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Every fall, much of Ontario is a cornucopia of colour, with green landscapes turning into a vibrant array of scarlet, yellow and burnt orange. But how does that colour transformation happen? It all has to do with how trees get their nutrients. Here are some facts about the process:

Leaf colour compounds

green leaf

Chlorophyll

Chlorophyll, the dominant leaf pigment of spring and summer, 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.

Carotenoids

Change in the fall starts from carotenoids, which are triggers for yellow, gold and orange leaf colours. Xanthophylls 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 is the pigment responsible for the orange of autumn leaves, such as the bright orange of sugar maples.

orange leaf
green leaf

Anthocyanins

Unlike carotenoids, which exist in leaves all summer but are masked by chlorophyll, red anthocyanins are freshly produced by trees as fall ramps up. The leaves with the highest sugar content contain anthocyanins, which are byproducts of extra sugar formed late in the summer and cause leaves to turn glorious red and purple, such as in red maples and red oaks.

Tannins

The presence of tannins means leaves will turn brownish or tan. Tannins are typically seen in oak leaves in the fall and seep out of other tree leaves as they are dying, making their brown pigments characteristic of very late fall.

orange leaf

SOURCE: Various / Infographic by Tammy Hoy

How does it work?

Leaves change colour through a process called photosynthesis, which means “putting together with light.” Throughout the spring and summer, each green leaf is a tiny factory that converts light into energy for the tree. Inside leaves are small bodies called chloroplasts, which contain chlorophyll – when water and carbon dioxide come into contact with this green pigment, a chemical reaction occurs, which produces sugar and oxygen.

In the fall, as the weather becomes cooler and deciduous trees begin to go into survival mode for the winter, the chlorophyll in leaves starts to break down, making other compounds in the leaf visible. Different tree varieties have different compounds in their leaves, resulting in the variety of colours in the fall.

Getting it right

What weather conditions influence fall leaf colour?

  • Daylight
  • Temperature
  • Precipitation
  • Soil moisture

Best conditions for fall foliage:

  • Wet growing season followed by an autumn with many sunny days, dry weather and cold, frostless nights.
  • Fall foliage is sparkling after several bright fall days and more muted during rainy spells.

Worst conditions for fall foliage:

  • Drought conditions during late summer and early fall can trigger an early shutdown of trees, causing leaves to drop early.
  • A hard frost can kill the process within the leaves, leading to poor colour and early leaf drop.

Why do trees lose their leaves at all?

Stems, twigs and buds are equipped to survive extreme cold but tender leaf tissues would freeze in winter, so plants must either toughen up and protect their leaves or dispose of them.

Fun fact

One theory is that red pigment acts as a sort of sunscreen for the leaves, staving off damage from bright autumn sunlight and allowing the leaves to stay on the tree for longer than they would if they were to remain green or even yellow. Another is that the bright red colour could deter insect pests from feeding on leaves, or that red attract birds that feed on (and spread) the trees’ fruit.


Advertising feature produced by Globe Content Studio. The Globe’s editorial department was not involved.

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