Urban forestry

Professional Tree Climber (arborist: Zack Weiler) climbing a willow tree in Port Elgin, ON. Canada

A showy urban forestry featuring autumn foliage in Sydney, Australia.

Urban forests mitigate the effects of urban heat island through evapotranspiration and the shading of streets and buildings. This improves human comfort, reduces the risk of heat stroke and decreases costs to cool buildings.[2] Urban forests improve air quality by absorbing pollutants such as ozone, nitrogen dioxide, ammonia, and particulate matter as well as performing carbon sequestration.[3] Urban forestry can be an important tool for stormwater management as trees absorb and store rainwater through the canopy, and slow down and filter runoff with their roots.[4] Other benefits include noise control, traffic control, and glare and reflection control.[5]

A 2018 study asked low income residents of Philadelphia "how often they felt nervous, hopeless, restless, depressed and worthless."[6] As an experimental mental health intervention, trash was removed from vacant lots. Some of the vacant lots were "greened", with plantings of trees, grass, and small fences. Residents near the "greened" lots who had incomes below the poverty line reported a decrease in feelings of depression of 68%, while residents with incomes above the poverty line reported a decrease of 41%. Removing trash from vacant lots without installing landscaping did not have an observable mental health impact.[6]

Urban forests in the built environment affect urban wildlife in several ways. An urban habitat can impact wildlife behavior significantly and can alter the ecology of urban wildlife, influencing these organisms’ behavior. The interactions between humans and wildlife and the impacts of urbanization on these wildlife populations influence cities across the world.

Disturbances in urban forests are known for occurring more frequently and with higher intensities than in nature. Changes in the urban landscape can lead to greater competition for resources among species on fragmented areas of land, leading to more stress for urban wildlife.[7] Urban wildlife is also exposed to warmer temperatures as well as higher levels of pollution as cities alter the natural environment significantly.[7]

The construction of urban infrastructure requires deforestation, leveling, and other activities that lead to habitat fragmentation, reduced genetic diversity, and changes in behavior.[8] Urban wildlife is also exposed to higher amounts of toxic substances, including heavy metals, road treatments, or pesticides from lawns that can lead to abnormal reproduction or development.[7] Consumption of prey species by domesticated pets, such as dogs and cats, also leads to an increased mortality rate in urban habitats.[8] Urban forests are essential to creating habitats for wildlife within cities, and many species have adapted to living in the disturbed conditions of the built environment by utilizing urban green-spaces.[9] Research has shown diverse green-spaces to be better suited for wildlife. For example, in Krakow, Poland, the species richness of owls was higher in parts of the city with varied land uses than more homogeneous areas.[10] Additional support for land-use diversity in urban areas is provided in a study showing the importance of leaving dead and decaying trees on the landscape for wildlife habitat.[9]

Urban forests can alter natural diets by providing dietary supplements to wildlife in the form of fruit or nut-producing ornamental plants, trash, or even domestic pets like cats.[11] By examining coyote scat and using stable isotope analysis, it was estimated that about 22% of the scat or 38% of the urban coyote diet was from human-created sources.[11] Wildlife is also attracted to urban forests for their increased surface waters due to reduced runoff in these areas.[11] Having wildlife interacting around humans in urban areas can create conflicts between humans and animals. A case study in Aspen, Colorado observed the foraging habits of bears, tracking their movements using GPS collars, and found that bears visited forested areas in the city with fruit-bearing trees for food.[12] Alternatively, in a study on the behavioral ecology of urban deer populations, the authors discussed the difficulty of managing this species due to its positive public perception as an aesthetically pleasing animal.[13] Proper species selection, placement of trees, and other urban forest management strategies can be utilized to mitigate human-wildlife conflicts in cities.[13]

Urban forestry provides potential habitat for urban wildlife. In addition, it creates great opportunities for observing wildlife to the general public.[14]

Urban forest related events such as planting festivals can significantly reduce social isolation problems, enhance people's experience and raise environmental awareness. Urban forests also encourage more active lifestyles by providing space for exercise and are associated with reduced stress and overall emotional well-being. Urban forests may also provide products such as timber or food, and deliver economic benefits such as increased property values and the attraction of tourism, businesses and investment.[15] Street trees, if managed and cared for, are beneficial in creating sustainable and healthy communities.[16]

The City of Denver Department of Parks and Recreation website[17] hosts interactive online tools that allow residents to view the financial impact to their neighborhoods directly related to healthy tree planting. In the Washington-Virginia Vale neighborhood the city website cites 2,002 individual trees as having been planted and maintained by the City Forester. These trees are believed to bring in an annual ecosystem benefit of $159,521. This is mostly wrapped up in property benefits, which cite a contribution to this total of $143,331. The majorities of these trees are between 0 and 12 feet tall and are a mix of mostly Elm, Maple, Pine, and Locust species.[18]

Trees serve an economical function within the urban forest, providing various monetary benefits. It is estimated that there are around 3.8 billion trees in urban areas around the United States, equating to $2.4 trillion in overall structural value.[19] In addition, environmental and social benefits such as air quality, climate change, water flow, real estate, and even community well-being can be quantified to determine their economical impact.[19] Examples of the economic values created by the urban forest includes an annual $4.7 billion of air pollution removal, $3.8 billion in carbon sequestration.[19] Additionally, recreational experiences have the potential to surpass $2 billion in annual value.[20] Furthermore, while these are national estimates for the United States, it is necessary to note that all of these estimates may vary by location.

The value of an urban forest is estimated by quantifying social and ecosystem services, then assigning those services monetary worth, which are often based on market value.[21] Modeling tools, such as i-Tree, are used by urban foresters to accurately assess the effects of an urban forest’s structure; this information is used to quantify ecosystem services and ultimately the economic value of the forest across a variety of locations.[19] By creating these models, urban foresters are able to quantify and communicate the value of the urban forest to stakeholders and the general public. These evaluations can be used to influence the amount of money allocated to tree management by the government and general populace.[21] Trees may live a long and healthy life if they continue to receive proper management in the form of maintenance and pruning, which sustains the value of the urban forest.[19] Moreover, after death, trees have the potential to remain profitable to the community— if utilized correctly.

Typically, wood products such as lumber and wood pellets are associated with rural forestry and logging. Annually, urban forestry creates 14.8 metric tons of wood waste in the United States through pruning and removal.[22] Within urban forestry there are initiatives to use this waste as wood products such as fuel, lumber, art, and more.[22] These initiatives seek to extend the value of urban trees after their lives.[23] One such initiative is the Virginia Urban Wood Group, a nonprofit with the mission to, “enhance the quality of life through the Stewardship of our Commonwealth’s urban and community trees.”[24] The Virginia Urban Wood Group promotes the production and sale of wood products sourced from urban wood waste.[24] The group connects governmental and commercial professionals such as arborists, municipal foresters, mills, carpenters, and more.[24] Another group contributing to the urban wood waste industry is Wisconsin Urban Wood. This group collects suitable removed trees from local businesses and arborists and sells the wood to local mills.[22] While urban lumber may not be as high of a grade quality as forest grown lumber, these products are suitable for smaller projects such as woodworking and artisan furniture.[24] Some localities use their urban lumber to reduce costs on amenity construction— they use their wood to build their picnic tables and benches.[23] Additionally, some urban wood initiatives seek the use of reclaimed wood to decrease the use of freshly cut lumber.[22]

A street tree is any tree that is growing in a city right-of-way, whether between the sidewalk and the curb or in an unimproved right-of-way.[25]

Urban forest assessment is a strategy that is used within broader management and planning operations that allows urban foresters to better understand and care for the forest resource at hand.[31] It allows aspects of the forest, such as ecosystem services and benefits, species composition, canopy distribution, and health, to be monitored and predicted for current and future management needs.[32] Data from urban forest assessments can prove to be useful in not only providing information for foresters but in quantifying benefits that can show members of the public the importance of preserving and protecting trees in urban forest settings. Urban forest assessments are becoming integral to trees in urban communities as they plan and care for their trees, an example being found within cities like Tallahassee, Florida[33] that have incorporated assessment into their urban forest master plan. Within the United States, the USDA Forest Service has provided resources[34] to inform foresters and community members about the importance of these assessments and the benefits to conducting them.

Urban forestry planning and management methods are key to creating and maintaining an urban forest that produces sustainable benefits for the surrounding community. Stakeholders, such as individual citizens, local volunteer groups, and political figures, can oftentimes be involved in the urban forest planning and management processes within municipalities.[35] Urban forest assessments have the potential to increase urban forest economic, social, and cultural benefits to the community.[35] Diverse stakeholder groups allow a comprehensive plan to develop with unique elements brought to attention by each group. Things included in an urban forest plan include land use, transportation, infrastructure, and green space because they all affect the urban forest structure.[36] It will be determined per municipality why each of these is of certain importance and vice versa, as well as the proper actions to be taken to protect the urban forest function and role in the area.

An assessment must first be completed before any benefits are gained. There are generally two basic ways that urban forests are assessed. The bottom-up approach is a field inventory completed by crews on the ground.[31] This process is detailed and can provide useful forest information needed for management decisions. The top-down approach utilizes aerial and satellite imagery to discern canopy cover, plantable space, and impervious surfaces at a low cost.[31] There are different tools available to complete these assessments. i-Tree is a set of tools cooperatively created and maintained by the USDA Forest Service and other organizations. i-Tree Eco is commonly used for bottom-up approach assessment, and uses the field data collected by the user to quantify value and benefits of the trees.[37] The i-Tree software also has tools helpful to top-down approaches. i-Tree Landscape uses National Land Cover Database (NLCD) along with other layers to provide information about canopy cover, plantable space, ecological benefits, and more.[38] i-Tree Canopy allows the user to interpret aerial and satellite imagery to determine land cover on a smaller scale than landscape.[39]

Cambridge Urban Forestry (Massachusetts)

Tree warden laws in the New England states are important examples of some of the earliest and most far-sighted state urban forestry and forest conservation legislation. In 1896, the Massachusetts legislature passed the first tree warden law, and the other five New England states soon followed suit: Connecticut, Rhode Island, and New Hampshire in 1901, Vermont in 1904, and Maine in 1919. (Kinney 1972, Favretti 1982, Campanella 2003).

As villages and towns grew in population and wealth, ornamentation of public, or common, spaces with shade trees also increased. However, the ornamentation of public areas did not evolve into a social movement until the late 18th century, when private individuals seriously promoted and sponsored public beautification with shade and ornamental trees (Favretti 1982, Lawrence 1995). Almost a century later, around 1850, institutions and organization were founded to promote ornamentation through private means (Egleston 1878, Favretti 1982). In the 1890s, New England's "Nail" laws enabled towns to take definitive steps to distinguish which shade trees were public. Chapter 196 of the 1890 Massachusetts Acts and Resolves stated that a public shade tree was to be designated by driving a nail or spike, with the letter M plainly impressed on its head, into the relevant trunk. Connecticut passed a similar law in 1893, except its certified nails and spikes bore the letter C. (Northrup 1887).

The rapid urbanization of American cities in the late 19th century was a concern to many as encouraging intellectual separation of humanity and nature (Rees 1997). By the end of the 19th century, social reformers were just beginning to understand the relationship between developing parks in urban areas and "[engendering] a better society" (Young 1995:536). At this time, parks and trees were not necessarily seen as a way to allow urban dwellers to experience nature, but more of a means of providing mechanisms of acculturation and control for newly arrived immigrants and their children (e.g., areas to encourage "structured play" and thus serve as a deterrent for youth crime) (Pincetl and Gearin 2005). Other prominent public intellectuals were interested in exploring the synergy between ecological and social systems, including American landscape architect Frederick Law Olmsted, designer of 17 major U.S. urban parks and a visionary in seeing the value of including green space and trees as a fundamental part of metropolitan infrastructure (Young 2009). To Olmsted, unity between nature and urban dwellers was not only physical, but also spiritual: "Gradually and silently the charm comes over us; the beauty has entered our souls; we know not exactly when or how, but going away we remember it with a tender, subdued, filial-like joy" (Beveridge and Schuyler 1983 cited in Young 2009:320). The conscious inclusion of trees in urban designs for American cities such as Chicago, San Francisco, and Minneapolis was also inspired by Paris's urban forest and its broad, tree-lined boulevards as well as by the English romantic landscape movement (Zube 1973). The belief in green cover by early park proponents as a promoter of social cohesion has been corroborated by more recent research that links trees to the presence of stronger ties among neighbors, more adult supervision of children in outdoor areas, more use of the neighborhood common areas, and fewer property and violent crime (Kuo et al. 1998, Kuo and Sullivan 2001, Kuo 2003).

Many municipalities throughout the United States employ community-level tree ordinances to empower planning officials to regulate the planting, maintenance, and preservation of trees. The development of tree ordinances emerged largely as a response to the Dutch Elm Disease that plagued cities from the 1930s to 1960s, and grew in response to urban development, loss of urban tree canopy, and rising public concern for the environment (Wolf 2003). The 1980s saw the beginning of the second generation of ordinances with higher standards and specific foci, as communities sought to create more environmentally pleasing harmony between new development and existing infrastructure. These new ordinances, legislated by local governments, may include specific provisions such as the diameter of tree and percentage of trees to be protected during construction activities (Xiao 1995). The implementation of these tree ordinances is greatly aided by a significant effort by community tree advocates to conduct public outreach and education aimed at increasing environmental concern for urban trees, such as through National Arbor Day celebrations and the USDA Urban and Community Forestry Program (Dwyer et al. 2000, Hunter and Rinner 2004, Norton and Hannon 1997, Wall et al. 2006). Much of the work on the ground is performed by non-profits funded by private donations and government grants.

Policy on urban forestry is less contentious and partisan than many other forestry issues, such as resource extraction in national forests. However, the uneven distribution of healthy urban forests across the landscape has become a growing concern in the past 20 years. This is because the urban forest has become an increasingly important component of bioregional ecological health with the expanding ecological footprint of urban areas. Based on American Forests' Urban Ecosystem Analyses conducted over the past six years in ten cities, an estimated 634,407,719 trees have been lost from metropolitan areas across the U.S. as the result of urban and suburban development (American Forests 2011). This is often due to the failure of municipalities to integrate trees and other elements of the green infrastructure into their day-to-day planning and decision-making processes (American Forests 2002). The inconsistent quality of urban forestry programs on the local level ultimately impacts the regional context in which contiguous urban forests reside, and is greatly exacerbated by suburban sprawl as well as other social and ecological effects (Webb et al. 2008). The recognition of this hierarchical linkage among healthy urban forests and the effectiveness of broader ecosystem protection goals (e.g., maintaining biodiversity and wildlife corridors), highlights the need for scientists and policymakers to gain a better understanding of the socio-spatial dynamics that are associated with tree canopy health at different scales (Wu 2008).

In the UK urban forestry was pioneered around the turn of the 19th century by the Midland reafforesting association, whose focus was in the Black Country. England's Community Forests.[52] programme was established in 1990 by the then Countryside Commission as a pilot project to demonstrate the potential contribution of environmental improvement to economic and social regeneration. Each Community Forest was established as a partnership between local authorities and local, regional and national partners including the Forestry Commission and Natural England. Collectively, this work has formed the largest environmental regeneration initiative in England. In the mid 1990s the National Urban Forestry Unit (NUFU) grew out of a Black Country Urban Forestry Unit and promoted urban forestry across the UK, notably including the establishment of the Black Country Urban Forest.[53] As urban forestry become more mainstream in the 21st century, NUFU was wound up, and its advocacy role now carried on by organisations such as The Wildlife Trusts and the Woodland Trust.

Nanjing Vertical Forest Project designed by Stefano Boeri Architetti is currently under construction. 600 tall trees, 200 medium-sized trees and 2,500 cascading plants and shrubs will be planted on the building facades.[54] It is expected to absorb 18 tonnes of CO2 while providing 16,5 tonnes of Oxygen annually.

A 99 km long and 100 m wide forest belt surrounding the city was completed in 2003. The heat island issue has been significantly reduced.

Another pilot project by Shanghai Municipal Agricultural Commission aims to convert 35% of the total area of Shanghai to urban forest. A forest network of two rings, eight lines, five zones, multi-corridors, multi-grids, and one chain was introduced in the project, which means planting two ring-shaped forests, an inner ring 500 m wide by 97 km in length surrounding the central district, and an outer ring 180 km long in suburban land, eight longitudinal forest belts 1000 m wide along expressways and major rivers, five large forest parks about 30 km2 each in area scattered in the suburbs, multiple green corridors 25 to 500 m, grids of forests along the seashore and in industrial areas, and one chain linking various habitats.[55]

Curitiba is internationally known as a pioneer city in conservationist efforts. Since 2006 Curitiba has the RPPNM project, allowing owners of relevant native areas within the city to turn them into privately owned natural reserves in exchange of being able to transfer that area's constructive potential somewhere else. This means instead of building on an area of Atlantic Forest, the owner of such can add what could have been built there somewhere else, allowing the building to which the building potential was transferred to surpass the usual urbanistic height and density limit, thus preserving the forest and zeroing the urban impact. The project won 2006's UNEP-Bayer Young Environmental Envoy programme.

Following urbanization in Europe, rapid city expansion resulted in forests being kept to the edge of cities, making the only urban greenspaces privately owned by monarchs, religious establishments, and other positions of power.[56] Over time, as democracies began to gain emerge, the public was able to express interest in public recreational areas. Urban forest development was initially dictated by the wealthy and upper class society, yet in the second half of the 19th century, direct government intervention increased.[56] At the same time, more urban greenspaces began opening to the public. The development of urban greenspaces led to a need for management of these areas, leading to the urban forester professions becoming commonplace.[56] Forestry experts then became more involved in forest and green services management as localities and national forest services became responsible for these areas.[56]

According to a study published in the Scandinavian Journal of Forest Research, an average of 53% of forest lands within any Danish municipality are owned by the municipality itself.[57] While this number varies respectively as the size of a municipality increases and decreases, this average serves as a general statistic. When compared to the other Scandinavian countries, Denmark’s municipalities are unique in that they regularly buy and sell land to the private sector. This exchange of land results in various owners of the green spaces that reside within Denmark’s urbanized areas. Only around a quarter of municipalities in Denmark have woodland policies in place for managing their urban forests. The others either have a stand-alone policy (around 20%), or no policy at all (roughly 30%).[57] In fairly recent years, the budget for parks and tree maintenance in most places seem to be steadily dwindling. Sweden as well has transitioned into more of a conservation and active management mindset. In Sweden, the urban forests and green spaces are classified into five zones based on size and use.[58] After classification, recommendations for future improvements and management strategies are formed. In addition to urban zone classification, the use of i-tree inventory is also used for the assessment and management planning of their urban green spaces.[58] Swedish municipalities are constantly innovating and adapting their managing strategies for the old growth forests in central urban areas and the younger forests on the outskirts.

Most of the species in Scandinavian urban forests are native, with a majority of people stating their preference for native species.[59] Common species include Norway Spruce (Picea abies), Scots Pine (Pinus syl vestris), Silver Birch (Betula pendula), and Moor Birch (Betula pubescens).[59] Urban forests also tend to be fairly irregular in age and tree placement, however general favor tends to be shown towards older trees.[59] Visibility is rated as a priority in the design of these places, and is a common issue faced by managing officials.[59] Between surveys conducted across Finland, Denmark, and Sweden, approximately 53% of Urban Canopy cover is managed directly by municipal governments, while the rest is under private ownership.[60]

With over 75% of Canadians in urban areas, urban forests play an important role in the daily lives of Canadian citizens. Urban forests provide numerous environmental and health benefits to the people of Canada.[61] Over time, the use of urban forestry in Canada has changed. In the 1960’s, Dr. Erik Jorgensen of the University of Toronto, coined the oxymoronic term “urban forestry” while assisting a master’s student with his curriculum.[62] However, after this milestone in the urban forestry community, urban forestry faded to the background with few accounts of urban forestry being practiced. As urban forestry started gaining recognition globally and the importance of urban forestry was realized, Canada began creating Urban Forest Management Plans (UFMPs). These plans focus on maintenance, improving canopy cover, enhancing tree species diversity, and educational programs, without focus on economic or environmental services urban forests provide.[63] Today, Canada is conducting studies to address the gaps within their urban forestry programs. Because urban forestry is practiced under different departments, labels, and disciplines, the true extent of urban forestry in Canada is unknown.[61]

The University of Toronto during the 1960’s was home to some of the most significant forest pathology developments of the decade. Two professors at the university (Dr. Jorgensen, and media professor Marshall McLuhan), were given the catalyst to pioneer the discipline of “urban forestry” when the crisis of Dutch Elm Disease threatened 90% elm mono-culture at the university.[64] What made this new discipline different from prior urban tree management strategies was the sense of scale. Prior to the 1960’s urban trees were managed on a tree-by-tree basis.[64] The Dutch Elm Disease finally convinced forest pathologists at the school to consider the urban forest on a systems level, where small changes can create forest-wide effects if not properly managed. In 1962 this thinking gave Jorgensen a convincing enough argument to secure funding for the world’s first “Shade Tree Research Laboratory” in an old dairy plant that the university owned. By 1965 the University of Toronto had its first official urban forestry course, called “the Study of Urban Forestry”, taught by Dr. Jorgensen.[64] Only one year later department head, Dean Sisam, applied the term to the previously known courses of “arboriculture and parks management”, three years following that the university began creating diplomas for urban forestry; producing seven graduates by 1982.[64] The University of Toronto’s program has continued and has grown significantly into current times, inspiring many other institutions to offer a similar diploma as the discipline diffused across the globe.

Erik Jorgensen began as a forest pathologist for the federal government in Denmark, he then moved to Toronto in 1959 to begin studies on Dutch Elm Disease (DED). Which at the time was spreading through North America at extreme rates and killing thousands of Elm trees in its path.[65] He was a professor of Forest Pathology at the University of Toronto throughout the 1960s.[65] While being interviewed for a newspaper article in 1969 he defined Urban Forestry as “a specialized branch that has as its objective the cultivation and management of city trees”.[64] He continued his career at the University of Toronto and his laboratory became increasingly devoted to shade tree research in Canada.[64] Jorgensen continued to define and justify the importance of Urban Forestry through his conference papers published in the Shade Tree Research Laboratory throughout the 70’s and 80’s.[64] He ended up leaving the University in 1973 to lead a National Urban Forestry program in Ottawa, Canada.[65]

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