The elm bark beetle kills 15 mature elm trees in Winnipeg every day. The emerald ash borer that arrived in late 2017 could kill every ash tree in the city in the next decade.
Winnipeg has the unfortunate distinction of being the only city in North America at war with both of these tree-killing insects at the same time, and the two species they destroy make up almost two-thirds of the city’s public trees.
Our urban forest is on the cusp of devastation, but a sense of public urgency has yet to set in. In Ottawa, more than 100,000 ash trees have been lost in the eight years since the emerald ash borer’s arrival. Our day is coming. As we battle the invaders, management of our street trees will mean replanting more diverse species as old ones are lost, but we should not undervalue efforts to preserve our existing older trees.
The economic, environmental and social benefits of street trees increase exponentially with size, and it can take generations for the value of a lost mature tree to be regained. A replanted basswood, as an example, will be less than half of its mature height and breadth after 30 years of growth. A disease-free American elm can live up to 300 years, and doesn’t reach full maturity until 150 years. With salt and other environmental pollutants in today’s city, most replanted trees will never reach the scale of our current canopy.
Because of this, investment made to preserve larger trees will have significant payback over several decades. The costs will be high, but there are few public expenditures with greater return on investment than protecting our street trees.
The majestic beauty of leaf-filtered light streaming through the cathedral arch canopy of towering street trees inspires a sense of place that emotionally connects people to their neighbourhoods and cities. They create a comfortable pedestrian experience by providing shade and wind protection, while establishing an intimate pedestrian zone that separates cars and people.
This perception of safety is enhanced as street trees define the road edge and enclose a driver’s field of vision, which instinctively calms traffic and lowers driving speeds. A 2008 study by the Texas Transportation Institute found that the presence of large street trees lowers the average cruising speed of vehicles in suburbs by almost 5 km/h.
This more comfortable pedestrian environment translates into economic benefit. A study from the University of Washington determined that when shade trees are added to a retail street, the positive impact on the experience makes people stay longer and shop more, increasing spending by as much as 12 per cent on average. It also found that the attraction to mature street trees can increase residential property values by as much as 15 per cent. In Portland, Ore., this was calculated to be worth US$15 million in added property tax revenue for the city.
Street trees also improve physical and mental health. A large tree can remove up to 25 per cent of nearby airborne fine particulates and gaseous pollutants emitted by vehicles and buildings. Large mature trees remove 70 times more urban air pollution than the small trees planted to replace them. Recent findings published in the Urban Forestry and Urban Greening Journal indicated that across 86 Canadian cities, the combined reduced air pollution from urban trees saves the health-care system $227 million annually, avoiding 22,000 incidences of acute respiratory symptoms.
A Toronto study, Neighbourhood Greenspace and Health in a Large Urban Centre, found exposure to trees can be psychologically and physiologically restorative. The study asked 31,000 residents in different communities to assess their perceived state of health.
When the data were overlaid on tree maps, they revealed that having 10 or more mature street trees on a block had the equivalent self-reported health benefits of feeling seven years younger. People in poorer areas with plentiful street trees reported health that resembled affluent neighbourhoods, and those living on wealthier streets without trees reported health like that of people in poorer areas.
As trees absorb carbon from the atmosphere, they can play an important role in reducing a city’s contribution to climate change. Once again, size matters. As trees age, their climate benefits increase disproportionately, with as much as 50 per cent of their carbon accumulation occurring in the last quarter of their lives, and almost three-quarters in the last half. The cooling effect of shade trees also reduces the energy requirements for the air-conditioning of buildings, which in turn reduces GHG emissions. A study by the City of Sacramento determined that large street trees can reduce summertime energy use by as much as 35 per cent, twice as much as medium-sized trees, with small trees having very little effect.
With climate change increasing the frequency of high-volume stormwater events, street trees are playing a more important role managing runoff and relieving pressure on municipal infrastructure.
Large street trees can absorb as much as 60 per cent of the water in a rainfall, with capacity being directly related to the size of the tree’s canopy and root system. Losing our large trees would likely have catastrophic effects on Winnipeg’s combined sewer system — pipes that collect both raw sewage and rainfall, with a replacement cost pegged at more than $4 billion.
Winnipeg without its majestic street tree canopy is difficult to imagine. The economic, social and environmental impact would be devastating. The battle to preserve as many large old trees as possible, and to replace the ones that are lost, will take a significantly increased commitment from private citizens and all levels of government.
Their benefits show that a city’s trees are not merely leafy decoration, but should be considered an essential piece of urban infrastructure, no less valuable than roads, sidewalks and sewer lines.
Brent Bellamy is a senior design architect for Number Ten Architectural Group.
Brent Bellamy is senior design architect for Number Ten Architectural Group.