Why are the leaves on Montreal's trees so green in October?

After an unseasonably warm late September, trees across the Island of Montreal have held onto their green foliage slightly longer than usual.

Unseasonably warm September could be cause of slight delay in leaves changing colour, McGill professor says

Leaves are starting to turn, slowly, at the Morgan Arboretum in Sainte-Anne-de-Bellevue on Monday. (Navneet Pall/CBC)

After an unseasonably warm late September, trees across the island of Montreal have held onto their green foliage slightly longer than usual.

By Oct. 2, Montrealers can normally expect to see more colours in the trees — but this year, we've only seen a few reds, oranges, and yellows so far.

According to Jim Fyles, a forest ecology professor and director of the Morgan Arboretum, our city's trees have remained green a bit longer due to higher-than-average temperatures.

"As long as the temperature is really warm, the [tree's] demand for sugar is very high," Fyles told CBC Montreal's Daybreak.

Because of the need for energy, trees will keep their leaves green for as long as possible — chlorophyll, which causes the green pigment, allows trees to absorb light and turn it into sugar through the photosynthesis process.

Maple sugar trees at the Morgan Arboretum are suppose to be changing colours and falling. Instead, they are green and looks like canopy from July. (Navneet Pall/CBC)

Trees have other reasons to keep their foliage for longer — to make sure that sugars are stored away to survive the long, harsh Quebec winter.

This morning, Environment Canada issued frost warnings in the Laurentians and the Eastern Townships and other parts of Quebec.

"Those are the things that will make those colours really develop," Fyles said.

Why leaves change colour

Because of changes in length of daylight and temperature, leaves stop their food-making process in the fall. The chlorophyll is broken down, causing the green to disappear and the yellow and orange to become visible. 

No leaves on the road at the Morgan Arboretum. Usually, it's filled coloured leaves by now. (Navneet Pall/CBC)

Reds are produced as a new pigment, acting as a sort of sunscreen, according to the most recent theory, Fyles said. Another popular theory argues that the red colour is meant to ward off insects. 

The changing colours are signals the tree is preparing for dormancy, Fyles said.

Though he says it's hard to say how the colours will manifest themselves this year, they should begin to develop over the next week, particularly if we see some cooler nights.

With files from CBC Montreal's Daybreak