The Canadian sugar maple, famed for its syrup and blazing red colour in the fall, is in danger of being crowded out by invasive Norway maples in Montreal, botanists say.

The city's Mount Royal was originally densely wooded by sugar maples, hickories and red oaks, but Norway maples are taking over, according to a census of the trees.

The sugar maple was once the dominant species in the forests of southern Quebec. Norway maples are a more recent addition. The trees were planted on the streets of Montreal and Toronto during efforts to reforest the cities in the 1960s and '70s.

Norway maples are fast-growing, pollution-resistant trees that are now considered an invasive species in the Ontario and the U.S.

The two species look alike for most of the year.

But come fall, sugar maples turn red and orange while Norway maples turn yellow and ochre. That's one reason why the streets of Montreal put on a less-flamboyant show compared with the forests of the Laurentians or the Eastern Townships.

The shift in tree populations was first noticed by people working on Mount Royal.

That led the city to fund a tree census by Prof. Jacques Brisson of the Plant Biology Research Institute and graduate student Joëlle Midy. They aimed to determine the extent of the problem and to look for long-term solutions.

Among trees 10 years or older, 4,200 sugar maples and 1,200 Norway maples were found. But the ratio reversed in younger trees, with three times as many Norway maples found than sugar maples, the pair said.

Brisson recommends that workers on Mount Royal learn to recognize the two species, so they can remove seedlings of Norway maples. 

Another possibility, he said, is to plant young sugar maples to ensure the species will continue to be part of the city's forest 50 years from now.