The Science of Fall Colours

This post is adapted from a fall colours presentation by Toronto and Region Conservation Authority’s (TRCA) Community Learning team. Discover more Community Learning events.

This is a great time to enjoy a hike and take in the fall colours at one of our conservation areas across the Greater Toronto Area (GTA).

The riot of orange, red, and yellow leaves is one of nature’s most glorious displays – a burst of vibrant colours filled with warmth and light, even as the days grow shorter and the temperatures cooler.

tree displays yellow fall colours
tree displays orange and red fall colours
colourful fall leaves line a street at Black Creek Pioneer Village

The science behind the yearly spectacle of fall colours is equally amazing.

It’s all about photosynthesis – the process by which plants turn sunlight and carbon dioxide into sugars, the food that fuels their natural activities.

Leaves get their normal green colour from a pigment called chlorophyll. Chlorophyll allows plants to absorb energy from sunlight, making it crucial to photosynthesis.

During the spring and summer, when there is plenty of sunlight, plants make a lot of chlorophyll.

Chlorophyll is good at absorbing yellow, blue, and red light from the sun, but it reflects green light, which is what makes leaves appear to our eyes as green.

In the fall, deciduous trees begin to go into survival mode, anticipating the long winter ahead. With less daylight and colder nights, they shut down their food production centres and stop making chlorophyll.

As chlorophyll breaks down, other pigments start to show their colours!

Autumn Walk

While carotenoid pigments are present in spring and summer, they are overpowered by chlorophyll. But in the fall, they get to shine. Carotenoids are less sensitive than chlorophyll and can continue smaller scale photosynthesis in the cooler temperatures.

  • Xanthophylls, a subclass of carotenoids, are the pigments responsible for the yellow of autumn leaves. Their presence can be seen in beeches, ashes, birches, aspens, and some oaks.
  • Beta carotene, another carotenoid, is the pigment responsible for the orange of autumn leaves. You can see their handiwork in the bright orange of sugar maples.

fall colours at Kortright

THIS WEEKEND: Enjoy the Fall Colours at TRCA’s Kortright Centre

What about the red you see in on the leaves of some trees, like the red maple? This is the result of another type of pigment: anthocyanin, which also gives apples their familiar red colour.

Unlike the carotenoids, anthocynanin isn’t present in leaves during the spring and summer. Production is kick-started in the fall, as sunlight converts sugars trapped in the leaves into anthocyanin.

Fall colours, trail

If it seems like the fall leaves are more spectacular some years than others, it’s not just your imagination.

While the changing colours of the leaves are mainly the result of reduced sunlight, temperature and climate help to determine how vibrant the colours are.

In general, we see the most vibrant fall colours when:

  1. The growing season is wet, producing a lot of high-quality leaves, and is followed by …
  2. … a dry, sunny fall (producing a lot of sugars) with cold, frostless nights (trapping them in the leaves so they amplify the colours).

On the other hand, drought conditions during late summer and early fall can trigger an early “shutdown” of trees as they prepare for winter, causing leaves to fall early without reaching their full colour potential.

Walking on an autumn trail

If you’re planning to head out this weekend to catch the fall colours, the trails at TRCA’s conservation parks and lands offer great opportunities to savour the spectacle. Read more here – and enjoy the show!