Arboriculture & Urban Forestry 36(1): January 2010 planting lists of 25 communities), Cercis occidentalis (15), Quercus lobata (14), Calocedrus decurrens (12), Sequoia sem- pervirens (11), and Platanus racemosa (9). This contrasts with urban areas in the eastern United States where communities arose in locations originally covered by dense and diverse for- est lands. Perhaps by necessity, early California horticulturists looked abroad for tree species suitable to the state’s signature Mediterranean climate. In addition, California was heavily pro- moted in its early history as a land whose equable climate could grow anything. In a survey of urban forest diversity, California and Florida both exhibited significantly higher levels of diver- sity than 28 other states (American Forestry Association 1990), suggesting the mild temperatures and long growing seasons of both states are conducive to growth of a broad range of species. Standards for Urban Forest Diversity This evaluation of urban forest diversity has focused on the spe- cies richness of municipalities (i.e., the number of taxa present in the municipal forest). In this context, there are no appropriate standards that should be applied in urban forest management. The municipal forest of any community is part of a human-created environment, and its diversity will be the product of the desires of the consumers (residents, organizations, and local government agencies) as expressed in the goals and objectives of the urban forest management plan (Dwyer et al. 2003). The decision to add new species to the inventory will revolve around the desire to enhance the aesthetic beauty of the urban forest environment and to contribute to the emotional health of the community. Assuming that overall cover and health of the trees are the same, these are the attributes which differentiate the municipal forests of the com- munities in Table 1, and make each of these communities unique. Some attention has been given to the appropriate concentra- tion of species in a sustainable urban forest. At issue is avoiding over-representation of single taxa (cultivars, species, genera) in order to prevent significant loss due to unanticipated disease/pest outbreak or environmental fluctuation. Calls made to avoid over- planting have suggested limiting single species to 5%–15% of a community’s inventory (Barker 1975; Grey and Deneke 1989; Moll 1989). More recently, this has been formalized in the 10-20- 30 rule (Santamour 1990; Santamour 2004), whereby no more than 10% of all individuals should come from a single species, 20% from a single genus, and 30% from a single family. This approach does not appear to be rooted in solid ecological prin- ciples, other than recognition that greater evenness of species occurrence (i.e., reduced concentration) may mitigate against devastating losses (Raupp et al. 2006). However, in an extreme case, the rule could be satisfied by no more than ten species, each of which contains 10% of the total number of individuals. Obvi- ously, such a scenario is unlikely to happen. However, it is often the case that most trees in the urban forest come from only a few species even though they may not cross the 10% threshold. The balance, and most, of the diversity is represented by only a few individuals of each species (Lesser 1996). This lack of eq- uity reduces the effective species diversity of the urban forest (Ricklefs and Miller 2000). The primary effect of the 10-20-30 rule is to provide a buffer against over-representation of one or more species. However, even if a single species is marginally within the 10% rule or a single genus within the 20% limit, an outbreak of disease or pest attack could leave a significant hole 23 in a community’s urban forest (Raupp et al. 2006). Furthermore, many significant diseases or pests are not limited to a single species or even genus (e.g., sudden oak death, giant white fly). Of the 17 communities in this study that provided frequency data with their inventory species list, six contained no species with an importance of greater than 10% of the total number of trees. The remaining 11 communities contained only a single spe- cies with importance greater than 10% and five of those were below 15%. In two extreme cases, communities had a single species that accounted for 23.8% and 20.1% of all invento- ried trees, respectively. This planting intensity seems excessive given Arizona ash’s (Fraxinus velutina) weak wood (Gilman and Watson 1993), in the first case; and crapemyrtle’s (Lager- stroemia indica) susceptibility in some micro-climates to fun- gal leaf spot and powdery mildew (Hagan 2004), in the second. Formulating standards for diversity of a community’s urban forest may be of limited value and, further, may be a disser- vice. Certainly, the need to avoid over-representation of single taxa or taxonomic groups is fully justified. However, setting standards to avoid over planting may also provide communi- ties with the false sense of their having attained as diverse an urban forest as reasonably possible. Those standards give no guidelines for under-representation of species that are impor- tant to the taxonomic diversity of a community, but are at risk of loss from lack of management, climatic extremes, or disease. CONCLUSIONS AND RECOMMENDATIONS The data resulting from this survey suggest several conflicting conclusions. On a statewide basis, the existing diversity of Cali- fornia urban forests is high. California communities lie at a unique juncture of soils, climate, and horticultural history that has led to the wide array of species successively grown there. In spite of high existing diversity, there is cause for concern about the future. Approved planting lists include only 29% of the number of trees already in the ground. Certainly, many species are inappropriate for reasons of susceptibility to disease, shallow roots, weak wood, litter, etc. However, there are multiple benefits of a diverse urban forest (Dwyer et al. 1992; Nowak and Dwyer 2007), which are important motivators in seeking new introductions to plantings in public places. The introduction of new or nontraditional species faces many obstacles (Saebo et al. 2005), not the least of which is institutional inertia because urban foresters and landscape de- signers understandably gravitate toward species with known at- tributes and proven performance records (Lesser 1996). The ap- proved planting lists alone suggest many species already present in some California communities may be excellent candidates for other communities. Because they are already “in the ground,” their qualities can be evaluated and appropriateness determined. How- ever, this is inadequate substitution for experimentation with new, previously untested species or cultivars. The benefits of a diverse urban forest are multiple and extend beyond the functional rea- sons of stability (i.e., resistance to pathogens and environmental stress). A diverse urban forest contributes to the aesthetic ambi- ance that defines and enriches a community (Dwyer et al. 1991). That diversity also holds potential for greater understanding and appreciation of the global village that our world has become. In light of this analysis, the following steps are recommended to enhance diversity of a community’s urban forest: * Share approved planting lists among communities, and ©2010 International Society of Arboriculture
January 2010
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