Urban Forestry - Functions and Values

Functions and Values

Function, the dynamic operation of the forest, includes biochemical cycles, gas exchange, primary productivity, competition, succession, and regeneration. In urban environments, forest functions are frequently related to the human environment. Trees are usually selected, planted, trimmed, and nurtured by people, often with specific intentions, as when a tree is planted in a front yard to shade the driveway and frame the residence. The functional benefits provided by this tree depend on structural attributes, such as species and location, as well as management activities that influence its growth, crown dimensions, and health.

Urban forest functions are thus often oriented toward human outcomes, such as shade, beauty, and privacy. As prominent "things," arranged in distinctive formations, trees command a symbolic and material presence that informs how places and landscapes are imagined. This link that humans have to trees has been theorized by Kellert and Wilson (1993) to be a genetically based emotional need to be close to trees and other greenery. According to their the "Biophilia Hypothesis," millions of years of human survival and evolution depended on our ability to cope with the natural world; learning what was safe and dangerous involved the imprinting of strong positive and negative emotional reactions to various natural stimuli. Although 21st century American society is no longer as dependent on nature for day-to-day survival, Kellert and Wilson suggest that closeness to the natural world is still critical for psychological well-being. The complex symbolic and emotional ties that humans have with trees have important implications for the importance of sound urban forest management practices that impact not only quality of life on an ecological level, but on the human and cultural level. People develop emotional attachments to trees that give them special status and value. Removing hazardous trees can be difficult when it means severing the connection between residents and the trees they love. For many, feelings of attachment to trees in cities influences feelings for preservation of trees in forests (McPherson 1998).

The value that people place on trees is especially evident with respect to big trees. There has always been a public fascination with large trees, especially the largest specimens of trees that reach a mature height of greater than 40 or 50 feet (i.e., Champion Trees) (Barro et al. 1997, Dwyer et al. 1991). Moreover, the ability of big street trees to create a ceiling of branches and leaves over all or part of a street impacts the scale of changing shadows cast by the trees, sunlight filtration, and other human-scale considerations that provide a changing visual environment (Zube 1973, Jones and Cloke 2002). In their qualitative study of Denmark residents’ perceptions of the importance of the urban forest, Hansen-Moller and Oustrup (2004) found that the scale of urban trees was one of the main conditions of an "ideal" urban forest, through its volume, height, and ability to envelop a person, thus creating a barrier from the outside world.

Urban forests bring many environmental and economic benefits to cities. Among these are energy benefits in the form of reduced air conditioning, reduced heating by shading buildings, homes and roads, absorbing sunlight, reducing ultraviolet light, cooling the air, and reducing wind speed (McPherson 1994; McPherson & Rowntree 1993; Simpson & McPherson 1996; Coder 1996; Wolfe 1999; Hastie 2003; Lohr et al. 2004). There are also economic benefits associated with urban trees such as increased land, property, and rental value (Morales et al. 1983; Anderson & Cordell 1988; Wolf 1998; Dwyer et al. 1992; Mansfield et al. 2005; Orland et al. 1992; Hastie 2003; USDA Forest Service 2003, 2004). Well-maintained trees and landscaped business districts have been shown to encourage consumer purchases and attract increased residential, commercial and public investments (Wolf 2004, 2007). Trees located in business areas may also increase worker productivity, recruitment, retention and satisfaction (Kaplan & Kaplan 1989; Kaplan 1992; Wolf 1998). Urban forests also improve air quality, absorb rainwater, improve biodiversity and potentially allow recycling to 20% of waste which is wood-based

The social and even medical benefits of nature are also dramatic. Urban poverty is common to areas lacking green spaces. Visiting green areas in cities can counteract the stress of city life, renew vital energy and restore attention, and improve medical outcomes. Simply being able to see a natural view out of the window improves self discipline in inner city girls

Having regular access to woodland is desirable for schools, and indeed Forest kindergartens take children to visit substantial forests every day, whatever the weather. When such children go to primary school, teachers observe a significant improvement in reading, writing, mathematics, social skills and many other areas.

Various methods are available to capture the value of urban trees, each designed to analyse a specific type of green space (individual trees, parks, trees on golf courses etc.). The following are examples of studies that have used these different approaches, along with their respective constraints.