Arboriculture & Urban Forestry 33(5): September 2007 351 ber of individuals representing a variety of species in differ- ent age classes (Sanders 1981) and stability as the low prob- ability that the number of functional trees will decline to the point of disrupting management and diminishing the benefits they provide (Richards 1982). If too many street trees are affected by a lethal stressor, the high cost associated with the removal of these trees as well as the disruption to the opera- tion of the city forestry department resulting from the sudden need to adjust priorities can be devastating to the city budget as well as the operations of the department responsible for these trees. Thus, the goal becomes not necessarily diversity, because this effort may not improve stability, but instead limiting the use of a species to forestall disruptions to budgets if wide- spread losses occurred within a short time period. The con- cept of limiting the use of a species appears to have been first discussed by Barker (1975) who proposed a list of species for “liberal use” as street trees but not to exceed more than 5% of the total street tree population. He also suggested another category of “limited use” species whose use should be limited to no more than 2% of the total and a “candidate use” cat- egory that restricted use to less than 0.3%. Miller and Miller (1991) modified the criteria to reflect the limited number of species suitable to the urban environment and proposed that the liberal use species not exceed 10%, limited use species not exceed 5%, and candidate use species not exceed 2% of the street tree population. Clark and others (2003) incorpo- rated this concept of limitation into their model of urban forest sustainability in which the vegetation resource is one of the components. The specific criteria they identified included an uneven age distribution and a diversity of species follow- ing a guideline of limiting a species’ use to 10% of the entire street tree population. This concept of limiting the use of a species is gaining renewed interest as a result of the relatively recent discovery of the emerald ash borer (Agrilus planipennis) in the Mid- west. This close relative to the bronze birch borer (A. anxius) became established in southeastern Michigan, U.S. sometime during the 1990s, apparently arriving in ash packing material from Asia. The emerald ash borer appears to be a secondary stressor on native ashes (Fraxinus spp.) in Asia, but has be- come a primary stressor on ashes native to North America both in this country as well as when they are planted in Asia (Hermes et al. 2005). The insect has been responsible for the loss of more than 15 million ash trees in Indiana, Michigan, Ohio, and southeastern Ontario (Poland and McCullough 2006) with new established infestations such as the recent one in Illinois being discovered as detection efforts expand. The projected costs of removing infested ash trees, if the insect does become established across the United States in a sce- nario reminiscent of the removal efforts associated with Dutch elm disease, is estimated to be between $20 and $60 billion (Cappaert et al. 2005). Much of the Midwest and Great Plains has a high presence of ash as street trees as well as on the adjacent private prop- erty. The concern in many communities, particularly those located on the Plains states such as South Dakota, is the potential effect the emerald ash borer will have on their street trees. The response to this threat should not be as simplistic as merely curtailing the use of ash in the landscape by sub- stituting one species or genera for another, or communities will be only repeating the mistakes made with Dutch elm disease management. This already may be occurring with the shift in popularity from ash to the Freeman maple cultivars (Acer × freemanii) attributable in part to the widespread con- cern about the future of ash and the desire for another rela- tively fast-growing shade tree. Instead, the focus should be on categorizing community forests in regard to their stability in response to lethal stressors and then adjusting planting efforts to reduce this hazard over time, hazard being defined as the vulnerability of a forest to a particular stressor given that the stressor is present (Coulson and Witter 1984). The assump- tion here is that given international trade, a lethal stressor should be assumed for any given genera regardless of whether such stressor is present or even known at the time. The South Dakota Division of Resource Conservation and Forestry has been collecting community street tree data for approximately 14 years. The intent of this project is to pro- vide baseline data of the species, age, and condition of se- lected community forests across the state. One of the objec- tives was to determine our reliance on limited species. Al- though the emerald ash borer was not yet recognized as a threat when the project began, there were concerns about the possible loss of a key species, much as what began occurring in the state after the detection of Dutch elm disease in South Dakota in the late 1960s. MATERIALS AND METHODS The project began with a preliminary survey of several smaller communities across the state from 1992 to 1994 to develop a sampling technique. The communities to be sur- veyed were selected through a two-way stratification proce- dure (Cochran 1977) with the two criteria being location and city size. The cities were divided into two strata, East River or West River. These are common designations in South Da- kota that divide the state into areas east or west of the Mis- souri River. This boundary is more than a physical division of the state because the soils and climate, in terms of precipita- tion, differ on either side of the river (Hogan 1995). The communities were further stratified based on population with the three strata following the South Dakota Municipal League (2004) divisions into classes 1, 2, or 3 communities based on a population of 5000 or greater, 501 to 4999, and 500 people or fewer, respectively. The allocation to each stratum was proportional to population and the communities selected within each of the strata were random with this notable ex- ©2007 International Society of Arboriculture
September 2007
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