172 McPherson and Peper: Urban Tree Growth Modeling Arboriculture & Urban Forestry 2012. 38(5): 172–180 Urban Tree Growth Modeling E. Gregory McPherson and Paula J. Peper Abstract. Selecting, locating, and managing trees to provide ecosystem services are becoming increasingly important facets of munici- pal and consulting forestry. The science of urban tree growth modeling is fundamental to quantifying these services. This paper describes three long-term tree growth studies conducted to evaluate tree performance because repeated measurements of the same trees produce criti- cal data for growth model calibration and validation. Several empirical and process-based approaches to modeling tree growth are reviewed. Modeling is more advanced in the fields of forestry and pomology than in urban forestry. The USDA Forest Service’s reference city research has developed over 1,800 growth equations from measurements on more than 17,000 trees in 16 cities. The database is a valuable source of informa- tion that reflects regional differences in species composition, climate, soils, site conditions, and management practices. Several examples illustrate how differences in local climate and management practices can influence growth of a single species and the resulting value of services. Further advances in urban tree growth modeling are needed to inform the design, management, and modeling of high performing landscapes. Key Words: Allometry; Ecosystem Services; Predictive Equations; Tree Growth; Urban Forest. Selecting, locating, and managing trees to produce ecosys- tem services are becoming increasingly important facets of municipal and consulting forestry (Young 2010). This trend is driven from the top down and the bottom up. An increas- ing number of “green consumers” are demanding more from their landscapes than aesthetic appeal. At the same time, more policies, rules, and regulations are promoting green infrastruc- ture, including trees and the soil that supports them (Benedict and McMahon 2006; Tzoulas et al. 2007). The potential to stack and sell the services trees produce is motivating arbor- ists to begin thinking of trees as a solar-powered biotechnol- ogy, wherein their appearance is only one facet of overall value. The science informing the design, selection, and management of high performing trees is relatively limited. Few long-term tree growth studies have been conducted. Although arboriculture lit- erature is rife with studies that focus on effects of tree growth regulators (Sachs et al. 1986; Burch et al. 1996) and a variety of individual environmental stressors on growth (Goodfellow et al. 1987; Clark and Kjelgren 1990; Grabosky and Gilman 2004; Nielson et. al 2007), the relative effects of multiple stressors re- mains largely unknown. Much of the research has been carried out on young trees in controlled settings that are very different from the heterogeneous conditions found in cities (Sjöman and Nielsen 2010). The paucity of literature on tree growth science is rivaled by the dearth of urban tree growth modeling studies. Com- pared to the breadth and depth of growth modeling in the fields of forestry and pomology, urban tree modeling is in its infancy. The purpose of this paper is to describe the current state of arboricultural knowledge concerning tree growth model- ing. To begin several long-term tree performance studies are described because repeated measurements of the same trees produce critical data for growth model calibration and valida- ©2012 International Society of Arboriculture tion. Two theoretical approaches to modeling tree growth are described, and their application in arboriculture and urban for- estry are presented. The analytical potential of the USDA For- est Service’s reference city database, which contains measure- ments of more than 17,000 urban trees, is rendered with several examples. Researchers conclude by advocating for greater in- terdisciplinary collaboration to advance tree growth modeling and renewed investment in long-term performance monitoring. TREE PERFORMANCE EVALUATION Long-term performance evaluation of tree species and cultivars is fundamental to tree growth modeling. Repeated measurements of tree size and health provide real- world data for modeling tree growth. Thorough descriptions of site conditions and management activities can be used with multivariate statistics to explain their influence on growth and performance. For centuries, foresters have been measuring the effects of site factors and management interventions on stand dynamics. The Forest Inventory and Analysis program has been monitoring U.S. forests since 1928 (Smith 2002). Recently, For- est Inventory and Analysis plots have been located in some ur- ban areas. However, long-term studies of urban tree growth first began in the U.S. a half-century ago by arboreta, universities, and foundations. In the mid-1960s, Dr. L.C. Chadwick of Ohio State University and Mr. Bif Stapes of the Davey Tree Expert Company began evaluating street tree species in five Ohio cit- ies, as well as trees planted in research plots. Now called the Street Tree Evaluation Project, the study expanded to include 89 revisited sites and continues to supply valuable “then and now” information on survival and growth, as well as photographic records of visual impacts as trees mature (Sydnor et al. 1999).
September 2012
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