How and Why do Leaves Change Colour?

Written by: Karlee May, Digital Media Coordinator

We’re deep in the heart of the fall colours during this time of year. The maples, birches, oaks, and sumacs are flaunting and flourishing a spectacular splendour of colour across Halton Region. How do deciduous, broad leafed, trees produce the annual autumnal pageantry? What is the science behind the seasonal effect?

Red Maple

Red Maple

The Science

The initial cue for the leaves to change are the shortened days and cooler temperatures. To complete photosynthesis, plants need light, water, and soil. Both light and water are scarce resources during winter. Additionally, broad leafed plants are vulnerable to frost. To promote the survival of the plant, the leaves must die, seal off the branch, and fall to the ground. Once the weather cools and the period of light during the day shortens, it’s the beginning of the end for nutrient production. Trees shut down photosynthesis, and begin to ‘hibernate’ for the winter.



Winter Supplies

Trees produce an excess amount of glucose (also known as sugar, and a plant’s favourite food!) during the summer when they receive the most amount of light. Once the days begin to dwindle, glucose production is shut down, and the excess is turned into starch. The tree sends the starch to the roots for storage where it will stay until the spring warmth awakens new growth.

The Science of Colour

Once the supply of nutrients to the leaves stops, the internal water supply is cut off and prevented from escaping. This process is called abscission. Since water stops flowing, and photosynthesis stops producing, then the chlorophyll recedes from the leaves. Chlorophyll is the chemical that infuses leaves with a green colour. When the chlorophyll recedes, the remaining pigments of the leaves start to show through, causing the gradual change in colour. Carotenoids turn the leaves orange; xanthophyll turns the leaves yellow; and, anthocyanin turns the leaves red and purple.



Read the Leaves

When you’re walking in Halton Parks, try to read the leaves by their fall colours, to identify the tree species. Oaks become red, brown, or russet. Hickories are golden and bronze. Aspen and Yellow Poplars become a beautiful golden yellow, dogwoods a purplish red, and beech turns a light tan.

White Oak

White Oak

There is a great diversity of colour within similar species, too, like maples. Red maples are a brilliant scarlet, sugar maples are orange-red, and black maples become a glowing yellow.

When you’re hiking in the parks with friends, family, and children, play a game to see who can identify the tree by the brilliant autumn colour of its leaves!


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