Why do leaves change colour and fall off in autumn?

Autumn is a time of outstanding natural beauty. As the trees turn vibrant shades of yellow, orange and red, we're treated to a final burst of colour before the onset of winter.

Read on to discover why leaves change colour, why they fall off and why, in some years, the autumn colours are more spectacular than ever.

Why do leaves change colour in autumn?

Leaves change colour because deciduous trees prepare for winter by breaking down the green chlorophyll used in photosynthesis. This process gradually reveals other pigments contained within the leaves.

The pigments that colour leaves are produced by leaf cells to help them obtain food. They fall into three main groups: chlorophyll (green), carotenes (yellow) and anthocyanins (reds and pinks).

The concentration of these pigments occur at different levels in different tree species, which is why some species, such as beech, are known for brighter autumnal displays than others.

Why is autumn colour better in some years than others?

The depth of colour seen in autumn leaves is influenced by the blend of chemical processes and weather conditions. Therefore, as the weather conditions in autumn vary from year to year, so too do the intensity of the leaf colours.

• 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 during dry conditions. More anthocyanin is produced and consequently leaves are redder.

• 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 turn 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. 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.