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Fall colours

The First Nations Garden in fall
Photo: Jardin botanique de Montréal (Michel Tremblay)
First Nations Garden in autumn.

Why do leaves change colour in the fall?

And why do birch leaves turn yellow, while maple leaves turn gold, orange, red and green?

In the fall, shorter days and cold nights stimulate the production of ethylene, a hormone responsible for causing the leaves to drop off. A cork plug gradually forms and blocks the channels that carry nutrients to the leaves. They survive for a while longer by using up their own reserves.

When the essential minerals are no longer available, chlorophyll (green pigment) disappears, leaving its place to the yellow pigments (carotenes and xanthophylls) that are normally masked by the green. This is the case of the white birch.

For maples, the cork plug is not completely watertight and doesn’t completely block the flow of raw sap into the leaves. As a result, sugar builds up in the leaf tissues, causing the formation of red pigments (phenols and anthocyanins). These pigments change colour depending on the acidity of the soil: bright red in acidic soil, and purple in alkaline soil. At the same time, the percentage of (green) chlorophyll gradually declines, revealing the (yellow) carotene. This is why we see some maple leaves that are still green while others are yellow or red.

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