58 Martin et al.: Open-grown Crown Width Equations for Three Oak Species Arboriculture & Urban Forestry 2012. 38(2): 58–63 Predictive Open-grown Crown Width Equations for Three Oak Species Planted in a Southern Urban Locale Nicholas A. Martin, Arthur H. Chappelka, Edward F. Loewenstein, Gary J. Keever, and Greg Somers Abstract. Models that predict ecosystem services in urban areas are useful tools to urban forest managers. Predictive open-grown crown width equa- tions were developed for three oak species common in urban forests in the southern United States. Tree crown form is an important component of these equations; however, there are few predictive equations available for urban, open-grown trees. The species used were Quercus lyrata Walt. (overcup oak), Quercus nuttallii Palmer (Nuttall oak), and Quercus phellos L. (willow oak). The study authors believe that these are the first predictive open-grown crown width equations developed for these species in the southern U.S. Diameter at breast height (DBH) (independent variable), DBH2 variable), and average crown width (dependent variable) data were used to create the predictive crown width equations and yielded R2 (independent values of 0.96, 0.94, and 0.91 for overcup, Nuttall, and willow oaks, respectively. These equations can aid urban landscape and utility planners by providing a means to predict crown dimensions at varying trunk diameters. Field time could also be minimized by reducing the need to measure crown width and with time, these equations could be used to validate species specific equations (e.g., leaf biomass) for these and other southern urban-planted tree species. Key Words. Dimensional Relationships; Open-Grown Crown Width Equations; Predictive Crown Width Equations; Quercus lyrata; Quercus nuttal- lii; Quercus phellos; Urban Trees. Tree measurements such as tree diameter at breast height (DBH), total height, height to the live crown, and crown width can pro- vide vital information on their own and they provide crucial data for other calculations such as leaf area and leaf biomass (Nowak 1996; Peper et al. 2001a; Peper at al. 2001b). These measure- ments are important to urban forest managers, arborists, research- ers, and planners because they aid in the development of man- agement strategies and practices (Peper et al. 2001a; Peper et al. 2001b). DBH, crown width, leaf area, and other information from trees also aid in assessing ecosystem processes such as evapo- transpiration, light interception, and atmospheric deposition (Nowak 1996), and can help in developing predictive equations for pollution uptake (Peper et al. 2001b). Tree measurements are vital when determining ecosystem services and utilizing crown equations makes it possible to determine benefits such as carbon sequestration and air pollution removal. Predictive open-grown crown width equations could also accelerate data collection in the field by only requiring DBH to be measured and using predictive open-grown crown width equations to estimate the crown width, thereby reducing the amount of actual field work. Urban shade trees are vital to our environment and offer many benefits, most of which are dependent on size (Frelich 1992). Limited research has been conducted on open-grown, pre- dictive crown equations for urban trees (Nowak 1996; Peper et al. 2001a; Peper et al. 2001b; Peper and McPherson 2003), es- pecially for specific regions. However, researchers in the tradi- tional field of forestry have developed numerous equations that include DBH, biomass, and crown width (Krajicek et al. 1961; Ek 1974; Hasenauer 1997; Lhotka and Loewenstein 2008). ©2012 International Society of Arboriculture Although some of these equations have been used for urban trees, additional validation is lacking (Peper et al. 2001a). Tree canopy architecture differs between open-grown and restricted canopy (i.e., closed canopy, next to a building, etc.) conditions. When grown in the open, a tree’s canopy can reach its full size and not be restricted; however, in a forested situation, tree canopies compete for limited growing space and, therefore, may not reach maximum expansion potential. However, other factors, such as soil, light, moisture, and crown loss due to storms or pruning, can also be limiting factors for crown width. Dimensional relation- ships for urban trees have been calculated on trees with crowns that were full and healthy in New Jersey, U.S. (Fleming 1988), on healthy trees in St. Paul and Minneapolis, Minnesota, U.S. (Frelich 1992), on trees with full tree crowns in excellent con- dition in Chicago, Illinois, U.S. (Nowak 1996), on street trees in Santa Monica, California, U.S. (Peper et al. 2001a), and on street trees in Modesto, California, U.S. (Peper et al. 2001a; Peper et al. 2001b). The research conducted by Peper et al. (2001a; 2001b) aided in the development of predictive crown width equa- tions for urban trees in regions with longer growing seasons, varying locations, and broader ranges of condition; however, with the available knowledge, there are no equations available for southern United States tree species planted in urban locales. The goal of the study authors was to develop predictive open- grown crown width equations for three commonly planted urban oak species in the southern U.S.: Quercus lyrata Walt. (overcup oak), Quercus Nuttallii Palmer (Nuttall oak), and Quercus phel- los L. (willow oak); using diameter (DBH) and crown width data collected during a 100% tree inventory to produce the predictive
March 2012
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