filled with non-native plants and trees which are not well suited for supporting local and regional biodiversity compared to native plants (Burghardt et al., 2008; 2010; Tallamy, 2004; Tallamy and Shrophsire, 2009; Tallamy et al., 2010;). Non-native plants support fewer insects (e.g., caterpillars). In fact, there are often five times more species and 22 times more insects in native-plant-only areas.
Most insect herbivores are specialized to eat particular plants (Ehrlich and Raven, 1964) and can develop and reproduce only on the plants with which they share an evolutionary history. Insects that are specialized to eat one plant cannot eat other plants. Ninety percent of all phytophagous (i.e., herbivorous or plant-eating) insect species can eat
plants in only three or fewer families. Most can tolerate only a few closely related species (Bernays and Graham, 1988).
Insects play a significant role in supporting biodiversity because they are eaten by many animals (e.g., birds, frogs, fish, etc.). 96 percent of terrestrial birds eat insects when making and raising babies. For example, the Carolina chickadee rears its young exclusively on caterpillars, all of which are typically collected within 50 meters of the nest. A chickadee pair brings 390-570 caterpillars to the nest per day (Brewer, 1961). Chickadees feed their young for 16 days before they fledge. This means that to rear one clutch, the parents must catch 6240-9120 caterpillars. Reproduction is the limiting factor for future bird populations and food availability limits reproduction.
The solution to supporting biodiversity in urban areas is not simply to plant native plant species. Some native plants are not as successful as others in sustaining biodiversity. There should be a ranking system of all native plants for this purpose.
There are several key questions related to urban forests’ role in biodiversity. Are urban forests ecological traps? Does bird reproduction, for example in restored urban ecosystems, exceed losses from mesopredators (e.g., cats), toxins, window strike, and road kills? What do we do about trophic cascades caused by the loss of top predators (e.g. there is an overpopulation of deer because most of their predators have been removed). Is the claim that native plants cannot survive in hostile urban environments valid?
Given that urban ecosystems are growing and wildly dispersed, we need to find ways to sustain biodiversity within urban ecosystems. As urban forestry science continues to mature, sustaining biodiversity should be considered one of its primary goals.
Urban Greening: Health Benefits and Caveats of the Urban Forest
Shubhayu Saha, Centers for Disease Control and Prevention
A wide array of studies have identified a range of health benefits directly and indirectly associated with urban forests. Some of the potential long-term beneficial health outcomes include physical activity, improved cardiovascular health, and better quality of life. In a systematic review, better access to parks, trails, and sidewalks is found to be associated with increased outdoor physical activity (Ferdinand, et al., 2012). Though the evidence linking access to green space and obesity prevention is tenuous, the American Heart Association recommends development of trails, parks, recreational opportunities and green spaces within communities. Self-rated quality of life was found to improve with density of public parks (Parra et al., 2010).
Studies have found several mental health benefits to be associated with urban forests. Children with greener play settings exhibited less severe ADHD symptoms (Kuo and Taylor, 2004). Residents in neighborhoods with greater walkability are found to be less hypertensive (Mujahid et al., 2008). There is also weak evidence to support that greater green space is associated with fewer depressive symptoms (Miles et al., 2011).
Research also documents several environmental health benefits to be associated with urban forests. For example, urban trees effectively remove large amounts of airborne pollutants, improving air quality (Nowak et al., 2006). Urban green space can reduce runoff and improve water quality (McPherson et al., 2011). Both tree planting and green roofing have been shown to be effective strategies to reduce ambient temperature in highly urbanized areas (Rosenzweig et al, 2009).
There is a growing recognition of the potential role of urban green space in fostering social capital and promoting environmental justice. For example, participation in an urban greening program was found to be associated with community empowerment and social
cohesion (Westphal, 2003). In a study in Baltimore, inequitable spatial distribution of parks in relation to race and ethnicity was assessed as a reflection of urban environmental inequality (Boone et al., 2009).
Empirical assessments of how urban forests affect health outcomes pose analytical challenges since the pathways linking the two are numerous. There are direct effects where closeness to nature has intrinsic healing effects. On the other hand, some pathways involve an intermediate step where urban forests either need to change an exposure (like air pollution) or behaviors (like active use of trails) that lead to beneficial health outcomes. Measuring some of these aspects requires pooling expertise from multiple disciplines, as well as recognizing that all variables are commensurate in scale. Cross-sectional studies have limited applicability in drawing causal inferences between urban forests and health outcomes. Given that performing randomized control trials with urban forest interventions and health are practically infeasible, statistical techniques (e.g., propensity-score matching), natural experiments, and carefully designed case-control quasi-experimental studies are necessary to increase the evidence base on this issue.
One needs to be aware of some of the unintended consequences of public policies designed to utilize the health benefits from urban forests. There is a policy push towards urban greening as an effective adaptation strategy to combat an increase in extreme summertime heat. However, Jenerette et al. (2011) found an increasing positive correlation in canopy cover and household income over time in Phoenix, implying that poorer neighborhoods had less tree cover and subsequently less of the heat mitigation effect. Urban greening projects have also been associated with a rise in pollen-related respiratory illnesses like asthma and allergic rhinitis. To lessen the allergy impact when planting urban trees, species biodiversity should be increased, the overuse of male pollinating species should be avoided (Carinanos and Casares-Porcel, 2011), and species with low allergenicity should be planted (Ogren, 2000).
An essential requisite in expanding the evidence base linking urban forests and health outcomes is developing a data repository that allows researchers and practitioners to conduct such analyses. The Centers for Disease Control recently launched the National Environmental Public Health Tracking Network,10 which is a system of integrated health, environmental exposure, and hazard information and data from a variety of national, state, and city sources. Suitably-created indices of data on urban forests could be linked with a wide range of health outcome data available through the Tracking portal to facilitate research in this field.
In conclusion, urban forests have a multitude of health benefits, but there are significant challenges. Consideration should be given to health guidelines in any urban tree-planting project. There are also obstacles to long-term monitoring of environmental health through, for example, installation of pollen monitors or tracking variables of urban forests. More resources need to be invested in developing protocols to systematically merge remotely sensed ecological data with spatially referenced health datasets.
Some points raised in the open discussion that followed this panel’s presentations:
- It is common to lose large numbers of trees very quickly at neighborhood scales (e.g., from a major storm). These events may provide opportunities for “paired” neighborhood studies, to look at realtime differences. However, it would take many
years to see evidence of differing ecosystem service outcomes; such studies would require long-term observations.
- Additional studies could determine whether there is a correlation between an increase in biodiversity and enhancement of human health and wellbeing.
- “Horticultural therapy11” may be a logical consideration when measuring the mental health benefits of urban forests.
- Climate change is shifting the natural range of many tree, animal, and bird species.
- “Cultural ecosystem services” is an important consideration within the field of urban forestry.
- Some regulatory agencies may be prohibited from examining benefits of urban forests if these benefits fall outside their mission.
- The District of Columbia (DC) Park Prescription Rating Tool is an example of a tool with the goal of tracking environmental health benefits.
TOOLS FOR ECOSYSTEM SERVICE EVALUATION: MODELS AND METRICS
Moderators: Molly Brown, NASA; Marie O’Neill, University of Michigan
The first two panels discussed the services of urban forests that are quantified through modeling tools, remote sensing, GIS, and other mapping and monitoring technologies. However, uncertainties and challenges remain in developing standard, widely accepted methods for making such estimates.
Panelists were asked to discuss: (1) key gaps in our ability to model, measure, and monitor ecosystem services, and (2) current capabilities for assigning quantitative economic value to these services and strategies for improving these capabilities (in order, for instance, to allow for rigorous cost/benefit analyses and policies that compensate and incentivize land owners for good forestry conservation and planting practices).
Urban Forestry Models
David Nowak, USFS
i-Tree (www.i-Treetools.org), which was released in 2006, is a software suite from the Forest Service that provides urban forestry analysis and benefits assessment tools. It is a collaborative effort and brings users together around one integrated model that assesses many of the functions or ecosystem services of the urban forest. There are approximately 20,000 i-Tree users. i-Tree programs are currently working to integrate with other models such as Biome-BGC12 (Ecosystem process model from the University of Montana that estimates storage and flux of carbon, nitrogen and water), CENTURY13 (Soil Organic Matter Model from Colorado State University), BenMAP14 (EPA’s Environmental Benefits Mapping and Analysis Program), and Silvah/NED15 (a USDA program that emphasizes the analysis of forest inventory data from the perspective of the different forest resources).
About 25 percent of i-Tree users are from outside the United States (Figure 2.6). In 2012, i-Tree released a version for Canada and Australia. In theory, the model could be used anywhere, but in reality, there are challenges with international usage due to differing data formats among different countries.
11 The engagement of a person in gardening activities, facilitated by a trained therapist, to achieve specific therapeutic treatment goals.
FIGURE 2.6. i-Tree Tools desktop application users by country as of April 2013. The United States has the most users, followed by Canada, and Australia. Source: www.itreetools.org.
The framework of an ecosystem service model begins with quantification of the structure of the forest (i.e., the composition of the forest including tree species and density). The function and values of forest resources cannot be calculated without structural information. Urban forestry managers can manipulate the forest structure (what to plant and where), which in turn directly affects forest functions and values. Ideally, they should start by identifying the functions they want to attain, and from that information determine what kind of forest structure is needed to attain those functions.
Models can be used to calculate how trees and the surrounding landscape influence air temperatures. Understanding tree impacts on air temperatures (which is more difficult to accurately model than surface temperature) is critical because it feeds back into many ecosystem services (air pollution, human comfort, stormwater runoff). Air temperature is calculated from regression-based and physical process-based approaches (e.g., EPA’s Weather Research and Forecasting [WRF] Model), which tend to be meso-scale models. The challenge is to use physical process-based approaches to model air temperatures (i.e., simulate the underlying processes that affect air temperature) at the micro-scale within a city.
At this time, i-Tree estimates the effects of trees on building energy use through look-up tables based on various model runs for U.S. regions. The numbers from the tables are based on tree size, and distances and directions from a building. A current challenge is developing a system that is more interactive with building energy models. One goal is to link i-Tree to DOE’s Energy Plus model, which would make it more dynamic. This will require users to provide more information about building types.
There is some social data that can be used in the models. Census data are being incorporated into the model, but there is a desire to develop equations that link urban tree structure to social benefits. Current estimates focus on who is underserved in terms of tree cover and on populations at higher risk to air pollution and heat stress. More equations are needed that link structure to various functions (e.g., human health benefits).
The air quality component of i-Tree is broad scale and estimates pollution removal by trees and VOC emissions. Some current challenges related to air pollution include linking i-Tree with a more integrated modeling framework, developing fine-scale modeling, integrating secondary effects (energy and temperature effects), improving particulate matter (PM) modeling, estimating pollen loads, and linking to regulations.
There are many water quality models including HSPF (Hydrological Simulation Program—Fortran), BASINS (Better Assessment Science Integrating point and Nonpoint Sources), SWMM (Storm Water Management Model), RHESSes, and i-Tree Hydro. Challenges related to urban hydrologic modeling include: making the models more user friendly for local and program managers, capturing water quality measures and procedures, obtaining water quality data for calibrating and verification, linking to pollution reduction credits, and developing more fully distributed models.
Models capture the storage and sequestration of GHGs, particularly carbon dioxide, via biomass equations and growth rates. They also estimate energy impacts on carbon emissions. Future goals are to expand outputs beyond carbon dioxide, gain a better understanding of urban equations for biomass and growth, improve the modeling of tree effects on energy use, and capture tree species influences on albedo and atmospheric conditions (e.g., moisture).
A module is currently being built to estimate tree effects on exposure to ultraviolet radiation. It will be based on simulating shadows and sky view. Current challenges include utilizing Light Detection and Ranging (LIDAR) data, linking to human health, and capturing diverse atmospheric conditions.
Modeling biodiversity, nutrient cycling, and urban soil conditions is limited at this time. Models can estimate tree species diversity, leaf area and biomass, and some soils information. The challenge is to incorporate even more soils data, link structural data to nutrient cycles, and link to forest nutrient and soils models (e.g., BIOME-BCG, CENTURY).
The modeling of wildlife impacts is still in development. Currently nine bird species will be represented in the model, which is small relative to the total number of bird species. Eventually, modelers would like to capture many more species, develop regional equations, and integrate existing wildlife models with urban data.
Various studies on noise exist, but i-Tree does not currently address this topic.
Researchers are currently investigating conversion factors for urban tree biomass to products and fuel production. It is a challenge to capture mortality rates, pruning debris, storm debris, and market data. For example, urban areas tend to discard substantial amounts of wood. How do we encourage this resource to be more fully utilized?
Incorporating monetary values into the model is fairly straightforward. For example, the value of carbon comes from the Interagency Working Group on the Social Cost of Carbon. Users are free to add or adjust for their own values if they do not like i-Tree values. Monetary values are straight multipliers. Water effects are one of the most difficult services to assign a dollar value.
In conclusion, many areas of modeling can be and are being improved. The framework exists to integrate science and models, which will ultimately lead to a more robust integrated systems approach.
Mapping the Urban Forest from Above
Jarlath O’Neil-Dunne, University of Vermont
The use of aerial monitoring to study tree canopy was motivated by two questions from local forest managers: (1) How much tree canopy do we have now? (2) How much room do we have to plant trees?
Accurate estimates of tree canopy are important, especially when the social context is considered. Within any given city, the land is managed by thousands of individual land owners. Quantifying and modeling tree cover at the scale of the land ownership parcels could help motivate residents to maintain or increase their tree canopy.
It is difficult to map trees in urban areas. Shadows from tall buildings can hide trees. The use of LIDAR data can help address this problem. Mapping tree canopy at high resolution allows for studies to be conducted on multiple scales, from parcel or jurisdiction to watershed. For example, studies can begin with individual households and aggregate up to neighborhood level and city level. Or studies can assess larger metropolitan areas and look across several jurisdictions, up to entire watersheds.
FIGURE 2.7: Crime and Tree Canopy in Pittsburgh, PA. This map shows per capita crime and the percent of existing tree canopy at the neighborhood level. There is an inverse relationship between crime per capita and the percent of existing tree canopy. For example, in Highland Park, with its 49 percent tree canopy, there were three crimes per capita in 2010, as compared to 13 crimes per capita in Larimer, where the tree canopy is 22 percent.
SOURCE: Jarlath O’Neil-Dunne. http://dx.doi.org/10.6084/m9.figshare.716318.
Maps need to be affordable, have a high degree of accuracy, and have excellent cartographic representation to be useful to decision makers. It is important to note that mapping does not replace fieldwork. Field inventories provide unique information (e.g., tree species and condition, etc.), that cannot be effectively acquired through overhead mapping. However, unlike field inventories, remotely sensed data can provide a complete census of the tree canopy. High-resolution land cover maps can help resource managers prioritize areas for tree canopy preservation, maintenance, restoration, and plantings. That being said, they will never replace on-the-ground site surveys, as numerous factors go into planting a tree.
These maps do show what areas in the city have a high vs. low percentage of tree canopy and how tree canopy overlays with other variables of interest. For example, tree canopy and crime are closely associated (Figure 2.7).
Mapping larger areas can help address watershed issues across county boundaries. Tree canopy maps can also help city managers and their staff understand ownership patterns, which is important because residents are the primary owners of land where trees can be planted. Many city managers want to increase their cities’ tree canopy by planting street trees, but residential areas (not just streetscapes) as a whole provide the most opportunity for increasing tree canopy.
Mapping of tree canopy can also be used in outreach and communication efforts. Mapping different demographic groups and their geographic spread can help city managers develop tactics to reach out to different groups in different places. Researchers can do a change detection analysis which helps city managers understand where changes in tree canopy are occurring and what the drivers may be. Maps can also be used for pest management, but it is very expensive.
Finally, although there is not a mandate to share the data, it is important to move toward a policy of openly shared local and regional data.
The Role of Urban Forestry in Public Health
Laura Jackson, EPA
EPA recently developed EnviroAtlas, a mapping application that allows users to view and analyze multiple ecosystem services nationally and in specific communities. The beta-release of EnviroAtlas is planned for late Spring of 2013, with the first public version available in Fall of 2013.
A key purpose of EnviroAtlas is to communicate how ecosystem services have an impact on human health and well-being. The following science questions were considered in developing EnviroAtlas:
- How can we effectively quantify and communicate the production of the goods and services we receive from ecosystems?
- What is the supply of those services in relationship to the demand and future demand?
How do drivers of ecosystem services such as land use change (e.g., road development), climate change, and pollutant loads impact the delivery of ecosystem services?
- At the screening level, where does it make sense to invest or prioritize land and water restoration, conservation, or use?
- If we invest in green space, can we reduce the costs of gray infrastructure while also gaining other co-benefits?
- How can we promote the incorporation of this type of information into decision making?
- How can we demonstrate how these services explicitly relate to human health and well-being?
The utility of ecosystem services and green infrastructure to buffer impacts from climate change and extreme events is a key message for the public health community. Furthermore, the loss of ecosystem services is frequently disproportionate in low-income neighborhoods, contributes to cumulative community burdens, and is aligned with the public health concept of social stressors in weakening resiliency and increasing vulnerabilities.
The community component of EnviroAtlas is a high-resolution analysis of 50 cities and towns along gradients of interest (e.g., location, population size, demographics, and health and environmental ranking). Mapped metrics calculated for EnviroAtlas by the Forest Service include ambient air pollutants removed, water runoff reduction and filtration, ambient temperature reduction, carbon storage and dollar valuation, and health benefits of urban air filtration. EPA is developing additional metrics and qualitative information about the following topics: near-road tree buffers and adjacent residential population, vulnerability to heat stress and other localized climate-related hazards, homes and schools with limited green window views, and physical and mental health benefits of access to natural amenities.16
Where possible, EnviroAtlas estimates environmental value in units of public health and well-being (e.g., senior longevity, chronic illness, hospitalizations, days missed from school or work, self-reported happiness) which can all be converted to dollar amounts. However, research on the role of the natural environment in human well-being has not been uniform; variability in study designs and in the selection of specific dependent and explanatory metrics makes it difficult to conduct a metadata analysis for many of these issues. At a minimum, EnviroAtlas provides fact sheets that qualitatively describe the current state of knowledge. EPA will continue to move toward quantitative analyses where possible.
BenMAP is the EPA Office of Air’s model for estimating the human-health benefits of criteria air pollutant rules. It uses data from air quality models and estimates the change in population exposure to certain ambient air pollutants. Based on this information, the model estimates changes in the incidence of a variety of health outcomes. Finally, it places a dollar value on changes in the incidence of health outcomes. Forest Service calculations for EnviroAtlas-Communities include BenMAP estimates at the Census block-group scale.
One significant environmental health issue is the effects of living near roads. Elevated pollutant concentrations (e.g., carbon monoxide, nitrogen oxides, particulate matter mass, benzene, and metals) have been measured near roads. Living, working, or going to school near major roadways has been associated with numerous adverse health effects. These include respiratory and cardiovascular effects, adverse birth outcomes, premature mortality, and cancer. A significantly large portion of the U.S. population lives near large roads, and of those who do not, many work or go to school near large roads.
Can near-road vegetation buffer air pollution? Models and fieldwork suggest that tall, dense vegetation has the potential to improve near-road air quality. However, results vary depending on wind speed, direction, seasonality, road design, and traffic conditions. Barrier type, depth, gaps, and edge effects are also important. Wind tunnel studies and computational fluid dynamics models have respectively shown that roadside vegetation can obstruct ultrafine particles and dilute pollutant concentrations. Field studies show there can be significant buffering of pollution, but the results depend on many variables (including tree type, height, wind conditions). EnviroAtlas is mapping near-road tree buffers, but it is
16 Please refer to EPA’s Eco-Health Relationship Browser at http://www.epa.gov/research/healthscience/browser/introduction.html.
still too soon to do simple predictive calculations. Qualitatively, it appears that having no tree cover is worse for near-road ambient air quality than having a buffer.
In the future, Dr. Jackson would like to replicate published findings on eco-health associations, refine metrics and thresholds for eco exposures,17 conduct meta -analyses (which requires more replicable studies), and conduct more studies to determine causation (i.e., animal studies) and mechanistic pathways (e.g., of how green space alleviates stress). There are key data needs for studying the effects of urban forests on public health: public health data at sub-country scales, morbidity data (e.g., chronic disease, mental health), school performance, and prescription drug sales. Collaborations among the Department of Health and Human Services, the Department of Education, local health departments, local school districts, regional pharmacies, and schools of public health could help address some of these data and analysis needs.
Some points raised in the open discussion that followed this panel’s presentations:
- Currently United States Geological Survey (USGS) is attempting to do nationwide LIDAR data collections; it is important that forest-appropriate data is captured.
- Improving public health studies would help quantify the benefits of urban forests.
- Better models could assess the negative outcomes of trees in a larger context, such as allergy impacts.
- i-Tree can quantify the influence trees have on stormwater runoff, which is important for both regulatory credit design and regulatory project review.
- Some regulators recommend using i-Tree-type data over a 20- or 40-year time span because tree benefits will change over time. However, these calculations are difficult to do because tree mortality data are scarce.
MANAGING THE URBAN FOREST
Moderator: Gary G. Allen, Center for Chesapeake Communities
Given that urban forests are increasingly being viewed as critical to sustaining environmental quality and human well-being, there has been significant growth in the number of urban areas across the United States declaring ambitious goals for expanding their tree canopy. Some cities are going one step further and are attempting to include large-scale tree planting as an official measure in air and water quality control plans. Governance issues of the urban forests is further complicated by the different (and sometimes competing) interests and priorities of the federal, state, and local organizations and private individuals who own and manage the land in cities.
Panelists were asked to discuss: (1) 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, determining planting locations) and (2) opportunities for enhancing collaboration and coordination among federal agencies, academic researchers, and other stakeholders.
17 The amount of exposure to ecosystems a person needs to receive various services (and disservices). For example, how long does a person need to sit in a park to relieve stress?
Air Quality and Urban Forestry
Janet McCabe, EPA
Sustaining urban forestry programs is a significant challenge, and it is becoming especially challenging for some states, given budget constraints. Therefore, it is important to explore how urban forestry programs could provide the added benefit of helping cities and states comply with Clean Air Act regulations. Some benefits of trees are well known (e.g. reducing local temperatures). But some less direct benefits are underappreciated. For instance, a yard that has more trees will need less mowing, thus reducing emissions from that activity. Cars parked under shading trees will be much cooler and have less evaporative emissions. Planting programs can also be designed for reducing emissions by, for example, focusing on large trees that absorb more pollution or on low-maintenance trees (given that the maintenance efforts themselves lead to emissions).
EPA recently launched “Ozone advance/PM advance” for areas that are already meeting current clean air standards, but are close to non-compliance or are expecting growth that will jeopardize future compliance. So far, 31 communities have signed up. Through this program, EPA offers partnerships, information resources, and tools, without any formal expectations or mandates for improvement. Communities can use these resources to help expand community engagement, identify new activities to improve air quality, and expand urban forestry programs.
EPA also provides support for areas that are not meeting current air quality standards. EPA just revised the national standards for PM, and state governments are now in the process of identifying which areas will not meet the new standards. EPA will formally designate areas not in compliance. States with areas that are not in compliance must begin the State Implementation Plan (SIP) process, which is a lengthy process of state planning and EPA approval with the end goal of complying with the Clean Air Act. Under this process, national mandates may drive some actions, but there are opportunities for states and cities to identify their own measures. The question therefore is, can urban forests be part of a SIP? Perhaps, but it would be challenging. In order to be counted in a SIP, a measure has to be quantifiable, enforceable, permanent, and surplus (i.e., not already required for other reasons).
Several cities, including Houston, Baltimore, Sacramento, and New York, have proposed using urban forests in their SIPs. But none have yet been approved by EPA. Houston came close, but the quantification requirement has proven to be a challenge. Cities like the idea of including urban forests in SIPs, but EPA needs to find ways to use these nontraditional programs in the SIP. It would be valuable to have this additional air pollution mitigation measure in the tool box since numerous cities have already undertaken many of the reasonable measures that are available.
Climate change is another major issue that EPA considers in the context of urban forestry. EPA does calculate the impact of trees in their annual GHG inventory. They estimated that in 2011, urban trees stored 69 million metric tons of carbon (EPA, 2013). EPA also acknowledges that the local cooling effect of trees leads to less energy demand for air conditioning, resulting in lower emissions. The role of trees in mitigating UHIs is also of great interest to EPA18.
In conclusion, there are some significant challenges in the regulatory structure, but EPA is committed to encouraging innovative, multi-benefit programs so that in the future, cities can receive regulatory credit for their expansion of the urban forest.
From Street Trees to Sustainability: Science, Practice, Tools
Morgan Grove, USFS
Up until now, most urban forestry research on benefits and services has focused on improving science and tools for general planning measures. But research is needed in quantifying the ecosystem services of urban forests so they can be used in a regulatory context.
The quantification of urban tree benefits has led to interest and demand for tree planting goals. There have been a number of cities declaring ambitious planting goals (typically a symbolic number like 1 million trees). However, does a city have enough plantable space for 1 million trees? How does a city prioritize available sites? Assessments are needed to quantify existing and available plantable space at the decision-making scale. Three questions should be asked when prioritizing where to plant trees in any given city: Where is it biophysically feasible to plant trees? Where is it socially desirable to plant trees? Where is it economically likely to plant trees?
City leaders often ask if they can reach their tree-planting goal exclusively by planting public street trees. This is not possible. The opportunities for increased tree planting are largely in residential areas, which is an extremely distributed set of individually owned land parcels. How do city leaders work with the new “forest landowner” (i.e., the private urban homeowner) to produce a public benefit? What happens when private landowners ask to be paid for the benefits they are providing?
Any particular organization usually has insufficient funds to achieve and maintain a significant urban tree canopy goal. Tools are needed to identify opportunities for coordination and collaboration among the various organizations that have an interest in urban forestry. Coordination and collaboration requires an understanding of the types of organizations, their preferences, categories, and areas of interest, and how the organizations are linked.
Stakeholders and local agencies should work together to develop priority areas for tree planting based on the benefits the organizations would like to attain. Every city department with potential relevance should answer the following three questions: Do you have any regulatory requirements that might involve planting trees? What variables would you use to decide where to plant? How do you share that information? Many city agencies and nongovernmental organizations (NGOs) have overlapping missions related to tree planting. Analyzing and mapping the data from the different agencies based on areas of interest (e.g., a watershed, a neighborhood, etc.) allows scientists to provide individual maps tailored to the different stakeholders. Areas of overlapping interest can be identified when the individual maps are compared.
Such an analysis was conducted in Baltimore. Among the various departments, the highest priority that emerged was reducing impervious surfaces, followed by mitigating the UHI and identifying opportunities for stewardship. Most groups were focused on street trees, with very few groups focused on utilizing residential lands to increase the number of trees.
There were numerous affinities among the groups, based on metrics such as where they work, what they work on, or areas of interest. Understanding stewardship networks is key to addressing the question of which groups are most likely to want to work together. Stewardship mapping illustrates how organizations are working together, or how they may need to.
In Baltimore, most groups were neighborhood-focused, and only a few were city-wide. There was a lot of redundancy among different groups’ goals which encouraged them to
focus on more cooperation and collaboration. These kinds of relational databases can help city leaders determine how to achieve that 1 million tree goal (or whether it is feasible).
The next big step is to think about goods that will ultimately come out of the benefits and services. For instance, a lot of the wood biomass coming out of cities is going to landfills. The “Baltimore wood project19” is focused on assessing optimal uses for all that wood.
In conclusion, the next major phase in urban forestry will be a shift in focus from street-tree planting to sustainability in a broader sense by including goals that are social, economic, and environmental.
Management Challenges and Opportunities: City of Trees
Mark Buscaino, Casey Trees
A recent tree canopy study by Nowak and Greenfeld (2012) showed tree canopy decline in many U.S. cities over the past 10 years with equal increases in impervious surface cover. Following this national trend, Washington DC’s canopy declined 2 percent from 2006 through 2011; historically, aerial photos show that DC’s canopy was 50 percent in 1950 compared to 36 percent today.
In short, arboricultural and urban forestry professionals are failing at keeping our cities green, and development pressures will only make our task more difficult. How can this be reversed?
There are several steps that need to be taken to increase urban tree canopy in cities across the United States. First an inventory of the extent and condition of the urban forest is needed so realistic canopy goals can be determined. While these assessments are becoming more common, many jurisdictions lack resources to conduct them. Another challenge is the lack of national standards for monitoring tree canopy—technology changes so rapidly that jurisdictions often receive conflicting data. A national inventory clearinghouse would greatly facilitate efforts and raise local success, and 10-year interval urban canopy change data at the 1-meter level for all major U.S. cities should be the standard provided by the USFS Forest Inventory and Analysis National Program.20
Once inventory data are available, canopy goals should be set and clearly communicated to the public in easily understandable terms. Until better guidance is available, goals will be set based on what is attainable, but this will do nothing to reverse the national trend of canopy decline. We must answer the question of what is optimal to truly make a difference. More research is needed to help jurisdictions nationwide determine appropriate canopy goals that are based on the multiple benefits of trees—environmental, economic, social, human health, etc., as well as climate constraints of the various regions. When known, this information could change the face of urban areas from coast to coast, and perhaps globally as well.
Achieving these goals requires devising strategies by city leaders, agency heads, nonprofits, interest groups, and others (Figure 2.8). Tree protection laws and regulations form the foundation for canopy goal attainment and shift our culture’s understanding of what is and is not acceptable behavior. From these laws flow other initiatives, but without them it is doubtful that canopy goal achievement will be successful or, even if attained, long-lasting.
FIGURE 2.8 Washington, DC is a good example to highlight the many organizations involved in achieving tree canopy goals, and the land types impacted. DC is complicated due to the fact that a large portion of land (about 30 percent) is owned by the federal government, with that too divided up to several agencies. DDOT (District Department of Transportation); UFA (Urban Forestry Administration); DGS (Department of General Services); DCOP (Department of Human Resources); DDOE (District Department of Environment); NGO (Non-governmental Organization); GSA (General Services Administration); NPS (National Park Service). SOURCE Mark Buscaino, Casey Trees.
Progress on goal attainment needs to be conveyed clearly and consistently. Accomplishing this communication function has been made easier in recent years with e-media and similar outlets, but reporting is also controversial, and national reporting lacks consistency to be useful. A national registry should be published of urban area canopy and impervious surface levels, as well as progress toward meeting urban canopy goals. A national tree report card based on easily verifiable metrics is another option for reporting. Without such reporting, most goals, once achieved, will have no staying power. Communication is critical to long-term success.
Finally, goal attainment requires periodic data collection and information review to ensure progress is being made and the process stays on track. A feedback loop should be incorporated into the broad strategy to ensure success.
Some points raised in the open discussion that followed this panel’s presentations:
- Mr. Buscaino indicated that tree mortality is not a major factor when setting canopy goals. The key is to design an effective maintenance plan.
- Models of air quality impacts of trees are not yet sufficient to be used as a basis for regulatory decision making. States are asking EPA to allow the usage of new and alternative tools.
- National standards for assessing urban tree canopy goals would be useful, but one could argue that guidelines for local-level efforts would be even more helpful.
- Giving high priority to addressing research needs in a regulatory context could help pave the way for cities to receive regulatory credit for expansion of their urban forests.
In closing the plenary session, Mr. Allen said that the most significant threat to urban forests is not the longhorn beetle or the emerald ash bore, but rather the changing demographics of our communities. More and more people are moving to urban areas. Local governments are trying to accommodate this growing urban population, which often leads to incompatible objectives.
As an example, Mr. Allen cited his local jurisdiction in Maryland, which recently adopted an urban canopy goal of planting 20,000 trees in the next decade, partly in response to a Chesapeake Bay program that advocates for local governments in the watershed to set canopy goals. But at the same time, this community also adopted an electrical reliability standard in response to residents’ concerns about power outages due to storms, especially from falling trees. To address these concerns, in less than 18 months the local utilities cut down 30,000 trees-more than the total number of trees slated to be planted in the next 10 years. It is a significant challenge to encourage local stewardship to replace the trees that were cut down for valid electric service reliability reasons (or other local social goods). This example illustrates how the numerous services provided by local governments can be incompatible, and at times, a threat to urban trees. Mr. Allen urged the workshop participants to take a look at service objectives in their local area and determine whether or not they are compatible with preservation, protection, and enhancement of urban forests.
Finally, Mr. Allen noted that the frontier of ecology can be found in urban areas where daily decisions are made about how we live, learn, move, and play. The workshop participants are among the pioneers in this young field. Their work will help focus new research and determine the next steps toward our growing knowledge base. Although much was learned at the workshop, many issues remain, and ultimately it is clear that a broad and challenging agenda lies ahead for urban forestry.
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