Arboriculture & Urban Forestry 33(5): September 2007 353 South Dakota street tree populations in the surveyed com- munities ranged from a low of 0.13 to a high of 2.41 street trees per capita. The average was 0.40 street trees per capita but with differences among and within the three community population classes (Table 2). A total of 22,390 street trees representing 33 genera and 62 species were inventoried in the 34 communities included in the survey (Table 3). The most common species used state- wide as a street tree is green ash at 35.3% of the total street tree population. The next most common is American elm (9.1%) followed by crabapple cultivars (Malus spp.) (7.7%), silver maple (A. saccharinum) (5.8%), and hackberry (Celtis occidentalis) (5.3%). The ranking and species changed with community popu- lation size (Table 4). Green ash was still the most common species regardless of the population size of the community, but it became more abundant in the smaller communities. Siberian elms also became more abundant in these commu- nities. Cottonwood (Populus deltoides) and Colorado blue spruce (Picea pungens), two species often prohibited from planting in larger communities, are common street trees in smaller towns. The surveyed communities in South Dakota have slightly less than half of their green ash population with a diameter smaller 30.5 cm (12.2 in) (Table 5). American elm, as a comparison, has only 4.9% of their street population in the 2.5 to 30.5 cm (1 to 12.2 in) categories. Two other most common street trees, silver maple and hackberry, had only approximately one-fourth to one-third of their trees in the 2.5 to 30.5 cm (1 to 12.2 in) categories. Crabapples, also one of the top five most common species, had all the trees in the 2.5 to 30.5 cm (1 to 12.2 in) categories, but this is to be expected given the mature height of the cultivars in this species. Wray and Prestemon (1983) defined full stocking as street tree populations having spacing between stems of approxi- mately 15 m (49.5 ft), which was modified to 13.7 m (45.2 ft) for South Dakota because this is the common spacing in our communities. The communities included in this survey had a range of full stocking between 18.5% and 74.6% with an average of 45.6%, meaning slightly more than half of the planting sites are not occupied, particularly in the smaller communities (Table 1). This does not necessarily mean that planting all available sites would double the population of street trees; in fact, it would be less than double. It was Community class Class 1 Class 2 Class 3 SE standard error. ©2007 International Society of Arboriculture Mean city population 23,662 1256 174 common to find residents that had planted street trees closer than the typical spacing of 13.7 m (45.2 ft) and some prop- erties had tree spacing less than 4.5 m (14.9 ft) along their portion of the street. More information regarding heights, interference with power lines and sidewalks, and other characteristics for many of these communities and others are reported in Mason (2006). DISCUSSION The primary reason for promoting street tree diversity is from a management, rather than an ecological, need. Diversity does not reduce the probability that a species will become infected or infested by an exotic stressor, but limiting the use of a species does reduce the potential for a stress to signifi- cantly affect management plans and budgets as a result of the loss of a host species. Much of the discussion of measuring diversity has been centered on species, typically a 10% limi- tation (Grey and Deneke 1986), but genera may be a more useful indicator when the concern is lethal stressors. Many of our exotic stressors are not limited to a species but to a genus. Dutch elm disease is not limited to American elm, although it is regarded as the most susceptible, but other North American species are affected as well as many European species (Sin- clair and Lyon 2005). The emerald ash borer appears to suc- cessfully attack many North American ashes, although there is some variation in susceptibility (Hermes et al. 2005). The threat posed at the genera level is possibly linked to the herbivore–plant relationship and geographic isolation of these relationships until recent time (Liebhold et al. 1995). The most common genera had their origin when the conti- nents were connected by land bridges. Since that time, the close coevolution of stressors and hosts that occur in a for- ested region, often limiting the stressor to a secondary role, may work to the disadvantage of genera when the stressor is introduced into another forest region with the same genus present but with species that lack defenses (Gibbs and Wain- house 1986). A community forestry goal of a 10% limit on a single species could give a false indication of stability if, for example, the street tree population was 10% green ash, 10% white ash, and 10% black ash, all susceptible hosts to the emerald ash borer. This concern was addressed in a proposal by Santamour (1990) for a 10–20–30 limitation with no more than 10% of Table 2. Street tree characteristics by municipal league community population classes for the 34 surveyed communities. n 3 11 20 Trees per capita mean ± SE 0.29 ± 0.14 0.53 + 0.26 0.76 ± 0.57 Planting sites per capita mean ± SE 0.41 ± 0.19 1.08 ± 0.42 1.65 ± 1.15 Percent full stocking mean ± SE 0.69 ± 0.01 0.49 ± 0.19 0.46 ± 0.15
September 2007
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