178 CONCLUSIONS Regulatory policies are creating markets for services like storm- water management, carbon storage, energy conservation, and air quality improvement. As rules and reporting become more quantitative, the influence of tree selection and performance on compliance and return on investment becomes more critical. Arborists, landscape architects, and contractors will be held ac- countable to regulators and investors, not just for tree survival, but for long-term performance. Tree growth modeling is fundamen- tal to quantifying the value of tree services and maximizing the health and productivity of trees. Ultimately, this science will pro- vide the information arborists need to modify sites, select trees, and apply management practices for peak plant performance. Long-term performance evaluation of trees is an integral component of tree modeling because it provides real-world data for calibrating models and explaining effects of site con- ditions and management activities on growth. Although there are several evaluation programs in the United States, they have not been integrated with tree growth modeling efforts. New long-term evaluation programs are needed that go beyond nar- rowly focused assessments (e.g., utility trees and elms) and encompass a variety of regional climates, growing condi- tions, and management regimes. Ideally, side-by-side compari- sons under controlled conditions are complemented by in situ field measurements that reflect a range of landscape situations. Modeling urban tree growth is in its infancy compared to modeling of forest and fruit trees. Empirical models, such as those found in i-Tree and L-Systems, project future tree dimen- sions based on growth equations derived from size measure- ments on large samples of trees. The USDA Forest Service ref- erence city database contains size data from more than 17,000 trees representing 171 unique species from 16 U.S. cities. It is a valuable source of morphological data that reflects regional differences in species composition, climate, soils, site condi- tions, and management practices. Mining these data for cor- relations between tree size, condition, and other variables will increase the accuracy and robustness of tree growth models. Process-based models translate physiological processes, such as photosynthesis and transpiration, into morphological growth. L-PEACH has successfully coupled water transport with growth and visualization, creating a promising platform for modeling ur- ban tree growth response to pruning and future climate stressors. Improved growth models will result in better estimates of carbon sequestration, water use, and the supply of tree biomass for future utilization. For example, green ash in Cheyenne, pro- vided one-third the value of services compared to the same trees in nearby Fort Collins, because of dwarfed growth from climate and soil influences. Only through a broader and deeper under- standing of relations between site conditions, management, and growth will urban tree modeling become more of a science and less an art. To that end, there is need for increased invest- ment in long-term urban performance evaluation and greater collaboration among tree growth modelers from all disciplines. McPherson and Peper: Urban Tree Growth Modeling LITERATURE CITED Allen, M.T., P. Prunsinkiewicz, and T.M. DeJong. 2005. Using L-systems for modeling source-sink interactions, architecture and physiology of growing trees: The L-PEACH Model. New Phytologist 166:869–880. Benedict, M., and E. McMahon. 2006. Green Infrastructure: Linking Landscapes and Communities. Island Press, Washington, D.C., U.S. Brasch, S., E.G. McPherson, and L. Linsen. 2007. Visualization of time- varying tree data. pp. 129-134. In: J.J. Villanueva (Ed.). VIIP ‘07 The Seventh IASTED International Conference on Visualization, Imaging and Image Processing. Acta Press, Anaheim, California, U.S. Burch, P.L., R.H. Wells, and W.N. Kline. 1996. Red maple and silver maple growth evaluated 10 years after application of paclobutrazol tree growth regulators. Journal of Arboriculture 22:61–66. Casey Trees and Davey. 2011. National tree benefit calculator. Accessed 12/01/2011. Clark, J.R., and R. Kjelgren. 1990. Water as a limiting factor in the devel- opment of urban trees. Journal of Arboriculture 16:203–208. Da Silva, D., R. Favreau, I. Auzmendi, and T.M. deJong. 2011. Linking water stress effects on carbon partitioning by introducing a xylem circuit into L-PEACH. Annals of Botany 108(6):1135–1145. DeVries, R.E. 1987. A Preliminary Investigation of the Growth and Lon- gevity of Trees in Central Park. New Brunswick, NJ: Rutgers Univer- sity. M.S. thesis 95 p. Fleming, L.E. 1988. Growth estimation of street trees in central New Jersey. New Brunswick, NJ: Rutgers University. M.S. thesis. 143 pp. Frelich, L.E. 1992. Predicting dimensional relationships for Twin Cities shade trees. St. Paul, MN: University of Minnesota, Department of Forest Resources. 33 p. Gerhold, H.G. 2007. Callery pear cultivars tested as street trees: Final re- port on a 12-year study. Arboriculture & Urban Forestry 33:153–156. Goodfellow, J.W., B. Blumreich, and G. Nowacki. 1987. Tree growth response to line clearance pruning. Journal of Arboriculture 13:196–200. Grabosky, J., and E. Gilman. 2004. Measurement and prediction of tree growth reduction from tree planting space design in established park- ing lots. Journal of Arboriculture 30:154–164. i-Tree Team. 2011. i-Tree. Accessed 12/01/2011. Jutras, P. 2008. Modeling of Urban Tree Growth with Artificial Intelli- gence and Multivariate Statistics. Montreal, Quebec: McGill Univer- sity. Ph.D. thesis 424 pp. Lawrence, A.B., F.J. Escobedo, C.L. Staudhammer, and W. Zipperer. 2012. Analyzing growth and mortality in a subtropical urban forest ecosystem. Landscape and Urban Planning 104(1):85–94. McPherson, E.G., J.R. Simpson, P.J. Peper, S.E. Maco, Q. Xiao, and P.J. Hoefer. 2003. Northern mountain and prairie community tree guide: Benefits, costs and strategic planting. U.S. Department of Agricul- ture, Forest Service, Pacific Southwest Research Station, Center for Urban Forest Research, Davis, California, U.S. McPherson, G., L. Costello, J. Harding, S. Dreistadt, M.L. Flint, and S. Mezger. 2009. National elm trial: initial report from Northern Cali- fornia. Western Arborist 35(3):32–36. Nielsen, C.N., O. Buhler, and P. Kristoffersen. 2007. Soil water dynamics and growth of street and park trees. Arboriculture & Urban Forestry 33:231–245. ©2012 International Society of Arboriculture
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