Tree canopy cover in Canadian communities By Mike Rosen
Urban forests—it’s personal “Canopy cover”, a conventional standard of measurement of an urban forest is defined by the National Capital Commission as “the layer of tree leaves, branches and stems that provide tree coverage of the ground when viewed from above”. Several techniques are used to measure it with a generally accepted opinion that 40 percent represents an “optimal” amount (according to city planning expert Jennifer Keesmat, for instance). Many factors constrain canopy cover including urban development, land use patterns (such as commercial versus residential), tree regulations, and climate—for instance, canopy cover in prairie cities tends to be less than in those which have surrounding forests. My own life has paralleled fluctuations of canopy cover. I grew up in Toronto in the 1960s where I learned to downhill ski at the now-defunct Don Valley Ski Centre—Whistler Mountain, it was not. Today, trying to find that hill is impossible. The Don Valley is now an amazing, closed canopy urban forest. The 70s was the start of my career in forestry. Armed with a Pioneer P26 chainsaw I cut firewood near Peterborough, Ontario, mostly dead American elms, killed by Dutch elm disease. I was unaware at the time that the landscape of North America was going through a tremendous change due to the muchvaunted role of elm in our communities. In 2018 my parents’ condominium in Toronto needed to replace the deteriorating 40-year-old membrane protecting its extensive, underground parking garage. This meant the removal of a row of impeccably managed 16-inch diameter hackberries. These personal stories all heralded historic changes in the urban forests of Ontario (and Canada). In the beginning The need to provide parks coincided with the industrial revolution, creating Canada’s great municipal parks (High Park, Parc du Mont-Royal, Stanley Park and so forth). Although the priorities were more around family amuse42 • 2021
Many people treasure the canopy cover that is a part of many cities.
ment, their creation coincided with the provincial and national park systems, which were created with more conservation-related themes. At the turn of the 20th century, roadside plantings began in many communities across Canada featuring various species: silver maple, little-leaf linden, Japanese cherry, crab apples and (the dreaded) Norway maple, often directed by those with little formal training. It was in the 20th century that communities in Canada began to urbanize and commercialize and hence take their trees honestly. The population of Toronto went from 208,000 in 1901 to over 522,000 by 1921. This same jump was seen in all the other cities of Canada. In the 1960s the pressures on urban trees continued with the introduction of Dutch elm disease. Erik Jorgensen, the University of Toronto professor, who coined and marketed the term “urban forest”, had a long-term effect on the management of trees in cities and towns. His leadership was sought to help manage the native elm population which was virtually eliminated from the landscape of urban (and rural) Ontario in a matter of about 20 years. Issue 4
After arriving from Denmark as a forest pathologist for the federal government, he joined the University of Toronto in 1959 and began a concerted program to study and combat DED. This led to the teaching of the first urban forestry course in Canada (1969) and the eventual development of treatments for DED. He was supposed to lead a national urban forestry program in Ottawa, but the government and academic priorities had him finish his career at the University of Guelph. This coincided with the training of urban foresters by Jorgensen and the explosive growth of the arboricultural trade and the International Society of Arboriculture. Foresters trained by Jorgensen included Bob Perkins (Oakville, Ontario), Mike Allen (Winnipeg), Bill Granger (Vancouver) and Lloyd Burridge (Windsor, Ontario). The onslaught of DED changed Canadian cities and towns dramatically. Elms were eventually replaced with other trees, notably green ash which, at the time, seemed to be resistant to the increasingly harsh reality of the environment of cities. Although no formal study was ever commissioned, from about 1970 to the early 1990s urban localgardener.net
