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 abcission, start when a layer of cells is formed between where the leaf stalk joins the stem. This layer, known as the abcission 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.