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Urban forestry is the care and management of single trees and tree populations in urban settings for the purpose of improving the urban environment. Urban forestry advocates the role of trees as a critical part of the urban infrastructure. Urban foresters plant and maintain trees, support appropriate tree and forest preservation, conduct research and promote the many benefits trees provide. Urban forestry is practiced by municipal and commercial arborists, municipal and utility foresters, environmental policymakers, city planners, consultants, educators, researchers and community activists.
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. Urban forests improve air quality by absorbing pollutants such as ozone, nitrogen dioxide, ammonia, and particulate matter as well as performing carbon sequestration. 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. Other benefits include noise control, traffic control, and glare and reflection control.
A 2018 study asked low income residents of Philadelphia "how often they felt nervous, hopeless, restless, depressed and worthless." 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.
Urban forestry provides potential habitat for urban wildlife. In addition, it creates great opportunities for observing wildlife to the general public.
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.
The City of Denver Department of Parks and Recreation website 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.
Urban forestry is a practical discipline, which includes tree planting, care, and protection, and the overall management of trees as a collective resource. The urban environment can present many arboricultural challenges such as limited root and canopy space, poor soil quality, deficiency or excess of water and light, heat, pollution, mechanical and chemical damage to trees, and mitigation of tree-related hazards. Among those hazards are mostly non-immediate risks like the probability that individual trees will not withstand strong winds (as during a thunderstorm) and damage parking cars or injure passing pedestrians.
Although quite striking in an urban environment, large trees in particular present a continuing dilemma for the field of urban forestry due to the stresses that urban trees undergo from automobile exhaust, constraining hardscape and building foundations, and physical damage (Pickett et al. 2008). Urban forestry also challenges the arborists that tend the trees. The lack of space requires greater use of rigging skills and traffic and pedestrian control. The many constraints that the typical urban environment places on trees limits the average lifespan of a city tree to only 32 years - 13 years if planted in a downtown area - which is far short of the 150-year average life span of trees in rural settings (Herwitz 2001).
Management challenges for urban forestry include maintaining a tree and planting site inventory, quantifying and maximizing the benefits of trees, minimizing costs, obtaining and maintaining public support and funding, and establishing laws and policies for trees on public and on private land. Urban forestry presents many social issues that require addressing to allow urban forestry to be seen by the many as an advantage rather than a curse on their environment. Social issues include under funding which leads to inadequate maintenance of urban trees. In the UK the National Urban Forestry Unit produced a series of case studies around best practice in urban forestry which is archived here.
There are many benefits, costs, and challenges to planning an urban forest. Urban forests provide both ecosystem services and disservices that are considered prior to planning. Urban forests provide services such as improved air quality, noise reduction, temperature mitigation, and stormwater mitigation when they are placed in the right spot. Urban forest planning is used to maximize the benefits that trees provide by thoughtfully placing them in the best locations. Challenges that are faced during planning include managing the disservices from trees and valuating their services, the loss/replacement cost of green infrastructure, and the cost of remediating gray infrastructure interference. A major loss of green infrastructure could alter the sense of place, community identity, and social cohesion of a municipality.
When planning an urban forest there are several practices that can be used. Many municipalities put plans for an urban forest into an official document such as a master plan. While not every city can implement an urban forest plan, it is possible to implement plans for specific areas, such as parks, that would help increase the canopy cover of a municipality.
During the creation of the urban forest management plan, criteria and goals are usually outlined in the plan early in the planning process. Determining criteria is done by assessing the current state of the urban forest and then incorporating criteria for performance goals into the management plan. Assessment is the first step in planning and provides necessary information on the forest extent, age distribution, tree health, and species diversity. Once the assessment is completed, the next step becomes deciding what criteria--or indicators--to incorporate into the plan so that there are set performance goals. Incorporating indicators into the management plan makes it easier to track the progress of the urban forest and whether goals are being met. Criteria/indicators typically focus on a category of urban forest management and usually include subjects such as:
? The urban forest vegetation and its characteristics such as canopy cover, age distributions, and species diversity.
? Having a community focus that involves industry cooperation, and community and stakeholder involvement.
? The planning of the urban forest and whether it is successful in the management and funding of the urban forest.
The incorporation of indicators into management plans are a strong aid in the implementation and revision of management plans and help reach the goals within the plan.
A key part of a master plan is to map spaces where trees will be planted. In the paper A methodology to select the best locations for new urban forests using multicriteria analysis, three different steps are outlined for determining planting areas. The first stage is an excluding stage, which uses a set of criteria to exclude poor locations and indicate potential locations for planting. Second is a suitability stage, which evaluates the potential locations to determine a more selective group of suitable spots. Finally, the feasibility stage is a final test to determine if the suitable locations are the most feasible planting areas with minimal site use conflicts.
The management of urban forest planning falls into many hands. During the writing process of a plan, the input from professionals and citizens are taken into consideration. When designing the plan and determining planting locations, landscape architects, arborists, and urban foresters provide valuable input and knowledge as to what trees to plant and where, in order to ensure an urban forest that is long lived and healthy. The public works department and planning commissioners also play a role in the process to make sure that no trees are planted where they may interfere with emergency practices, underground or above ground utilities, or safety of the public in any way. Planning for an urban forest involves input from a variety of people and the consideration of how trees affect the community they grow in.
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. 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. 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. 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.
Resolving limitations will require coordinated efforts among cities, regions, and countries (Meza, 1992; Nilsson, 2000; Valencia, 2000).