Why do leaves change colour and fall from trees?

Watching leaves change colour and fall from trees is a spectacular part of autumn. The rich oranges, vivid reds, and warm yellows transform our landscape and remind us of the changing seasons. If you’ve ever looked out of your window while drinking a warming drink and wondered why leaves change colour and trees lose their leaves, we’re here to answer your questions.

Why do leaves change colour?

Chlorophyll is the green pigment in leaves, used by plants and trees for photosynthesis (create glucose from sunlight, water, and nutrients from the soil).

As the nighttime temperatures drop and the days shorten, leaves receive less sunlight. This triggers the chlorophyll to break down without being replenished. Photosynthesis still happens on sunny autumn days using the remaining chlorophyll. This breakdown of chlorophyll reveals colourful pigments underneath, causing the trees to burst into gorgeous oranges, reds, and yellows.

These pigments are:

• Flavonoids, which create yellow leaves.
• Carotenoids (also found in carrots), which produces deep orange tones.
• Anthocyanins, which colour leaves in shades of red, purple and pink.

Individual leaves and trees change colour at different times. If the temperatures stay above freezing, and the weather is dry, bright and sunny through autumn, anthocyanin production increases and creates a stronger redder colour. In a mild wet autumn, this process is slower, and leaves will typically turn brown before they fall, as only the existing flavonoids and carotenoids will be present.

Why do trees lose their leaves?

Leaves are connected to their stem by a layer of cells called the abscission layer, which forms in spring when the leaves grow. Hormones produced by the tree called auxins keep the abscission layer intact, so the leaves stay attached to branches.

In autumn, hormones in trees begin to change. As days shorten and temperatures lower, auxin production decreases. This means the cells stretch within the abscission layer, creating fractures, which in turn break the layer, and leaves fall from the trees. A new protective layer of cells then grows over the exposed area once the leaves have fallen.

In the autumn and winter, losing leaves helps plants. As the tree slowly becomes dormant and the leaves get ready to fall, naturally produced abscisic acid seals the base of the leaves, stopping water reaching the leaf and trapping any chemicals left in the leaves. This helps trees retain water in their trunks and branches.

By dropping leaves, trees become dormant and so save energy to survive the shorter winter days. Having fewer or no leaves also means that they can weather winter storms, as strong winds can travel between the branches more easily without putting strain on the tree.

The fallen leaves still contain nutrients so don’t throw them away. They can provide a host of helpful benefits for your lawn and the local wildlife. As leaves fall to the ground they will rapidly break down and the nutrients returned to the soil, ready for use by the plants to grow in spring.