Why do leaves change colour?

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Our Envrionment by Katharine Fletcher

Did you know there’s an activity called “leaf-peeping”? That’s what tourism boards call our predilection for taking country drives amid the autumn colours, to admire the golden, scarlet, crimson and other hues of deciduous trees’ leaves.
But why do deciduous trees’ leaves change colour?

Our Envrionment by Katharine Fletcher

Did you know there’s an activity called “leaf-peeping”? That’s what tourism boards call our predilection for taking country drives amid the autumn colours, to admire the golden, scarlet, crimson and other hues of deciduous trees’ leaves.
But why do deciduous trees’ leaves change colour?
Chlorophyll
When leaves first emerge, they are green, and continue to be so until autumn because of the presence of chlorophyll, a pigment reflecting green.
‘Wild’, the Canadian Wildlife Federation’s magazine for children explains: “Chlorophyl is an important part of how trees feed themselves. Leaves absorb a gas called carbon dioxide from the atmosphere and draw water taken up by a tree’s roots. In the leaves, chlorophyll drives a chemical reaction that turns water and carbon dioxide into a sugar called ‘glucose,’ which feeds the tree.”
This process of making food from sunlight is called photosynthesis, where chlorophyll is key.
During the summer months, trees continue manufacturing chlorophyll because the sunlight is strong. However, come early September in our latitude, daylight hours decrease.
Other chemicals, other colours
Because chlorophyll production depends upon light, its reduction means other chemicals that possess different colour profiles take over. In fact, all the chemicals were always present. It’s only because chlorophyll stops being produced that green fades and other hues dominate.
Now, we know most carrots are orange. In fact, these vegetables get their name from “carotenoids,” the chemical that makes them that colour. So it’s not surprising that orange and yellow-coloured leaves also possess that chemical.
Anthocyanin is the chemical which enables leaves to become red and purple. Again, this has always been present in leaves, acting as a sort of sunscreen to protect them from the sun.
Deciduous trees’ leaf loss
Now, I’ve been discussing only deciduous trees, because it’s only those which lose their leaves upon winter’s approach that also turn colour.
But this begs another question: Why do they lose their leaves? We now understand that because the light has declined, photosynthesis also ceases (because there is no more chlorophyll in deciduous trees’ leaves). The trees now enter the plant equivalent of mammalian hibernation: they slow down during winter.
In order to survive, deciduous trees feed themselves through their root system, living on glucose (sugar) and water. ‘Wild explains’, “If they kept their leaves, the trees would use up resources they need to feed their trunks, barks and branches.”
Deciduous trees drop their leaves in late October or November, here in our region. They do this by growing a “cap” precisely where each leaf stem joins the twig, sealing the plant from each individual leaf and “pushing off” the leaf. Because “scar tissue” seals the place of attachment, there is no exposure damage to the tree.
Evergreen trees
Meanwhile, some trees have leaves we call “needles” — pine, tamarack, balsam fir, spruce, yew, and juniper. They remain green year-round because they are
northern species’ which have adapted to harsh climates where they must feed themselves as well as possible.
‘Wild’ explains: “The needles on evergreens are actually regular leaves that are rolled up tight. They hold water to help the trees photosynthesize during winter. That’s why the needles have waxy coatings, which help prevent the water inside from evaporating on warm, sunny days.”
Want to go “leaf peeping” this autumn? Now you’ll know why you’re seeing those glorious colours of autumn, contrasting so beautifully against white pine, spruce and other evergreens.