Why Leaves Change Colour in the Fall

Autumn has arrived and with it comes the beauty inherent in all the colours that are splashed throughout treetops everywhere. Yes, this is the time of year when deciduous trees begin to shed their leaves. Before they hit the ground though, most leaves go through a magical transformation, taking on hues of red, orange, yellow and combinations thereof. What is it that makes the colour of leaves change from their normal shades of green to a rainbow in the trees? We call it chemistry folks…


Leaves contain Chlorophyll. Chlorophyll is what makes a leaf its particular shade of green. It also provides food for the plant that it is attached to via photosynthesis, by capturing the sun’s rays and transforming it into energy. When the days get shorter and cooler in our Northern climes, trees get ready for winter hibernation. They no longer produce chlorophyll and it quickly disintegrates within the leaf structure.

Before the leaf completes its duties of providing nutrients to the tree, it usually goes through a magnificent colour metamorphosis though. That is due to a couple of other pigments contained within the leaf structure, known as anthocyanin and carotene. These pigments are always in a leaf, but are masked by the larger amount of chlorophyll. When chlorophyll production ceases, anthocyanin surges and its subsequent colour shines through the leaf. Typically anthocyanin appears as the reds and oranges of maples and sumacs, and carotene is responsible for the yellows of birches and aspens.

Anthocyanin and carotene levels are not the only thing responsible for what colour a leaf turns in the Autumn though. The amount of sunshine itself actually determines the colour a leaf will change. When there is little sunshine available, anthocyanin is less active. That means that you will typically see more yellows and browns. Warm, sunny fall days ramp up anthocyanin levels, so you can look forward to more red and orange displays. That means that the same tree might be red, orange or even yellow from one year to the next, depending upon the weather that particular Fall.

With that in mind, I suspect that we will be able to look forward to some pretty brilliant fall colours in these parts this year. We’ve had plenty of sunshine in Southwestern Ontario that I recall. What about you? What do the trees look like in your neck of the woods?

Late summer

Late summer (Photo credit: Wikipedia)

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10 Responses to Why Leaves Change Colour in the Fall

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