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Norway maple trees were planted in Montreal's Mount Royal Park in the 1960s and 70s, and their fruitful seeds have continued blowing onto the mountain from other parts of the city.

Andrej Ivanov/The Globe and Mail

The forest floor of Mount Royal Park in autumn is a many-coloured carpet of maple leaves: orange, yellow, and every kind of red. The scene looks ready for a patriotic postcard.

But woven into this Canadian tapestry is a foreign interloper.

To an untrained eye, it could be mistaken for the sugar maple that adorns our flag and litters Montreal’s famous park with dazzling mulch. But that resemblance is just one of the Norway maple’s cunning tricks.

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For all the European tree’s outward charm, foresters damn its name. They see the Acer platanoides as a dangerous fraud and bully, unworthy of being mistaken for the native Acer saccharum that gives us our syrup and most prominent national symbol. Experts in the field worry about the ecological cost of a Norwegian takeover – and the growing tide of other invasive species changing the Canadian landscape.

Mount Royal is a telling example of the problem. Norways were planted there in the 1960s and 70s, and their fruitful seeds have continued blowing onto the mountain from other parts of Montreal.

It has been a slow but steady invasion. A 2003 inventory of saplings on Mount Royal found three times as many Norways as sugars. Within 100 years, 25 per cent of the park’s trees could belong to the species, estimates Christian Messier, a professor of forest ecology and urban forestry at the Université du Québec en Outaouais (UQO) and à Montréal (UQAM).

Since the Norway is slower to change colour, and turns a fairly bland yellow, that takeover could tarnish the mountain’s spectacular fall palette, Prof. Messier warned.

“It’s a question of heritage,” he said. “Do you want the mountain to remain a sugar maple forest?”

The question is not limited to one park, nor are the dangers simply aesthetic. A recent study found fully 17 per cent of Montreal’s trees were Norway maples, making the species the most common in the city. They are the third most common tree in Toronto. Even the Parliament buildings in Ottawa are surrounded by the “Viking” maple, as one Ontario horticulturalist dubbed them.

Non-native species (everything from plants to insects to animals) make up a larger and larger share of Canada’s natural world. About 8 per cent of wild species in the country are already exotic and the number looks set to increase thanks to global trade, Prof. Messier said.

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“We’re at the beginning of a vast period of invasion from species like this, because of the growth of commercial exchanges, especially with China,” he said. “The Norway maple could just be the tip of the iceberg.”

The encroachment of invasive species has done serious damage to local habitats, said Eric Davies, a PhD candidate in forestry at the University of Toronto, seen here in the Montreal park on Oct. 29, 2020.

Andrej Ivanov/The Globe and Mail

Canadian cities began planting Norways en masse in the 1950s because they are a hardy breed – “so bullet-proof it’s hard to pass them up,” said Astrid Nielsen, an Ottawa-based forester – but that hardiness also makes them ferocious invaders.

The trees have a suite of qualities that help them conquer native Canadian species. They can tolerate the salty, compacted soil of our cities, and research suggests their roots are “allelopathic,” meaning they release chemicals that can suppress neighbours.

The Norway’s leaves also grow in thick, blocking sunlight from competitors in the undergrowth. And those non-native leaves repel Canadian insects, which is good news for the tree but bad news for other birds and animals that depend on them for food.

The encroachment of invasive species has done serious damage to local habitats, said Eric Davies, a PhD candidate in forestry at the University of Toronto.

“Historically, hiking through Canadian forests in the spring used to be bright, no leaves, just wildflowers, early butterflies, like the Mourning Cloak, and the burgeoning wave of the migratory birds,” he said in an e-mail. “Now, increasingly, our forests are dark, toxic, and lifeless.”

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Not content with dominating nature, the tree has taken up root in our national symbology, too. Canada Post has printed Norway maple leaves on its stamps and the Bank of Canada on its bills, as forestry communications consultant Peter Kuitenbrouwer pointed out this summer in The Globe and Mail (a newspaper whose logo is unmistakably a sugar maple leaf).

You can often tell a Norway by the seven lobes of its leaves (as opposed to the sugar’s five), its rounded buds (the sugar’s are pointy) and a milky discharge when you snap off one of its stems.

Eric Richard shows the difference in the leaves between the Norway maple, left, and a Canadian maple.

Andrej Ivanov/The Globe and Mail

Unfortunately the trees are also revealing other, more troublesome traits. They’re prone to poor structure, which gives them a tendency to lose limbs during storms. They can also have “girdling roots,” which means the tree sometimes essentially strangles itself to death. Norway maples are “not the ideal urban species we once thought they were,” Mr. Kuitenbrouwer said.

That doesn’t mean they will die out on their own. Cities have taken steps to nudge the stubborn intruder along. The City of Toronto stopped planting Norways and reduced their share of the street tree population from 22 per cent to 13.5 per cent in the past decade or so. In Montreal, the trees can no longer be planted on heritage sites like Mount Royal.

In 2006, Les amis de la montagne, a citizen group dedicated to caring for the park, started cutting down every Norway they found in a series of land parcels, before replacing them with sugar maples and other local plants.

An overall shift in urban forestry has driven the Norway out of fashion, as ecological variety and native species have become more highly valued, said Éric Richard, scientific advisor to the group.

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But the fight is far from over. “It remains an issue for the future of the forest,” he said.

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