York Urbanist

Urban Forest Values

June 10th, 2015

This is the most comprehensive evaluation of urban forest I have found to date. Herein following is an excerpt from Trees in Canadian Cities: Indispensable Life Form for Urban Sustainability See also: http://yorkurbanist.com/2015/04/24/trees-for-healing/   The graphic is from another source. Information abounds about the intrinsic and extrinsic values of Trees

Benefits of Trees A Suite of Urban-Forest Values (copied from http://www.mdpi.com/2071-1050/7/6/7379/html  )

Trees enhance the aesthetic beauty of the city. The idea that trees are pretty may be self-evident to any urban dweller as visions of green usually inspire calm and pleasant feelings. Indeed, city trees provide a wide set of visual pleasures, from the different colours of the leaves to the size and texture of tree trunks [47]. Trees also have sounds, such as when leaves rustle in the wind or a carpeted floor of fallen leaves is walked upon.

Trees give people joy through the senses. Trees provide shade. Shade is at the root of so many of the social, economic, and environmental benefits provided by trees in the city. It is a vital service in promoting people’s health, as shade trees block harmful ultraviolet radiation [48]. Shade trees, especially ones that are strategically situated where people congregate outdoors like in parks and on sidewalks, can help to mitigate the risk of skin cancer. Children are especially vulnerable to skin cancer, and thus trees in playgrounds and school yards have the potential of significantly reducing their exposure and risk of skin cancer. Through shade, trees sustain human health.

Trees help conserve fuel by reducing emissions from parked vehicles. Outdoor parking lots can be considered miniature urban heat islands with extensive impervious surfaces and low albedo. Vehicles parked in sunlight heat up and emit hydrocarbons into the air from the fuel in their tanks. Well-placed trees can reduce the amount of fuel that evaporates into the atmosphere from vehicles [49]. Thus, trees conserve fuel while concurrently helping to sustain the urban atmosphere. Trees cool the city environment. Due to their built infrastructure, cities frequently experience higher temperatures than the surrounding countryside—this is the urban heat-island effect [50]. Trees reduce ambient air temperatures by altering wind speeds, shading surfaces, and blocking solar radiation [51,52]. Trees also transpire water vapour into the air and, thus, cool it [53,54]. Consequently, the more trees there are in the city, the greater the cooling effect will be. This is especially relevant given expected climatic change and the associated warmer temperatures.

Trees clean the air. Air pollution is an issue for most cities, and urban trees help improve air quality. Gaseous pollutants like ground-level ozone, sulphur dioxide, nitrogen dioxide, and carbon monoxide are removed from the atmosphere by trees by absorbing them through their leaves [55]. Particulate matter is also removed from the air and stored temporarily on the plant surfaces until it washes off in the rain [56]. Overall air quality is better with more trees in the city.

Trees foster health and healing. Trees help purify the air and reduce rates of asthma and other respiratory illnesses in urban populations [57,58]. When we fall ill, or are convalescing from illness, trees can play a pivotal role in healing. Landmark research by Ulrich [59] showed that surgery patients recovered faster and better when the view through their hospital bedroom window was dominated by trees rather than another building. Research in Japan has shown clear health benefits from exposure to forest landscapes [60].

Trees enhance community safety. Security and safety are serious concerns in a city. Among the psychological causes of insecurity and feelings of danger, we find mental fatigue and elevated levels of stress. Although earlier research suggested that dense and naturally vegetated areas were perceived as insecure and threatening (e.g., [61]), these perceptions have been changing through the years with our understanding of how mental fatigue and stress are mitigated by green space and its most dominant feature–trees. More recent research shows that residents living in greener surroundings report lower levels of fear, fewer incivilities, and less aggressive and violent behaviour [62].

Trees increase property values. Trees in the city are a major component of a neighbourhood’s aesthetic appeal and benefit homeowners by adding monetary value to properties. For Portland, Oregon, Donovan and Butry [63] found that a large tree on a residential property can add some $9000 to the sale price of a house. Land owners are also helping others by having trees on their residential properties, because adjacent homes and even entire neighbourhoods benefit from the increased property value. This is an important message to communicate to homeowners, since there is frequently more available space to plant trees on private residential properties than next to publicly owned streets.

Trees reduce energy costs. In some places, roughly 5%–10% of urban electricity demand is spent on cooling/heating buildings. Trees around buildings and houses can act as heat insulators and heat absorbers, shielding buildings from a high-temperature environment through shade, or keeping buildings from losing their heat in winter by increasing the humidity of the surrounding area and slowing down wind [64]. Simulations in Canadian cities have shown that an increase in a neighbourhood’s tree cover by about three trees per house reduces the heating energy of that house by up to 10% and the cooling energy by up to 40% [65]. The annual savings in heating and cooling costs can reach the hundreds of dollars every year depending on house size.

Trees prolong the life of infrastructure. Trees help reduce the amount of maintenance and repair required for city streets, thus reducing costs against the city budget. The asphalt used to pave streets is made up of aggregate held together by asphalt cement. The asphalt cement is a petroleum product, which breaks down and evaporates in the sunlight, causing streets to crack and eventually crumble into potholes, which need to be repaired, or the whole street repaved, at great cost. McPherson and Muchnik [66] found that just a 20% shading of streets in Modesto, California, could save 60% of resurfacing costs over a 30-year period. This service provided by trees is a huge incentive for engineers and indeed all municipal managers to increase tree canopy over asphalt surfaces.

Trees capture and store carbon. Atmospheric carbon dioxide is one of the main drivers of climate change [67]. Its concentration in the air is rising largely because of the burning of fossil fuels like coal, oil, and gas [67]. Anything we can do to slow down emissions of carbon dioxide and increase the rate of its removal from the air will be good for the future of cities [68]. Trees capture carbon dioxide from the air and store the carbon in their trunks, roots, and branches. As long as a tree is healthy and growing, it stores increasing amounts of carbon. The more trees we have in the city, and the larger and longer they grow, the more carbon dioxide will be taken out of the atmosphere [69].

Trees slow down stormwater flow and consequently improve water quality. Urban land is covered mainly by roads, sidewalks, rooftops, and parking lots. Most of these surfaces are impervious and prevent rainwater from being absorbed directly into the ground. Consequently, stormwater and wastewater systems, as well as natural water bodies, are strained during heavy rains as runoff flows into them off impervious surfaces. Excessive runoff can lead to flooding, sewage spillover, and aquatic pollution. This is especially the case in cities like Halifax, where the older parts of the city have combined storm and sanitary sewers. City trees intercept some amounts of rainfall and retain it in their foliage for a period of time [70]. Trees, thus, provide a critical economic service in stormwater management. Trees provide employment opportunities. As they grow up into overhead wires, shed their leaves, grow new branches in undesirable directions, drop dead branches, or die, trees are cared for by municipal workers as well as private landscape contractors and other specialized tree caretakers. Salaries make up large proportions of the budget of tree-care organizations. The more trees there are in a city, the greater the amount of economic activity associated with their maintenance [71].

Trees represent smart societal investments because they demand expenditures on caring for vital urban green infrastructure. Trees support business activity. The services provided by trees provide tangible financial benefits to business owners. Research has shown that consumers perceive business districts with trees as better places to shop [72]. Moreover, consumers say they are willing to pay higher prices, travel further and longer, and shop longer and more frequently in areas with green streetscapes [73]. This not only benefits business owners, but also provides incentive for them to become more actively involved in the stewardship of urban trees.

Trees enhance recreational opportunities. City residents frequently visit treed areas for recreation. Recreation in these areas can be passive or active, ranging from gentle activities such as cultural events, walking, picnics, or tree climbing, to active sports such as running or biking [74,75]. The many types of urban forest formats, ranging from treed streets to dense and naturalized forest remnants, serve diverse recreational uses [76]. Trees enhance tourism. Among the few studies linking urban forests and tourism, Majumdar et al. [77] concluded that for Savannah, Georgia, the better the urban forest, the more attractive the city is for tourists. This seems a reasonable conclusion considering that most city residents would like more trees and better urban-forest management in their own cities [27]. All other things being equal, it seems fair to say that tourists would prefer to visit a well-wooded city as opposed to one with few trees.

Trees provide diverse foods. Trees have been a source of food for people throughout the ages. Urban settings are highly suitable for growing the full range of fruit and nut trees [78]. Additionally, there may be opportunities to pick edible berries and mushrooms that grow on the forest floor of treed parks and other naturalized areas. Thus, trees can contribute, even if in a small way, to food security in the city.

Trees conserve biodiversity. Biodiversity, in the simplest terms, refers to the full diversity of life on earth and includes the diversity of gene pools, species, communities and ecosystems. Trees themselves represent important elements of biodiversity, but they also serve as host and habitat for a wide range of other organisms. The ability of trees to contribute to urban biodiversity increases as one moves from single isolated trees to lines of trees along streets and lanes, and further to stands of trees in parks and other areas [79]. Urban forests can contribute immensely to biodiversity conservation through inclusion of the full range of native tree species in their full spectrum of ages and community associations [80]. Trees promote learning opportunities. Trees provide habitat for many kinds of wild organisms, including fungi, insects, lichens, birds, mammals, and other vascular plants. An excellent focus to start learning about terrestrial nature in the city is the trees. Indeed, there is no better place to learn about nature than to be in it [81]. Getting away from the city and out into natural forests can be costly and may even be impossible for some people. The alternative is to study nature in the city. Trees in the city can provide excellent opportunities to learn about the kinds of species and natural ecosystems there are in the countryside and the wilderness. Research has also shown that urban trees can enhance the learning capacity of learners [82].

Trees impart a sense of place. Feeling a sense of belonging in the city is important to its citizens. The vegetation of a locale can contribute strongly to this sense of place. The presence of trees transforms barren areas into pleasant, welcoming spaces that infuse the city environment with a positive sense of self [83]. It has been shown that well-kept treed neighbourhoods serve to strengthen the ties among residents, generating a sense of place and stewardship among neighbours [84]. This in turn generates important civic values such as a greater sense of safety and adjustment, more use of neighbourhood common spaces, and fewer incivilities [85].

Trees contribute to a sense of well-being. The main argument is that the more that people can experience nature, the better they feel, emotionally, mentally, and physically [11,86,87]. The lack of nature in cities means that many people cannot benefit from it as directly as they might. In many cities around the world, trees dominate the natural ecosystems. If we are to bring nature to the people in an urban environment, that means more trees, not just in total but also more trees in naturalized conditions (see [88]). A healthy urban forest contributes to a healthy and happy people. To sum up, we conclude that trees boast an extensive and diverse array of values to cities. We suggest that they are the greatest contributors to urban sustainability of all forms of plants. In absolute terms, the contributions of trees to urban sustainability are substantial. The more trees there are in the city, the better the city can serve as a good place to live.

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