28 Catton et al.: Evaluation of Bur Oak Decline The present-day environmental data examined in this study generally did not support the notion that seemingly detrimen- tal urban disturbances such as high amounts of concrete or building construction were directly involved in bur oak de- cline. In fact, according to the information collected on pres- ent-day site conditions, bur oaks can be declined in appar- ently favorable environments and remain healthy in condi- tions normally interpreted as stressful. This conclusion is generally counterintuitive and demonstrates the problems with trying to represent past disturbances by visually exam- ining present conditions. Nevertheless, from the tree ring data and the development history of Winnipeg after World War II, it is reasonable to assume that urban disturbances may have been predisposing and inciting factors involved in early stages of bur oak decline in Winnipeg. It would be interesting to study lag times between known predisposing or inciting factors and tree response in terms of both ring width growth patterns and crown dieback. SUMMARY This study demonstrated that tree age, direct physical distur- bances from urban development (i.e., wounding), and indirect disturbances such as impeded drainage patterns may be pre- disposing factors in bur oak decline in Winnipeg, whereas high precipitation levels may be an important inciting factor. Together these factors are likely the main reasons why some bur oaks in Winnipeg are affected by contributing factors in oak decline, in particular the two-lined chestnut borer. Based on the proposed decline model emanating from this study, it is recommended that not only should care be taken directly around bur oaks before and during urban develop- ment, but that consideration should also be given to wide- spread effects on drainage patterns, particularly in undis- turbed areas containing bur oaks. As well, because the inci- dence of decline increases with tree age, it is important to replenish the bur oak population by planting new individuals in urban landscapes, because young trees generally have more ability to adapt to stressful conditions than older speci- mens (Manion 1981). Acknowledgments. The authors thank Dr. N. Kenkel, Dr. T. Goh, and Dr. D. Punter for their input into the study; and Ashley Linden, Jane Dawson, Linda Pearn, and Erik Nielsen for technical assistance in the field and in the laboratory. The project would not have been possible without the generous financial support from NSERC, Mani- toba Hydro, the City of Winnipeg, or the cooperation from many private golf courses, cemeteries, and residential tree owners in the city of Winnipeg. LITERATURE CITED Abrams, M.D. 1996. Distribution, historical development and ecophysiological attributes of oak species in eastern United States. Annales Des Sciences Forestieres 53: 487–512. ©2007 International Society of Arboriculture Allen, M. 2000. Oak Decline in Southern Manitoba. The Manitoba Gardener, Early Spring 2000, pp. 38–39. Allen, M., and G. Kuta. 1994. Oak Decline. Natural Re- sources of Canada, Canadian Forest Service, Northwest Region, Forestry Leaflet 28, Edmonton, Alberta. Amorini, E., M. Biocca, M.C. Manetti, and E. Motta. 1996. A dendroecological study in a declining oak coppice stand. Annales Des Sciences Forestieres 53:731–742. Boone, R. 2003. A forest health study of bur oak (Quercus macrocarpa Michx.) and trembling aspen (Populus tremuloides Michx.) stands near a coal-fired generating station, southeastern Manitoba. M.Sc. Thesis, Department of Botany, University of Manitoba, Winnipeg, Manitoba. Catton, H.A. 2005. An evaluation of bur oak (Quercus mac- rocarpa Michx.) decline in the urban forest of Winnipeg, Manitoba, Canada. M.Sc. Thesis, Department of Plant Science, University of Manitoba, Winnipeg, Manitoba. Ciesla, W.M., and E. Donaubauer. 1994. Decline and Die- back of Trees and Forests: A Global Overview. FAO For- estry Paper 120. Food and Agriculture Organization of the United Nations. Rome, Italy. City of Winnipeg. 2004. Winnipeg’s Neighbourhood Profiles, 1996 Census Statistics Canada Data. http://www.winnipeg. ca/interhom/aboutwinnipeg/workinginwinnipeg/ econodemo.stm (accessed 5/16/04). Dafoe, C. 1998. Winnipeg: Heart of the Continent. Great Plains Publications Ltd. Winnipeg, Manitoba. Dunbar, D.M., and G.R. Stephens. 1975. Association of the twolined chestnut borer and shoestring fungus with mor- tality of defoliated oak in Connecticut. Forest Science 21:169–174. Dyer, S.M., and D.L. Mader. 1986. Declined urban sugar maples: Growth patterns, nutritional status and site fac- tors. Journal of Arboriculture 12:6–13. Environment Canada. 2002. Canadian daily climate data on CD-ROM—western Canada. http://www.climate. weatheroffice.ec.gc.ca/prods_servs/cdcd_iso_e.html (ac- cessed 6/20/04). Franklin, J.F., H.H. Shugart, and M.E. Harmon. 1987. Tree death as an ecological process. Bioscience 37:550–556. Fritts, H. 1976. Tree Rings and Climate. Academic Press. New York. Gaertig, T., H. Schack-Kirchner, E.E. Hildebrand, and K.V. Wilbert. 2002. The impact of soil aeration on oak decline in southwestern Germany. Forest Ecology and Manage- ment 159:15–25. Greve, U., Eckstein, D., Aniol, R.W., and F. Scholz. 1986. Dendroklimatologische untersuchungen an fichten unter- schiedlicher immissionsbelastung in Nordostbayern. All- gemeine Forst- und Jagdzeitung. 157:174–179. Haack, R.A., and R.E. Benjamin. 1982. The biology and ecology of the twolined chestnut borer, Agrilus bilineatus
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