Arboriculture & Urban Forestry 33(5): September 2007 355 Table 3. Street tree population of 34 surveyed South Dakota communities by family, genus, and species. (continued) Family Simaroubaceae Tiliaceae Ulmaceae Percent of total <0.1 5.6 19.0 Genus Ailanthus Tilia Celtis Ulmus Percent of total <0.1 5.6 5.3 13.7 Species altissima americana cordata mongolica occidentalis americana pumila the street trees in a community in any one species, 20% in any one genus, and 30% in any one family. However, serious stressors that cross genera are relatively rare and those that cross families even less so. Gypsy moth (Lymantria dispar) does attack trees across a wide range of genera and families (Johnson and Lyon 1988) as well as Verticillium wilt (Ver- ticillium dahliae and V. albo-atrum) and fireblight (Erwinia amylovora), two common urban pathogens (Sinclair and Lyon 2005). These are not commonly regarded as serious lethal stressors for street trees. There is probably little con- cern about the diversity of families used as street trees but not enough concern on the reliance on a limited number of genera. Because the primary function of limitation is to reduce the probability of a significant disruption of management, a 10% limitation on genera may be our best measure of stability. This is a reasonable threshold based on the impact a stressor will have on a community. As an example, a community of 16,000 in South Dakota may have ≈12,000 street tree sites at full stocking. If a single genus comprising 20% of full stock- ing, 2400 trees, and has an annual loss of 6% to 10% of the genus as a result of a lethal stressor or 140 to 240 trees, not an unrealistically high loss rate based on what has occurred with emerald ash borer (Witter and Storer 2004) or Dutch elm disease (Wallner and Hart 1971), this removal effort would require ≈2 months for a crew at an estimated cost in South Dakota of $40,000 to $60,000. Because a community of this size may have an annual forestry budget of $200,000 to $250,000 and a single four-person forestry crew, the impact on the city budget and resources would be significant. This 2.8 2.8 <0.1 5.3 9.1 4.6 expenditure may be too high to be absorbed by the city, and it is likely that state and federal resources would be required. The same scenario would most likely be occurring in sur- rounding cities at the same time; thus, it is in the state’s best interest to have inventory data. Note that this 10% rule is not 10% of the trees planted, but 10% of full stocking. The use of full stocking as the measure allows for more consistent comparison and keeps the focus on limitation rather than strictly diversity. For example, if a com- munity has 10,000 street tree sites of which only 60% are planted, the 10% rule would mean that no more than 1000 trees, not 600 trees, should be of any one genus. This use of a constant base prevents merely adding more trees of differ- ent species as appearing to increase stability. In this example of the community with 10,000 street tree sites, if 1500 were filled with ash and the other 4500 with other species, adding 2000 new trees, other than ash, could lead to the assumption that now the ash population has been reduced from 25% to 19% when, in fact, the same number of ash trees are present in the street tree population and there is no change in the hazard represented by the 15% of the full stocking in this genus. South Dakota communities fall short of full stocking as do other communities across the country. One survey of 22 com- munities found stocking ranged from 12.6% to 70.3% with an average of ≈38% (McPherson and Rowntree 1989). Another study of five communities found street tree stocking ranged from 8.9% to 66.3% (McPherson et al. 2005). McPherson and Rowntree (1989) categorized community street tree populations as strong dominance, codominance, Table 4. The five most abundant tree species in the 34 surveyed communities by percentage for the three municipal league community population classes. Class 1 (n 3) Species Green ash Crabapple American elm Hackberry Silver maple SE standard error. ©2007 International Society of Arboriculture Mean ± SE 32.0 ± 7.1 11.4 ± 6.6 9.7 ± 3.6 6.2 ± 2.9 4.3 ± 0.4 Species Green ash Silver maple American elm Siberian elm Hackberry Class 2 (n 11) Mean ± SE 40.8 ± 14.5 9.3 ± 4.4 8.1 ± 8.0 7.3 ± 7.1 4.1 ± 2.3 Species Green ash Siberian elm American elm Cottonwood Blue spruce Class 3 (n 20) Mean ± SE 42.3 ± 6.4 9.8 ± 5.1 8.1 ± 1.8 6.1 ± 2.2 5.0 ± 1.3 Percent of total <0.1
September 2007
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
