22 Muller and Bornstein: Maintaining the Diversity of California’s Municipal Forests to existing stands of native trees may lead to genetic contami- nation that will likely have lasting impacts. As a case in point, while London plane tree (Platanus × acerifolia) itself does not exhibit invasive characteristics, genetic interchange has seriously compromised native California sycamore (Platanus racemo- sa) populations (Whitlock 2003). Yet, 32 of the 42 responding communities maintained the London plane tree on their ap- proved planting lists. Similar concerns exist for oaks (Quercus spp.), which are notorious for interbreeding within subgenera. Patterns and Trends of Diversity Within California Communities The size of the inventories and approved planting lists suggest that diversity is well-represented within individual communities of California (Table 1). However, as might be anticipated, most of the inventories were concentrated on a small subset of the overall composition. For instance, in the community with the number of inventory species closest to the average (176), 26% of all inven- toried trees were accounted for by two species. The next three most abundant species accounted for an additional 20% of inven- toried trees. In this same community, 49 of the inventoried spe- cies (28%) were represented by three or fewer individuals, a par- ticularly low number considering potential threats from drought, disease, or development. If those few individuals were planted in close proximity, which is often the case, the susceptibility to any of these threats is further increased. Lesser (1996) found that 15 species accounted for approximately 50% of the planted trees in southern California communities. The reality is that, within individual communities, the concentration of trees among a very few number of species is even greater. On average, among the 18 communities that provided inventory density information, half of the street trees of each community were accounted for by fewer than nine species. This concentration of trees among just a few species suggests, while the number of species found in California communities appears high, diversity is at risk. The loss of a few individuals of an under-represented species may result in the loss of a species from the community. This raises important questions in the analysis of diversity in the urban forest. How many individ- uals of a given species are required to be considered a viable ad- dition to the inventory? Is risk of species loss from the inventory a viable consideration in evaluating urban forest diversity? These questions remain unanswered by those setting urban forest policy. It is also notable that, while communities expressed a desire to maintain diversity in their urban forests, their policies may yield considerably different results. In all communities where both ap- proved planting lists and actual inventories were available (17), almost four times as many species were already present than were approved for future planting. Thus, over the long-term, as species die and are replaced, actual diversity of a community’s urban for- est as measured in its inventory seems likely to decline. Given the considerably shortened life-span of trees in a street tree setting (Beatty 1991), this process of shrinking diversity may be occur- ring more rapidly than generally recognized. The anecdotal ob- servations of Santa Barbara’s loss of diversity are a case in point. The number of species on the approved planting lists was un- related to community dynamics. It was not correlated in any way to a community’s size or economic well-being (but see Hope et al. 2003). Approved planting list size, however, correlated with the number of species in the actual inventory. Seemingly, a his- ©2010 International Society of Arboriculture tory of large numbers of species in the ground is tied to a commu- nity’s willingness to maintain a large number of species in future plantings. Still, in the absence of concrete steps to assure contin- ued diversity of their urban forests, the future for all of the com- munities surveyed seems to be less diverse. Lesser (1996) noted a strong tendency towards planting a few favored species and similarly concluded that this would lead to a decline in diversity. Patterns and Trends of Diversity Among California Communities Diversity of both inventories and planting lists exhibited similar patterns among communities. Average inventories contained 185 species (range 95–408), a surprisingly robust number. However, in the composite, 632 species were represented in the inven- tories of one or more of the California communities surveyed. This is considerably higher than the approximately 410 species reported by Lesser (1996) in a separate survey of 21 southern California cities. The vast majority of the inventory species were found in only a few communities. Indeed, of the 18 communi- ties providing inventory data, all listed at least one species found in no other community. Over half of the inventory species (351) were found on the streets of three or fewer communities. The approved planting lists exhibited similar patterns. Among the 42 responding communities, planting lists contained an average of 49 species (range: 12–105). However, the combined lists of the 42 communities contained 309 species. Almost two-thirds of those (198) were found in five or fewer communities (Figure 1). Conversely, 34 communities had approved species that were found on the lists of only one other community. Thus, while di- versity within California communities may be considered high, it is even better-represented among communities. Similar patterns were exhibited among geographically close communities which share similar environments. For instance, eight of the commu- nities surveyed are from the Los Angeles basin, away from the coast. Their combined inventories shared 466 species (range: 43–357). Almost 60% of these species were found on the streets of three or fewer communities. While patterns of diversity among communities may reflect, in part, the array of environments rep- resented in California, a significant portion of that diversity re- flects the unique history of each community, which has resulted in a correspondingly unique subset of species in its urban forest. Utilizing Native Species in Urban Forest Diversity Interest in protecting and using native species in managed land- scapes continues to grow. While the question was not asked directly in this survey, one municipality described its efforts to emphasize native trees in its street tree plantings as a means to provide habitat for native wildlife. California is perhaps unique in that many of its cities are located in geographic areas that did not originally support diverse forest habitats. Consequently, the number of local native trees that can be used for street tree plantings is limited. Of the 632 species on the combined inven- tory lists, 44 are native to California. Of the 309 species on the combined approved planting lists, 31 are California natives. Most of these natives were poorly represented (few communities and few numbers of trees), and several of the inventory trees were of limited use in urban habitats outside of their natural range. Only a very few native species are used extensively in Califor- nia urban forests, including Quercus agrifolia (on the approved
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