Why do leaves change colour before they fall?
Most leaves include different quantities of yellow, orange and red colours all year round, but we see them only when the dominant green hues disappear in the autumn.
NAT U R E Australia has very few native deciduous tree varieties, and perhaps only one – deciduous beech or Fagus ( Nothofagus gunnii) – which can match the magnificent autumnal colours displayed by the more common deciduous varieties elsewhere in the world. But what makes such leaves change from green to yellow, orange, red or brown?
It is, of course, chlorophyll in the leaves of trees that makes them appear green. Chlorophyll is important for plants’ energy production, which takes place via photosynthesis, where the plant produces sugars based on light, water, and CO2. The plant also produces oxygen as a waste product for the plant, though a vital resource for most of life on Earth, including humans.
Other pigments are present alongside the chlorophyll, and although some of these are also included in photosynthesis, they are overwhelmed by the greens during the spring and summer. Chlorophyll is continuously destroyed by sunlight, but as long as the leaves grow, the tree constantly produces more.
When autumn comes, the production of the green pigment stops as the days become shorter and temperatures fall. As the remaining chlorophyll is broken down, the other pigments become more visible – yellow and orange carotenes, and reddish or purple anthocyanins. These reflect wavelengths of light other than green, and so provide such leaves with the beautiful autumn colours we see.
Norwegian scientists have shown that autumn colours are linked with the quantity of natural insect toxin that plants produce, finding that the clearer the autumn colours displayed by individuals of the downy birch species, the fewer insect attacks the trees suffered the following spring.