Maintaining the Diversity of California’s Municipal Forests

Municipal Policies Toward Diversity

The concept of diversity has gained currency in many realms of life. A large proportion of respondents (82%) indicated that maintaining diversity was an objective in managing their urban forests. Goals are easy to establish, whereas translating objective into action is more difficult. Fewer than half of those municipalities indicating a desire to maintain diversity had actually codified their objective into a concrete management plan. Ironically, codification in a management plan did not translate into increased diversity on the ground. Although there was a slight trend towards higher diversity in inventories and planting lists of municipalities with an objective of “diversifying their tree population,” this was not statistically significant (P > 0.05).

Had they been asked, any of the professionals surveyed would likely have acknowledged an accurate inventory is a critical tool in effective management of their urban forest. Yet, 20% of communities surveyed did not maintain a municipal forest inventory, and nine of the 39 with an inventory had not updated theirs within the past five years. Maintaining an accurate inventory is the first step in maintaining diversity of a municipality’s urban forest; however, it appears that many communities do not have this tool at their disposal.

Municipal Policies Toward Invasive Species

While concern for introduction of invasive species in urban forests has been expressed in some discussions (Alvey 2006; Avalos et al. 2006), this has not permeated urban forest planning to the extent diversity has. At issue is the possibility that species planted for horticultural use escape the bounds of the managed landscape and significantly alter adjacent native habitats. Only 12 of the 48 respondents (25%) have urban forest management plans that address the issue of invasive species. However, a critical comparison of all approved planting lists (from 42 communities) with lists of known invasive species (Cal-IPC 2009) suggest there is considerable room for improvement. While no species of “high” invasive capacity were listed, four species of “moderate” capacity had been approved, including Eucalyptus globulus (approved by 1 community), Myoporum laetum (1 community), Sapium sebiferum (23), and Washingtonia robusta (17). An additional eight species of “limited” invasive capacity were also listed: Acacia melanoxylon (3), Eucalyptus camaldulensis (1), Olea europaea (9), Phoenix canariensis (6), Prunus cerasifera (25), Robinia pseudoacacia (3), Schinus molle (10), and Schinus terebinthefolius (4). While the extent to which these species were evaluated for potential invasiveness in the context of the individual communities that listed them is unknown (e.g., geographic location and local habitat), it appears this aspect of considering a species’ appropriateness is often overlooked. Seven of the 12 communities with management plans that address invasive species provided approved planting lists. Surprisingly, all but one of those communities approved the planting of at least one of the above invasive species listed by Cal-IPC (2009), and one community approved the use of seven species from the same list. This data suggests the need for more rigorous evaluation of potentially invasive species at the local level, along with improved public awareness (Reichard 1997; Reichard and White 2001).

Genetic pollution of locally indigenous native species is perhaps equally important and must be addressed when considering diversity of the urban forest. Planting near-relatives adjacent to existing stands of native trees may lead to genetic contamination 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 racemosa) populations (Whitlock 2003). Yet, 32 of the 42 responding communities maintained the London plane tree on their approved 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 inventoried trees were accounted for by two species. The next three most abundant species accounted for an additional 20% of inventoried trees. In this same community, 49 of the inventoried species (28%) were represented by three or fewer individuals, a particularly 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 individuals of a given species are required to be considered a viable addition 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 approved 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 forest 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 occurring more rapidly than generally recognized. The anecdotal observations of Santa Barbara’s loss of diversity are a case in point.

The number of species on the approved planting lists was unrelated 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 history of large numbers of species in the ground is tied to a community’s willingness to maintain a large number of species in future plantings. Still, in the absence of concrete steps to assure continued diversity of their urban forests, the future for all of the communities 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 inventories 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 communities 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 diversity 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 communities 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 represented in California, a significant portion of that diversity reflects 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 landscapes 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 inventory 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 California urban forests, including Quercus agrifolia (on the approved planting lists of 25 communities), Cercis occidentalis (15), Quercus lobata (14), Calocedrus decurrens (12), Sequoia sempervirens (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 forest 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 promoted 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 diversity 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 species 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 communities in Table 1, and make each of these communities unique.

Some attention has been given to the appropriate concentration 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 overplanting 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 principles, 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. Obviously, 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 equity 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 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 species 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 inventoried 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 (Lagerstroemia indica) susceptibility in some micro-climates to fungal 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 disservice. 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 communities 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 important to the taxonomic diversity of a community, but are at risk of loss from lack of management, climatic extremes, or disease.