Winnipeg's urban forest a monoculture at risk of being wiped out by disease
CBC analysis using city tree inventory data reveals never-before-seen picture of Dutch elm disease
In the wake of increasing threats to Winnipeg's tree population, city officials undertook a massive project in 2009 to build an inventory of every tree on public space in order to help better protect its valuable urban forest.
"Part of managing an urban forest is you need to know what you have. It's important to have an inventory so you know what you're up against," says Martha Barwinsky, the city's forester.
Her team provided CBC with a copy of the tree inventory database containing nearly half a million trees located on public space, such as parks, boulevards and along river banks. With this data the CBC was able to paint a citywide picture of the impact of Dutch elm disease on neighbourhoods.
Each point on this map represents an elm tree that was cut down in 2014 due to infection from Dutch elm disease:
Of Winnipeg's approximately eight million trees, just over 200,000 are American elms. It is the largest mature elm population in North America. However, in 1975, when disease hit for the first time, Barwinsky said it was a wake-up call for everyone.
"On average over the past 10 years, it's been about 5,000 trees a year," she says.
The tree data shows disease is more rampant in older neighbourhoods in the heart of the city, where the concentration of elms is higher, such as River Heights, Wolseley, the West End and the North End. The map also reveals that Dutch elm disease also spreads rapidly along waterways, such as the La Salle River just west of St. Norbert.
In 2014, several hundreds of trees in La Barriere Park were removed due to an infestation.
A second wave of monoculture
In order to slow the progression of Dutch elm disease, the province declared a moratorium on planting elms in the 70s for nearly a decade, which in some ways led to a second form of monoculture, says Barwinsky.
"The Dutch elm disease legislation prohibited the planting of elms, so the next best tree was the green ash. So now we've got a second type of monoculture in those neighbourhoods," she points out.
She says this poses a problem, since most experts say it's just a matter of time until emerald ash borer is detected in Winnipeg.
The map below illustrates a random sampling of 2,000 green ash trees, the second most common tree in Winnipeg. The data reveals that although the tree was widely planted across the city, there are high concentrations in newer neighbourhoods, which according to experts are likely to become the problem areas should the emerald ash borer appear in Winnipeg.
Eventual disappearance of cathedral elm arches
Barwinsky says future generations will witness a very different tree landscape in Winnipeg.
In 2009, her team established planting guidelines to ensure proper tree diversity for developers building new neighbourhoods. The strategy aims to limit any one species to 25 per cent of the total tree population in any given area.
In neighbourhoods with higher rates of Dutch elm disease, such as River Heights and Minto, American Elms represent about 70 per cent of the tree population. However in new developments like Royalwood and Whiteridge, no single species accounts for more than 25 per cent of the total population.
"Any new development has diversity guidelines that they are required to follow to ensure we don't wind up with another monoculture where we have elm-lined streets and ash-lined streets" said Barwinsky.
"For the most part, we won't see unfortunately the streets with the cathedral arches and the monoculture. Aesthetically, it's not as pleasing. However, from a management perspective and healthy canopy perspective, it's very important," Barwinsky said.
"So we'll see a mix of ages, and mix of sizes and a mix of species in these neighbourhoods."
According to Jacques Tardif, a professor of forest ecology at the University of Winnipeg, the city's tree management strategy is sound.
"It is less expensive to control the spread of disease than it is to simply destroy the trees and to replant," he says.
However, he says more funds would help those responsible for managing the urban forest.
"Is that possible? Do taxpayers have the desire to pay for tree management? That's a whole other debate."
What is the most common tree in your neighbourhood?
Hover over the map below to find out:
Where is the oldest tree in Winnipeg?
It can be found in Point Douglas at the corner of Syndicate Street and Rover Avenue. This eastern cottonwood measures nearly 30 metres and its diameter measures close to 180 centimetres.