Autumn is a wonderful time of year where nature really impresses with the gamut of colour that appears in the trees and hedgerows around us. We can happily enjoy a shuffle through the golden Autumn leaves without ever considering where these colours come from, but did you ever wonder – how do Autumn leaves change colour?
The Science Bit
As many people know, the lovely green of most leaves is caused by the pigment chlorophyll… green in colour (obviously) and capable of using sunlight to convert water and carbon dioxide into energy (sugar) for the plant.
However, when the sunlight hours fade coming into winter, so too does the chlorophyll in the leaves of trees, or, to be more precise, the pigment begins to degrade and is not replaced. Once the green colour is gone other pigments that are often present in the leaf come into view:
- carotenoids are pigments responsible for the yellow/orange colour of leaves.
- anthocyanins are responsible for the redder colour of leaves.
- tannins are responsible for the brown colour.
There is, within this pigmented system, a sense of hierarchy, at least in part.
The Colour Change
Once chlorophyll is reduced, what happens to the colour of the leaves?
- Carotenoids are the pigments responsible for the orange colour of carrots. If carotenoids are present in leaves their colour tends to dominate leaving the leaves yellowy and orange.
- In the absence of carotenoid, anthocyanin is the dominant pigment. Anthocyanin (the same pigment found in red onions, red grapes, red apples and red cabbage) is a natural pH indicator, meaning that it can change colour depending on the levels of acids or bases/alkali in its environment. At the beginning of Autumn the levels of sugar in the leaves tends to be quite high, increasing the acid levels in the leaves, this strengthens the red colour of Anthocyanin if it is present.
- At the end of Autumn the leaves die off and the levels of carotenoids and anthocynins die off too, leaving another pigment to dominate… and this is the brown pigment of tannin, the same pigment that gives a cup of tea its colour!
7 Factors that Influence Leaf Colour
So why do some places have really spectacular autumn displays while others fall a bit short?
1. The type and age of the trees.There are plenty of trees that really add some vibrant colour displays from yellow to red to purple. Acers (maple trees) are one very good example.
2. The type of soil the trees grow in influence the general health of the tree, its leaves and the final colour display. Nitrogen level and the overall acidity of the soil are two important factors.
3. The summer weather will also influence the colours seen in the autumn, too little or too much sun earlier in the year will reduce the display when the leaves change colour. Droughts and floods can also have a negative impact.
4. Sugar levels in the trees at the turn of the season can be a big factor in the colour of the leaves.
5. The levels of air and soil pollution will affect the health of the tree and the uniform colouration of the leaves. Some species of trees are more resistant to pollutants than others.
6. The weather conditions in Autumn are also obviously very important. The best colour displays are favoured by dry sunny autumn days and cool, crisp autumn nights.
7. How long the leaves stay on the trees… if the stormy winds arrive too early in the season the leaves may fall before we get to enjoy their full spectrum of beauty.
So next time you are crunching through those leaves you may wonder why you are suddenly thinking of carrots and cabbages and cups of tea!!!