154 Day et al.: Contemporary Concepts of Root System Architecture of Urban Trees hydraulic lift, and avoidance of conflicts with pavement. Root architecture and its interactions with soil properties influence tree stability, which has serious implications in urban settings. This review has focused on the architecture of tree roots in the urban environment, particularly vertical and horizontal ex- tent. How does this knowledge guide future research? There are many unanswered questions that relate to management of urban tree root systems, but here the authors confine comment to basic research questions to provide a greater understand- ing of the characteristics of the urban tree root system. The au- thors propose the following as possible areas of future research: Root Architecture and Exploration of the Built Environment Although the rhizosphere is traditionally understood as the plane of contact between roots and soil where the soil environment is dominated by root activity, rhizosphere here is considered in a broader sense to be the root-soil ecosystem. If both roots and soil are present, they cannot be viewed in isolation. What below- ground situations allow for the greatest root exploration? Can this knowledge help advance techniques for avoiding tree-infrastruc- ture conflicts? Although initial investigations indicate that select- ing trees for their propensity for deep rooting may have limited success, further investigation is merited in this area because pos- sible benefits are considerable. In addition, investigation of the genetic control of other facets of root architecture that may confer an advantage to urban trees (e.g., root systems that are more fi- brous or regenerate more rapidly), may allow for selection of trees that can better exploit limited soil resources in urban settings. Resource Acquisition by Urban Tree Roots Urban trees may be exposed to long periods of drought, especially if global temperatures increase in the future. Urban species have the potential to grow deep roots, but are frequently limited by the soil environment. How can site design favor greater root explo- ration? If site design allowed for deep roots, would phenomena such as hydraulic lift allow for protection of vegetation during dry periods? Would drought tolerance be increased? Could access to nutrients and water be managed or engineered more effectively? Mechanical Stability If society is successful in growing large trees in the built en- vironment, then research is needed on what root architecture characteristics are essential for tree stability and how the de- velopment of these characteristics can be assured. In addition, engineered soils and designed substrates are increasingly com- mon and need to be evaluated in the context of tree stability. Acknowledgments. We are grateful to many people for their contribu- tions of literature, resources, and advice to this project. We especially thank Julia Bartens, Francesco Ferrini, Ed Gilman, Brian Kane, Greg McPherson, Paul Markworth, Tang Dai, Gary Watson, Larry Costello, and Lisa Richardson-Calfee. This project was supported in part by the International Society of Arboriculture and is part of the ISA Literature Review Series. LITERATURE CITED Adams, M.E. 1966. A study in ecology of Acacia mellifera, A. seyal and Balanites aegyptiaca in relation to land-clearing. Journal of Applied Ecology 4:221–237. Alberty, C.A., H.M. Pellett, and D.H. Taylor. 1984. Characterization of soil compaction at construction sites and woody plant response. Jour- nal of Environmental Horticulture 2(2):48–53. Baker, J.B., and O.G. Langdon. 1990. Pinus taeda L. — loblolly pine. pp. 497–512. In: Silvics of North America: Volume 1. Conifers. Vol. Ag- riculture Handbook 654. R.M. Burns and B.H. Honkala (Ed.). United States Department of Agriculture (USDA), Forest Service Balasubramanyan, S., and M.I. Manivannan. 2008. Root pattern stud- ies in acid lime in silt clay soils. The Asian Journal of Horticulture 3(2):241–245. Ballantyne, A.B. 1916. Fruit tree root systems: Spread and depth. Bul- letin 143. Utah Agricultural Experiment Station. Logan, UT. Bannan, M.W. 1940. The root systems of northern Ontario conifers grow- ing in sand. American Journal of Botany 27(2):108–114. Belsky, A.J. 1994. Influences of trees on savanna productivity: Tests of shade, nutrients, and tree-grass competition. Ecology 75(4):922–932. Betanowny, K.H., and A.M.A. Wahab. 1973. Eco-physiological stud- ies in desert plants. VIII. Root penetration of Leptadenia pyrotech- nica (Forsk.) Decne. in relation to its water balance. Oecologia 11: 151–161. Biswell, H.H. 1935. Effects of environment upon root habits of certain deciduous forest trees. Botanical Gazette 96(4):676–708. Black, K.E., C.G. Harbron, M. Franklin, D. Atkinson, and J.E. Hooker. 1998. Differences in root longevity of some tree species. Tree Physi- ology 18(4):259–264. Burger, D.W., and T.E. Prager. 2008. Deep-rooted trees for urban envi- ronments: Selection and propagation. Arboriculture and Urban For- estry 34(3):184–190. Burgess, S.S.O., M.A. Adams, N.C. Turner, and C.K. Ong. 1998. The redistribution of soil water by tree root systems. Oecologia 115(3): 306–311. Cable, D.R. 1977. Seasonal use of soil water by mature velvet mesquite. Journal of Range Management 30:4–11. Cheyney, E.G. 1932. The roots of a jack pine tree. Journal of Forestry 30:929–932. Clair, B., M. Fournier, M.F. Prevost, J. Beauchene, and S. Bardet. 2003. Biomechanics of buttressed trees: bending strains and stresses. Amer- ican Journal of Botany 90(9):1349–1356. Coile, T.S. 1937. Distribution of Forest Tree Roots in North Carolina Piedmont Soils. Journal of Forestry 35(3):247–257. Crook, M.J., A.R. Ennos, and J.R. Banks. 1997. The function of buttress roots: a comparative study of the anchorage systems of buttressed (Aglaia and Nephelium ramboutan species) and non-buttressed (Mallotus wrayi) tropical trees. Journal of Experimental Botany 48(314):1703–1716. Crow, P. 2005. The influence of soils and species on tree root depth. In- formation Note FCINO78 Forestry Commission Edinburgh 8 pp. Davis, E.A., and C.P. Pase. 1977. Root system of shrub live oak: implica- tions for water yield in Arizona chaparral. Journal of Water Conserva- tion 32:174–180. Dawson, T.E. 1993. Hydraulic lift and water-use by plants- implications for water-balance, performance and plant-plant interactions. Oecolo- gia 95:565–574. Dawson, T.E. 1996. Determining water use by trees and forests from iso- topic, energy balance and transpiration analyses: the roles of tree size and hydraulic lift. Tree Physiology 16(1–2):263–272. ©2010 International Society of Arboriculture
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