To counter this, some municipalities are implementing policies to ensure diversity. In Surrey, British Columbia, for example, the goal, says Greg Ward, the city’s manager of urban forestry and environmental programs, is to have no more than 20 per cent of one genus and 10 per cent of one species.
“There is no miracle street tree species,” says Stephen Smith of Toronto-based Urban Forest Associates. “Instead, it’s a question of what is the right tree for any particular site.” This is especially true in terms of size. On the one hand, small ones may make the most sense because they have the least impact on overhead wires. As Jeff Rietkerk, an urban forest technician in Esquimalt, B.C., puts it, “It’s not a good idea to plant a tree that grows to 80 feet where there’s a high-voltage power line 40 feet off the ground—otherwise you’ll be setting yourself up for a lifetime of pruning hassles.” As well, many homeowners have concerns over the impact these larger trees might have on driveways (when trees’ roots affect pavement, for example), and fears regarding big branches that overhang roofs.
In environmental terms, however, it’s the large canopy types, particularly those with big leaves, that provide the most benefits. As Andy Kenney, senior lecturer of urban and community forestry, faculty of forestry at the University of Toronto, explains, “Leaf area is the most important feature in terms of a tree’s ability to absorb air pollution, take in carbon dioxide, provide shade, intercept rainfall and create habitat.”
Ward has developed an approach to the size issue that juggles these competing factors: “All things considered, we plant the biggest tree possible. The environmental benefits are significant.” As he explains, 15 or 20 years ago, cities were planting small specimens, but that trend is changing. “I’ve seen that completely swing back to larger trees now.”
Size notwithstanding, encouraging homeowners to feel a sense of shared ownership is key to municipal success in street tree planting. “The challenge is to plant a species that will make people happy,” says Rietkerk, who admits to feeling some guilt about dictating what species is planted in front of people’s homes. He favours policies that allow residents to select from a list of options. While Ward agrees they need to look on these plantings as partnerships, he points out the challenges of involving homeowners in species selection. “When you’re planting 4,000 trees a year, it’s an administrative nightmare to consult everyone.” That said, Ward adds, “We let people know when we’re planting a new tree, and if a resident calls us, we’ll work with them.”