68 Sanders et al.: Size Expectations for Urban Trees with Regard to Designed Space Arboriculture & Urban Forestry 2013. 39(2): 68–73 Establishing Maximum Size Expectations for Urban Trees with Regard to Designed Space Jessica Sanders, Jason Grabosky, and Paul Cowie Abstract. One issue confronting the application of forest management principles to urban tree canopy management decisions is the lack of data correlating site, tree size, and tree age. Researchers tested whether terminal size (stem diameter) can be linked to site type for informed man- agement and design decisions. Data were considered from eleven New Jersey, U.S. communities. Diameter breast height (DBH) distribu- tion established regionalized service life expectancies of commonly planted species by site type and expected maximum DBH. The goal was to develop a method to identify trees approaching senescence within an inventory. Three common urban landscape site types were used: tree pit, planting strip, and unlimited soil. Thirty-one taxa were present in large enough populations to use in species-specific analysis. The species were classified into small, medium, and large size categories based on published growth expectations. The study authors developed DBH occur- rence percentiles, and DBH within the ninety-fifth were described as a maximum size range. There was a significant difference in maximum siz- es between planting site types. Regardless of the size class of the tree, the data showed reduced planting space resulted in reduced maximum size. Key Words. Age Class; Canopy Management; New Jersey; Senescence; Site Type; Urban Forest; Urban Tree Growth. Urban trees are both a public asset and a public risk. As a result, management planning is required to address and balance both appropriate public values and expenses. A tree inventory is necessary to best manage public trees. An inventory typical- ly includes the total number of trees, species composition, tree location, and tree condition to provide a better context for in- formed management decisions. Unfortunately, inventories often lack interpretive ability or an established method for effective management of age structure and sustainability within a mixed- species (and thus mixed maximum size) urban tree population. The process of plant selection to match urban tree species to varied, and often difficult, urban planting sites is a fundamental aspect of urban environmental design and management. How- ever, establishing expectations for performance over time is of- ten only implied as establishment success is the short-term goal and performance evaluation criteria are lacking. Urban areas are quite heterogeneous, and environmental stresses may vary considerably, even among adjacent planting sites. Berrang et al. (1985), studying more than 80 variables for 375 trees planted near the Consolidated Edison facilities in New York City, New York, U.S., found that excess soil moisture, mounding of soil on roots, soil salts, and overall root system size were the most important factors affecting a tree’s overall health. Chacalo et al. (1994) surveyed 1,261 street trees in Mexico City, Mexico, collecting data for seven different variables, concluding that problems with overall tree health could be attributed to plant- ing in inappropriate locations, overall species choice, and lack of adequate planning and maintenance. Both of these stud- ies concluded that planting selection becomes a site-specific challenge, and by extension, growth capability follows suit. ©2013 International Society of Arboriculture General wisdom might suggest that urban design forms should provide a metric for visual evaluation to assist manage- ment decisions. While there is a general understanding of soil bi- otic capacity and carrying capacity in the management of natural forested systems, the urban context of biotic capacity and carry- ing capacity is far behind the natural forested areas. Addition- ally, observations of tree performance in response to soil open- ing size (Grabosky and Gilman 2004; Day and Amateis 2011; Sanders and Grabosky 2013) suggest merit for an approach to understanding biotic capacity within an urban design context. Indeed, as major projects (e.g., PlaNYC, Million Trees Philadel- phia, Miami, Los Angeles) are increasingly evaluated within a context of sustainability, the ability to predict plant performance and longevity with relation to design choices is crucial for an appropriate program analysis. Planting spaces in urban design are usually reflective of three different sites: tree pit, plant- ing strip, and non-limited soil. ‘Non-limited’ included all trees where the area underneath the dripline of a tree was unpaved. An example would be the soil available to a tree planted in a park or in a lawn setting. ‘Planting strip’ had less available soil per tree, best described as the strip of ground where the avail- able soil is bordered on two sides by structures or pavement thus limiting the tree root zone. ‘Tree pit’ included a cutout, tree-well, or raised planter, where available soil was extremely limited, meaning the available soil was less than the drip line of the tree. Without a method of defining a useful end point to urban tree service life, there remains difficulty in developing a public acceptance or management plan for tree harvest and replace- ment in urban tree populations. The logistics of tree service life, removal, and replacement include the planning and timing of
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