Why autumn leaves turn the colours they do

Warm days, cold nights and leaf chemistry combine with specific tree varieties to create a spectacular spectrum of leaf colours in the autumn, says Sean Fox of the University of Guelph Arboretum.

Sean Fox of the University of Guelph Arboretum offers a science lesson behind those colours

Sean Fox talks fall leaves at University of Guelph Arboretum

3 years ago
Duration 3:06
The fall leaves this year are particularly vibrant. Sean Fox of the University of Guelph Arboretum explains why.

If you're thinking the fall colours on the trees this year seem especially vibrant, you're right.

Sean Fox is the manager of horticulture and curator of the University of Guelph Arboretum, where many of the trees have either changed colour or in the process of going from green to orange, red, yellow or even purplish-hues.

He says different tree species are predisposed to have different fall colours and those colours can be amplified in some years depending on weather and environmental conditions.

"What we're seeing this year, there's a late summer drought which causes those trees to start to shut down a little earlier. So they're not producing that chlorophyll. They're not replacing the chlorophyll in the leaves that they're constantly replacing all summer," he said.

"Then what you see is the pigments showing up when the chlorophyll leaves. So the carotenoids, the same colour that gives carrots the orange colour, those are orange and yellows. But the other thing that you see happening is on those really bright, sunny days, there are some species like those those really fiery red oaks and maples that when sugars are maintained in the leaves, they'll actually be converted to anthocyanins," he added.

"Those are the bright colours you see. And the really sunny, sunny, bright days are really great for producing those sugars or carbohydrates, converting to anthocyanins, but then the really cold nights trap them in the leaves longer so they amplify the colours."