200 but comments here are confined to basic research questions that can provide a greater understanding of the role of the tree root system in the ecology of the urban rhizosphere. The fol- lowing are proposed as possible areas of future research: 1. Biological community in the larger rhizosphere. Evidence is abundant that tree root systems cannot be fully understood sepa- rately from the microorganisms that inhabit the larger rhizosphere. Yet, we know little about these communities and how they develop in urbanized settings. The interactions that occur within the larger rhizosphere may not only influence tree growth, but also play a role in certain ecosystem services that trees provide, such as treatment of stormwater, that are now taking on a heightened importance. 2. Soil contamination. Urban infill development is growing in importance as societies seek to protect increasingly scarce agri- cultural and forested land from development and to rehabilitate previously developed land. Thus, professionals who work with urban trees will be increasingly faced with managing trees on sites that are undergoing rehabilitation, such as brownfields. Although considerable information concerning tree roots and contami- nated soils is available from phytoremediation and mine spoils research, this work focuses on maximizing extraction of contami- nants by trees and other plants with the intention of eventually harvesting the plant and safely disposing of it. Little is known about long-term challenges to growing trees in contaminated soils and the long-term effects of tree roots on contaminated soils. 3. Climate change. There is a growing body of scientific atmosphere is increasing, and urban heat islands are generat- ing ground-level ozone. All of these factors affect root growth and development either directly or indirectly through media- tion from the aboveground portion of the plant. What will be the responses of tree root systems in this altered environment? The urban ecosystem is under increasing scrutiny as so- evidence demonstrating that global temperature is increas- ing, atmospheric CO2 levels are rising, N deposition from the ciety strives to manage the environment in a sustainable way. Urban trees play a critical role in the urban environ- ment on many levels. As we increase our understanding of the complex processes at play in the rhizosphere, we will not only be able to better manage landscape trees, but also more fully benefit from their role in urban ecosystem processes. Acknowledgments. We are grateful to many people for their contribu- tions to this project, including Julia Bartens, Francesco Ferrini, Greg McPherson, Paul Markworth, Tang Dai, Gary Watson, and Lisa Richard- son-Calfee. We also thank the reviewers for many improvements to the manuscript. This project was supported in part by the International So- ciety of Arboriculture and is a part of the ISA Literature Review Series. LITERATURE CITED Abadía, J., F. 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