An evaluation of bur oak (KQuercus macrocarpa Michx.) decline in the urban forest of Winnipeg, Manitoba, Canada

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Catton, Haley Autumn
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Winnipeg is the only city in western Canada with a large, indigenous population of bur oak (Quercus macrocarpa Michx.). In the 1980s, many of the city's bur oaks 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 180 bur oaks visually rated as healthy, medium, or declined, based on crown dieback levels. Selected trees were located away from roads where harmful deicing salt is applied in the winter. The results indicated that many selected bur oak trees predated their surrounding urban development. Declined trees were significantly older and had more severe stem wounds and competition from surrounding trees than healthy specimens. Medium trees were generally intermediate between healthy and declined trees. No statistically significant differences in present-day tree size, foliar nutrient levels, nearby visible urban disturbances, or soil characteristics appeared between healthy and declined trees, although these results were not considered conclusive evidence that these factors were not related to bur oak decline in Winnipeg because present conditions do not necessarily reflect conditions at the time of onset of decline. Average annual growth ring widths of healthy and declined trees were similar in the early part of the 20th century; however, growth of declined trees became slower than that of healthy trees beginning sporadically in the 1940s and consistently from 1974 to the present. The divergence in growth rates coincided with a time of intense urban development in the city following World War II, and the separation between the two catagories was more intense in years with high precipitation levels during the early years of decline. Based on these data, it was suggested that decline of bur oak in Winnipeg had been occurring for decades before symptoms were noticed, and may have been caused by a combination of increasing tree age, disturbances from urban development detrimental to oak root systems, and waterlogged soils from the combined effect of impeded drainage and high precipitation levels. Urbanization, however, was not always detrimental to trees, as some showed increased growth rates following development, presumably from the removal of competing trees.