Arboriculture & Urban Forestry 36(1): January 2010 to distant lands. Today, those exotic curiosities and many sub- sequent introductions grace our urban forests and provide tan- gible connections to the ecosystems and cultures that make up our world. Yet, the origin, ecological characteristics, and cultural uses of species in our urban forests are rarely included in educa- tion programs. Knowing the Australian tea tree (Leptospermum laevigatum) was a reputed source of vitamin C used by Captain Cook to prevent scurvy in his crew brings an appreciation of world history as well as health benefits. Similarly, knowledge of present-day use of locust bean gum, derived from the carob tree (Ceratonia siliqua), as a thickening and emulsifying agent in processed foods ranging from sausage products to ice cream (Coppen 1995) fosters an understanding of the continued, and often unappreciated, dependence of our own culture on plant products. In this context, urban forests can be viewed as “con- servatories of the world,” providing a glimpse into the varieties of trees that peoples of the world see and use in their daily lives. These same conservatories can provide opportunity for educa- tional messages regarding protection of threatened species. For example, the seed of ivory-nut palms (Phytelephas spp.) has been used as a substitute for elephant ivory (Clay and Clement 1993). Urban forest diversity has not been comprehensively evalu- ated and the existing studies often come to contradictory conclu- sions (see, for example, McKinney 2002; Kühn 2004; Wania et al. 2004). Further, no longitudinal studies of diversity change over time have been conducted. Anecdotal evidence in one Southern California metropolitan area suggests that diversity may in fact be falling (Muller, unpublished data). Four editions of Trees of Santa Barbara have provided a species inventory for the metropolitan area that includes Montecito, Santa Barbara, and Goleta. During the period between the last two editions (Muller et al. 1974; and Muller and Haller 2005), the number of trees in the municipal forest (streets, parks, and publicly accessible gardens) declined by over 10% (from 470 to 420). While specific reasons for the loss of each species are not always known, multiple factors likely contributed to the overall decline, including lack of proper main- tenance (i.e., appropriate pruning or watering), drought, disease, and development. Another factor at play is that entrepreneurs and botanical institutions in California have not maintained histori- cal traditions of seeking new introductions for evaluation. This is not surprising considering the significant costs associated with such endeavors and is most evident in the lack of diverse and un- usual inventories in retail nurseries (Zipperer 2008). Plant iden- tification courses required of students majoring in horticulture or landscape architecture tend to favor the more common, easier to find species, thereby reinforcing the familiar. Public intolerance of trees that produce messy litter or have sidewalk-disrupting root systems is yet another factor. Finally, some of those species that were lost may simply have been inappropriate for Santa Barba- ra’s climate. Horticulture, after all, is a continuing experiment. The experiences in Santa Barbara raise serious questions about the motivation and long-term ability of communities to maintain high diversity in their municipal forest. Do communities value species diversity in their municipal forest? Are those values for- mally codified in a community’s urban forest management plan? Do communities help ensure the diversity of their municipal for- est through their list of approved trees for planting along streets and in public places? To evaluate these and other questions, a questionnaire was designed and distributed to determine policies towards species diversity in the municipal forest and to determine 19 limitations in maintaining diversity of these plantings. As part of this evaluation, planting lists of species approved for street tree planting and, where available, inventories of existing street trees for comparison were solicited. The focus of this evaluation was the street tree component of each community’s municipal forest. The municipal forest encompasses both street trees and parks, and is that portion of the urban forest that community agencies can most immediately influence through ordinance, planting programs, and maintenance. Since street trees account for the majority of the mu- nicipal forest, policies directed towards street trees will likely have the greatest and most immediate effects on urban forest diversity. METHODS A short questionnaire was sent to all 169 Tree City USA mu- nicipalities in California, with repeat requests to non-respondents sent five weeks after the initial inquiry. In all, 49 questionnaires were returned (Table 1) for a response rate of 29%. The survey was designed as an initial inquiry into municipal policies/prac- tices regarding diversity and asked questions regarding manage- ment of the municipal forest (Appendix). The combination of closed- and open-format questions allowed the collection of read- ily analyzed data while encouraging respondents to provide ad- ditional insights into their particular situation. While the question regarding invasive exotic species is tangential to the central thrust of the survey on attitudes towards diversity, it is a critical ele- ment of how a responsible community manages its urban forest. The questionnaire also asked respondents to provide cop- ies of “acceptable species” for street tree plantings and, if available, their street tree inventory as well. Forty-two mu- nicipalities provided lists of approved species and 18 mu- nicipalities provided their most recent inventory. Seventeen of these communities provided both approved planting lists and their most recent inventory. For tabulation purposes, cul- tivars were not considered to be distinct from the parent taxon although known hybrids were maintained as distinct taxa. Survey data were tabulated and are presented as the num- ber of respondents answering a question affirmatively or nega- tively. Inventory lists were summarized by number of taxa represented. For correlation purposes, municipality popula- tion and per capita income were obtained from the U.S. Cen- sus Bureau, State and County QuickFacts (U.S. Census Bureau 2008). Population estimates were for 2006 and Per Capita In- come estimates were for 1999. Correlation analysis was con- ducted between street tree diversity (number of taxa present) and independent variables (population, per capita income) as simple linear regression, with significance accepted at P < 0.05. Tree City USA recognizes municipalities that have made a commitment to their urban forest by maintaining: 1) a tree board or department, 2) a tree care ordinance, 3) a communi- ty forestry program with an annual budget of at least USD $2 per capita, and 4) an Arbor Day observance and proclamation. RESULTS The purpose of the survey was to determine policies and practic- es within California municipalities designated as Tree City USA regarding diversity in their urban forest. Of the 49 respondents, 40 (82%) indicated maintaining species diversity was an objec- tive in managing their urban forest (Table 2). However, fewer than half of those responding affirmatively (19; 48%) indicated ©2010 International Society of Arboriculture
January 2010
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