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Why do the leaves change colour in the fall?

October 17 2022 | Catalina Ville…

One of fall’s most beautiful features is how the colour of the leaves turn to warm shades of orange and gold. The sound of crunching leaves beneath our feet is pretty great too! Ever wonder why the leaves suddenly change colour at this time of year? Six-year-old Augustín did, and he asked us about it.

It’s true that colder temperatures play a part in how intense the colour of the leaves get and the hues they take on, but this is not the real reason they change colour. This happens because of the change in the amount of sunlight.

To better understand this, let’s look back at how plants feed themselves, that is, by converting the energy from the sun through a process called photosynthesis. The green colour of the leaves comes from a pigment in their cells called chlorophyl. This pigment is what plants use to capture light energy.

So, when the days get shorter at the end of the summer, trees have less sunlight to absorb. This tells them to slow their metabolism down and cut back on their photosynthesis. When this happens, the chlorophyl pigments go to rest and warmer colours start to bloom. These warmer colours come from other pigments in the leaves: xanthophylls generate the yellows while carotenes offer us those to orange hues.

You might wonder where those flamboyant reds come from, like the ones you see on Northern red oaks or certain kinds of maple trees. These are generated by anthocyanins. While those other pigments are found in the leaves all summer long, anthocyanins are only produced when the fall rolls around. 


It's not too late to come admire the beautiful fall colours at the Old Port of Montréal! 

Catalina Villegas Burgos
Profile picture for user Catalina Villegas Burgos

Catalina Villegas has a bachelor’s degree in physics engineering but found her calling in popularizing science and science outreach. This is what she has done at the Montréal Science Centre where she also has performed content research for its exhibitions and programs. Her other interests include art, caricature, origami, and literature.