Arboriculture & Urban Forestry 39(2): March 2013 maintenance activities, removal of trees, selection of new suit- able species, and location of planting stock for replacement, all while managing public relations, politics, and risk liabil- ity or management. Managing urban forests most often occurs over a time frame that exceeds any one manager’s tenure. Jim (1994) states that the many facets of managing tree replacement can be a daunting challenge to urban tree managers and Hitch- mough (1994) concedes that complexity and political nuances of the task are typical excuses for inactivity. Managing urban tree replacement is not a new problem (Solotaroff 1911). Pescott (1968), when addressing the planned replacement of trees, indicates that as early as 1954, in England, a government com- mittee of relevant experts was formed to advise the Ministry of Works on the felling and planting of trees in all types of loca- tions. Four decades later, Hitchmough (1994) claimed, “many landscape management organizations are not adequately pre- pared to cope with problems of this intensity and magnitude.” By developing a repeatable method to define and explain removal and replacement, a management conversation, and the requisite outreach program to inform the public, can be implemented. Sinclair and Hudler (1988) define the “decline” of trees, as opposed to natural senescence, as “a premature progressive loss of health, distinguished from the normal occurrence of senescence by premature debilitation.” However, there is no current definition of premature death due to the fact that there are no established definitions for normal rates of attrition and life expectancies for trees in the urban settings. Although anecdotal field wisdom ex- ists, lifespan has not been defined in the research record for most urban tree species within an urban context. Sinclair and Hudler (1988) further developed four major aspects of tree decline: three describing disease, pests, and environmental stresses; and the fourth describing synchronous cohort senescence (trees of similar age growing in groups have a tendency to display group behavior, such as shared patterns of senescence). Researchers of the cur- rent study sought to develop a better understanding within this fourth area of decline, acknowledging that the planting site types suggested earlier define an evaluation or management expecta- tion, but are associated with imposed environmental stresses. Trees in the urban context are planted in many different site typologies that cover a multitude of variables. Although complete site analysis could be performed on every tree, this is neither efficient nor does it provide a general model for assessing trees. Using available soil surface as a visual site type characteristic may allow managers to better assess the condi- tions of the current urban forest and provide better information for future plantings and design evaluation. Interpretation of tree size within a site can vary depending on the location. As a hypothetical example, a 51 cm DBH Quercus rubra is common and considered very large in a sidewalk zone in the study loca- tion, but it is very rare to see a 61 cm DBH tree in this zone. The same tree would be considered a mid-range size for the species if found as a park or yard tree within visual distance of the sidewalk zone. Cornus florida or Tilia tomentosa would have differing profiles. Choices of plant placement in design require an understanding of maturity and longevity expectations. Apparent available soil is certainly not the only variable that affects or informs the expectation of a tree’s overall health, longevity, and maximum size. However, it is an easily man- aged, recordable, and cataloged characteristic that can help predict what other variables may have an effect. The associa- 69 tion provides a prediction of tree growth behavior, reflective of the earlier observations of Grabosky and Gilman (2004), not a causation of the phenomena. In application, site type could later be combined with work suggested by Bond (2010; 2012) in examining the condition of the tree, especially in its char- acteristic profile in context with its expected maximum size. Urban trees are often planted with an expectation of near- ly zero attrition, in spite of ample experience to the contrary, thus skewing major portions of the urban canopy towards one age class and minimizing urban forest sustainability (Clark et al. 1997). As a consequence, researchers have little knowl- edge of what might be considered a reasonable life span for any number of common species, design situations, urban gradients, soil disturbances, or environmental ranges. Urban forestry has historically emphasized tree planting and sur- vival, with little attention directed to what constitutes mature and overmature trees in the urban forest, or how common de- sign responses affect ultimate tree size or service. Managing the urban forest within the traditional knowledge of forests, research- ers are unable to address stand longevity or a harvest interval. This study focuses on the harvest interval by deriving a methodology to define overmature trees, and provide a context for developing an urban size expectations to help define har- vest interval. The specific goals of this study were to develop a better understanding of senescence in urban tree populations and how different planting design choices might influence ma- ture size expectations. As a motivation for management, the repeatable method can begin to define and explain removal and replacement by establishing a definition for overmature trees and determine expected harvest intervals for species. MATERIALS AND METHODS The current study included community inventories collected throughout northern and central New Jersey, U.S., by a con- sultancy firm that was hired to develop the inventories as part of a state-wide program for the development of com- munity forest management plans (NJ DEP Division of Parks and Forestry 2011). Eleven communities were inventoried between 1995 and 2010; the resulting database is composed of approximately 45,500 trees and more than 280 taxa. Data collected included tree diameter at breast height (DBH), plant- ing site type, planting site area, tree species, tree genus, and maintenance recommendations. ANOVA was used to deter- mine if communities could be grouped. Percentiles on trunk DBH were then developed based on the multi-community data. Three specific sites were defined that were based on available soil: non-limited, planting strip, and pit. To avoid bias created by a single town’s maintenance and care, only the taxa that were found on all three site types across at least three inventories were analyzed and included in this study. In addition to groupings by available soil, trees were also grouped according to maximum height in a park-like setting (Table 1). Small species were con- sidered to be any tree in which maximum height at maturity was less than 9.1 m, medium tree species were greater than 9.1 m but less than 15 m, and large tree species were greater than 15 m (Gerhold et al. 1993; Hightshoe 1988; Bassuk et al. 2009). Since this study focused on maximum size, the trees in each species were ranked into DBH percentiles as an aggregated species grouping across all communities within each site type. For each ©2013 International Society of Arboriculture
March 2013
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