Why Do Leaves Change Colour In Autumn?
It’s that gorgeous time of year when the natural world treats us to a last burst of colour before the onset of winter. But why do leaves change colour and why is autumn colour better some years than others?
Leaf colour comes from pigments - natural substances produced by leaf cells to help them obtain food. The three pigments that colour leaves are chlorophyll (green), carotene (yellow and orange) and anthocyanin (red and pink).
During spring and summer chlorophyll floods the leaf, pulling in energy from sunlight and masking the other pigments with its greenness. As the days become shorter and cooler the production of chlorophyll slows down and eventually stops. Any chlorophyll remaining in the leaf breaks down and the green colour fades. The carotene pigments you can’t normally see are magically revealed, making the leaf look yellow or orange.
Meanwhile, a layer of corky cells forms across the base of the leaf stalk in preparation for shedding. This restricts the movement of sugars back to the main part of the tree. These sugars become trapped and concentrated in the leaf and are converted to anthocyanin, giving trees and shrubs such as maple, dogwood, sumac and acer their characteristic red, wine or purple colour.
A few trees, like oaks, barely colour up and go straight from green to brown. It all depends on the amount of chlorophyll residue and the concentration of carotene and anthocyanin in the individual species.
WHY IS AUTUMN COLOUR BETTER SOME YEARS THAN OTHERS?
It’s largely down to weather. Cold nights speed up the
destruction of chlorophyll and, provided the mercury doesn’t fall below freezing, they also stimulate anthocyanin production. Likewise, sunny days cause sugars to concentrate in the leaves and further stimulate anthocyanin.
The perfect recipe for stunning autumn colour is:
- A hot, dry summer
- Sunny, dry autumn days
- Cold, but not freezing, autumn nights
Cold, wet summers and cloudy, rainy autumns result in more muted colours. This year's heat and drought has given us a late bonus!