Arboriculture & Urban Forestry 36(1): January 2010 Table 2. Survey responses: policies and practices regarding diversity in the municipal forest of California Tree City USA communities. Number of cities responding Community has a goal of increasing diversity of urban forest Community’s goal of urban forest diversity is codified in a management plan Management plan addresses invasive exotic species Community maintains a street tree inventory Inventory is updated at intervals less than one year Inventory is updated at intervals of 10–20 years Community maintains a list of acceptable trees for planting on streets z species were a concern. y 49 40 (82%) 19 (48%) 6 (32%)z 39 (80%) 23 (59%) 6 (8%)y 45 (92%) An additional six respondents without management plans indicated invasive species plans existed. Thus, of the 49 respondents, 12 (24%) indicated invasive Six respondents updated inventories at intervals of 1–10 years; four did not specify. Street tree inventories were provided by 18 respondents. On av- erage, responding communities contained 188 species (range 43– 408) in their urban forest. While the high end of this range (408) seems extraordinary, two additional communities had inventories containing over 300 species. There was no significant correlation between inventory diversity and either community population or per capita income (P > 0.05). The combined inventories of all 18 communities included 632 species (Table 2). As in the planting lists, a small proportion of the species (72; 11%) were found in 75% of the communities (14 or more), while over half of the spe- cies (351; 56%) were found in three communities or fewer. Inter- estingly, there was a strong correlation between number of species in a community’s inventory and the number of species on its ap- proved planting list (r2 = 0.81; P < 0.01). On average, the number of species on the approved planting lists of individual communi- ties was 29% of the existing street tree inventory (range 10–58). In so far as could be determined from titles of respondents, all were employees of the municipality and had direct responsibil- ity for management of the municipal forest. While many were certified arborists and carried the title of Urban Forester, others may have had responsibility for street tree management includ- ed within a broader mandate (e.g., Public Works Supervisor). Table 3. Summary of species on the combined approved planting lists and street tree inventories of California, U.S., communities responding. Approved Planting Listsz Native Species Invasive Speciesy Street Tree Inventoriesz Native Species Invasive Speciesy were provided by 18 communities. y Number of Species 309 30 (9.7%) 12 (3.9%) 632 50 (7.9%) 16 (2.5%) z Approved planting lists were provided by 42 communities; street tree inventories Invasive species are defined by the California Invasive Plant Inventory Database (Cal-IPC 2009). DISCUSSION 21 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 objec- tive into action is more difficult. Fewer than half of those mu- nicipalities indicating a desire to maintain diversity had actually codified their objective into a concrete management plan. Ironi- cally, 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 mu- nicipal forest inventory, and nine of the 39 with an inventory had not updated theirs within the past five years. Maintain- ing an accurate inventory is the first step in maintaining diver- sity 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 for- ests has been expressed in some discussions (Alvey 2006; Ava- los 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 criti- cal comparison of all approved planting lists (from 42 communi- ties) 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 “moder- ate” capacity had been approved, including Eucalyptus globulus (approved by 1 community), Myoporum laetum (1 community), Sapium sebiferum (23), and Washingtonia robusta (17). An ad- ditional 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 individ- ual 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 spe- cies 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 com- munity approved the use of seven species from the same list. This data suggests the need for more rigorous evaluation of po- tentially 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 per- haps equally important and must be addressed when consider- ing diversity of the urban forest. Planting near-relatives adjacent ©2010 International Society of Arboriculture
January 2010
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