Why do leaves change colour and fall off in autumn?
Autumn is often a time of outstanding beauty. We're treated to a final burst of colour before the onset of winter. And some years, that colour is more spectacular than ever. Find out why.
Credit: David Rodway / WTML
Why do leaves change colour?
Leaf colour comes from pigments. These are natural substances produced by leaf cells to help them obtain food. The three pigments that colour leaves are chlorophyll (green), carotenes (yellow) and anthocyanins (reds and pinks).
Credit: Laurie Campbell / WTML
Why is autumn colour better some years?
The depth of colour is influenced by the blend of chemical processes and weather conditions.
- Cold nights: low temperatures destroy chlorophyll so the green leaf fades to yellow, but if temperatures stay above freezing, anthocyanin production is enhanced and the leaves take on a red colour.
- Dry weather: sugars become concentrated in the leaves, more anthocyanin is produced and consequently leaves are redder.
- Bright sunny days: although the production of new chlorophyll stops in autumn, photosynthesis can still occur on sunny autumn days, using the remaining chlorophyll. Sugar concentration increases, more anthocyanin is produced and the leaves are redder.
Why do trees lose their leaves?
The beginnings of leaf drop, also known as abscission, start when a layer of cells is formed between where the leaf stalk joins the stem. This layer, known as the abscission layer, is formed in the spring during active new growth of the leaf.
In autumn, hormones within trees begin to change. The most notable is auxin. During the active growing season, production rates of auxin in the leaves are consistent with the rest of the tree. As long as these rates are steady, the cells of the abscission layer remain connected, which in turn, keeps leaves attached.
But as days shorten and temperatures cool, auxin production in leaves starts to decrease. This triggers cellular elongation within the abscission layer. The elongation of these cells creates fractures, allowing the leaf to break away from the plant. The leaf is finally blown off by the wind or falls from its own weight.
Credit: Michael Cooper / WTML
What are the benefits of leaf fall?
The benefit of shedding leaves is that trees can preserve the moisture in their branches and trunk, instead of drying out and dying. And a tree without leaves is in a state of dormancy and needs less energy to remain alive.
Leafless trees are also better able to tolerate winter storms as strong winds can move through the branches more easily.
Watch our film on why leaves change colour
Nature's Calendar is our citizen science project that uses seasonal observations sent in by members of the public from their gardens or local area. These records help scientists understand the effects of weather and climate change on wildlife.
Learn more about trees and woods in autumn
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