18 Muller and Bornstein: Maintaining the Diversity of California’s Municipal Forests Maintaining the Diversity of California’s Municipal Forests Arboriculture & Urban Forestry 2010. 36(1): 18–27 Robert N. Muller and Carol Bornstein Abstract. Policies to promote urban forest diversity were assessed in 49 California, U.S. municipalities through a short questionnaire, followed by evalu- ation of street tree inventories and approved planting lists. While the majority of respondents (82%) indicated maintaining species diversity was an objec- tive in managing their municipal forest, fewer than half of those responding positively (48%) had codified the objective in an actual urban forest manage- ment plan. Protecting against invasive species was an objective for a minority of communities (24%). Street tree inventories, provided by 18 respondents, indicated a high existing diversity within the communities of the state of California (avg. 185 species per community; range 95–408). In communities where both inventories and approved planting lists could be compared directly, the number of approved species for future planting was 29% of the number of species in the existing inventory. This suggests that the future diversity of California’s urban forests may be at risk. In order to improve diversity of their municipal forests, it is suggested communities retain an experimental approach to evaluating new species, build productive collaborations with all stake- holders as well as other communities, and develop outreach opportunities to enhance public awareness of the multiple values of a diverse urban forest. Key Words. California; Diversity; Invasive Species; Municipal Forest; Urban Forest. In both natural and urban ecosystems, species diversity has been recognized as an important attribute of stability, providing resis- tance to diseases, pests, and environmental oscillations. In natu- ral settings, diversity has long been understood to be greatest in benign environments (Huston 1979), and is a product of various ecological interactions (Willis and Whittaker 2002). However, in urban environments these ecological rules do not apply and diversity is primarily a function of human habitat modifica- tion—the product of individuals and institutions whose collec- tive actions have created the landscape we call the urban forest. In metropolitan settings, the benefits of a healthy urban forest are multiple and include several tangible benefits of environmen- tal modification (Nowak and Dwyer 2007). Among these direct environmental benefits are cooling the urban heat island (energy conservation), improved air quality, reduced runoff, noise reduc- tion, and improved wildlife habitat (Gilbert 1991; McPherson et al. 1997). However, healthy urban forests also provide important benefits of social well-being for those who live in urban settings (Dwyer et al. 1992). These less tangible but equally important benefits include desirable habitat (translated as increased real estate values), improved health and well-being of residents, a greater sense of community, reduced domestic violence, and increased economic development (Sullivan and Kuo 1996; Kuo 2003; Elmendorf 2008; Heidt and Neef 2008; Kielbaso 2008). Many of these qualities have been translated into financial terms, which suggest significant economic incentives for maintain- ing a healthy urban forest (Tyrväinena and Miettinen 2000). Most evaluations have considered urban forests assets whose benefits are solely based upon the number and size of trees in the urban landscape [e.g., cooling the urban heat island or moderating stormwater runoff (e.g., McPherson et al. 1997)]. Benefits of di- versity in the composition of an urban forest are not often consid- ered, but if discussed, usually follow ecological reasoning, noting ©2010 International Society of Arboriculture the benefits for a sustained resource in the face of diseases, pests, or significant environmental change (Botkin and Talbot 1992; Clark et al. 1997; Saebo et al. 2005). These are important and real values. High species diversity ensures maximal protection from the impacts of environmental stress, pests, and pathogens, there- by enhancing overall stability (Frank and McNaughton 1991). In natural settings, increased diversity is correlated with increased system-wide stability (Schulze and Mooney 1993), and one might expect the same to hold for managed urban forests, as well. In the special case of urban forests, the human element adds additional dimensions to the importance of diversity. In the view of some, maintaining a diverse urban forest contributes to pre- serving global biodiversity of both species and genetic resourc- es (Löfvenhaft et al. 2002; Heynen 2003; Chen and Jim 2008). A diverse urban forest also provides an array of phenological events, such that leaf out, flowering, fruiting, and leaf fall occur throughout the growing season, which is virtually year-around in much of California. Additionally, high diversity provides nu- merous textures and colors in the landscape and silhouettes on the skyline. Taken together, these attributes of the urban forest contribute to an emotional sense of well-being that translates into reduced stress (Ulrich 1979), and increased rates of recovery from surgery (Ulrich 1984; Ulrich et al. 1991). These beneficial effects of nature on physical and mental health have been docu- mented in hospitals, prisons, and homes (Kaplan 1995). Trees in the urban forest inspire deep emotional feelings that border on the spiritual (Dwyer et al. 1991), and a diverse assemblage of species within the urban forest enhances that perception by creating individually identifiable trees within the broader matrix. Another element of diversity in the urban forest is often over- looked. During European exploration of the world in the 18th and 19th Centuries, exotic plants (including many trees) were collected as scientific curiosities and often served as an entree
January 2010
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