22 Catton et al.: Evaluation of Bur Oak Decline Arboriculture & Urban Forestry 2007. 33(1):22–30. An Evaluation of Bur Oak (Quercus macrocarpa) Decline in the Urban Forest of Winnipeg, Manitoba, Canada H.A. Catton, S. St. George, and W.R. Remphrey Abstract. Winnipeg, Manitoba, Canada, has a large, indigenous population of bur oak (Quercus macrocarpa Michx.). In the 1980s, many of these trees were showing signs of decline, a disease caused by a complex of abiotic and secondary biotic stressing agents. Potential causal factors were investigated by comparing various aspects of 120 bur oaks visually rated as healthy or declined based on crown dieback levels. The results indicated that many selected bur oak trees predated surrounding urban development and that declined trees were significantly older with more severe stem wounds and competition from surrounding trees than healthy specimens. Average annual growth ring widths of healthy and declined trees were similar in the early part of the 20th century. However, decline actually began decades before symptoms were noticed, coinciding with a period of intense city-wide urban development, as growth of declined trees was slower than that of healthy trees beginning sporadically in the 1940s and consistently from 1974 to 2001. During the early years of decline, the year-by-year separation in ring width between the two categories was significantly positively related to precipitation levels. This suggested that in wet years, declined trees may have been surrounded by unfavorable water-logged soils, possibly as a result of natural drainage patterns being impeded by urban development. Key Words. Dendrochronology; tree decline; urban development. Bur oak (Quercus macrocarpa Michx.) is a large, slow- growing, deciduous tree native to North America (Johnson 1990). Winnipeg, Manitoba (49°53N 97°09W) is located at the northwestern boundary of the native range of bur oak and is the only major city in western Canada to have a large, indigenous population of mature oaks in its urban forest. Many of these trees are located in the city’s parks and bou- levards as well as on commercial and residential properties. The present oak forest in Winnipeg has regenerated after intensive logging in the region during the early and mid-19th century (Ross 1856; Dafoe 1998; St. George and Nielsen 2002). Together with several other native tree species, the bur oak in Winnipeg’s urban forest provides functional, aesthetic, and economic benefits such as shelter, shade, beautification, and increased property values that improve the urban envi- ronment. In 1986, city foresters reported that many of Winnipeg’s bur oaks were showing signs of distress (Allen and Kuta 1994). Symptoms reported included crown dieback, epicor- mic shoot growth, and susceptibility to the two-lined chestnut borer (Agrilus bilineatus), a wood-boring beetle not known to attack healthy oaks. A preliminary investigation indicated that these symptoms were not caused by a single, aggressive primary pathogen, but instead were the result of a disease known as oak decline (Allen and Kuta 1994). In response to ©2007 International Society of Arboriculture this problem, over 1700 affected bur oaks were removed from the city between 1986 and 2000 (M. Allen, pers. comm.). More recently, similar stress symptoms have been reported in other species of oaks in urban parks in Toronto and Oakville, Ontario (Ric and Bykov 2002; Hashemi 2004). Tree decline can be defined as a premature, progressive loss of vigor not explained by an aggressive disease or insect, and has been observed to affect many tree species around the world (Ciesla and Donaubauer 1994). It is a complex disease involving environmental, tree, and pest factors (Houston 1974; Wargo 1996) and may be a natural ecologic response in trees growing in unsuitable conditions (Manion 1981). Manion (1981) created a widely accepted model for tree de- cline by classifying damaging elements into three groups: predisposing, inciting, and contributing factors. Predisposing factors are long-term, chronic, or slow-changing environmen- tal factors, including climate, air pollution, site conditions such as soil compaction or fertility, and tree characteristics such as genetic potential and age that weaken plants growing in suboptimum sites (Manion 1981). Inciting factors are those that are more acute or short-term in duration such as defo- liation, drought, frost, and mechanical injury. These events often cause drastic injuries and trigger further weakness and vulnerability in trees. Trees may attempt recovery after incit- ing factors have occurred but are often prevented by the
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