During spring and summer, many tree leaves look green because they contain a large amount of chlorophyll. Chlorophyll is a pigment that helps plants capture energy from sunlight. Plants use that energy to make sugars from water and carbon dioxide. This process is called photosynthesis.
What changes in autumn?
As autumn arrives in Canada, the days become shorter. Temperatures also begin to fall. For many deciduous trees, these changes are signals that winter is coming. The tree starts preparing to save energy and water.
The tree gradually stops making as much chlorophyll. As the green pigment breaks down, colours that were already present in the leaf become easier to see. Yellow and orange pigments, called carotenoids, were there during the summer too. The strong green colour simply hid them.
Where do red leaves come from?
Some trees also produce red and purple pigments called anthocyanins. These pigments can form when sugars remain trapped in the leaf during cool, bright autumn weather. That is one reason the same kind of tree may look more colourful in one year than another.
Not every tree changes colour in the same way. Maples are famous for bright reds and oranges. Birches often turn yellow. Oaks may become red, brown or a deep reddish colour. Weather, tree species and local conditions all affect the final display.
Why do the leaves fall?
Changing colour is only part of the tree's winter preparation. A special layer of cells develops where the leaf stem connects to the branch. Eventually the connection weakens, the leaf falls, and the tree seals the tiny opening. Losing leaves helps many trees reduce water loss during cold months when liquid water can be harder to obtain.
So an autumn forest is not simply “dying.” The colour change is part of a carefully timed seasonal process that helps trees survive winter and prepare for new growth in spring.