Autumn is a time of outstanding beauty, when the natural world treats us to a last burst of colour before the onset of winter.

For just a few weeks, the green leaves of deciduous trees and shrubs change colour to many shades of russet, red, golden, yellow, purple, black, orange, pink and brown.

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).

As summer turns into autumn, the shorter days and cooler nights trigger three major changes in the leaf which have consequences for its colour.

  • The days become shorter and production of green chlorophyll slows down and eventually stops. Existing chlorophyll in the leaf breaks down and the green colour fades.

  • Yellow and orange carotenes that you can’t normally see in spring and summer leaves (because they are masked by the green of the chlorophyll) become visible, making the leaf look yellow or orange.

  • A layer of corky cells forms across the base of the leaf stalk, in preparation for leaf shedding. This restricts the movement of sugars back to the main part of the tree. Sugars become trapped and concentrated in the leaf and are eventually converted to anthocyanins giving the leaf a red, purple or pink colour.

 

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.

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. Also, a tree without leaves is in a state of dormancy and needs less energy to remain alive.

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