Autumn Leaf Colours
The most striking thing about deciduous trees in autumn is not that they shed their leaves, it’s the fact that the leaves on some trees change their colour before they fall. Why does this happen – the leaves have been green all spring and summer? Why don’t the green leaves just drop off without changing colour and why on some trees do the leaves turn yellow and on others red. In fact most trees in Britain, for example Common Alder and Common Ash, shed green or brown leaves but about a third of all species turn either yellow or red.
In Britain our landscape consists of trees that originated in Europe, America, Asia and Australia. Most trees of European origin turn yellow with hardly any turning red. European tree species that turn yellow include the Norway Maple, Field Maple, Aspen, Silver Birch, Common Beech (yellow then brown), Common Hornbeam, Willows, Poplars and Limes. The Ginkgo introduced from China, has yellow autumn leaves. In America there are some native trees such as the Quaking Aspen, Tulip Tree, some Oaks, some Maples and some Hickories that are yellow in autumn but many more that turn red. Here are 8 examples of trees that turn yellow.
Not all conifers are evergreens. Some are deciduous and shed their leaves in autumn. Examples are the Swamp Cypress and the Dawn Redwood. For information on these trees click HERE
Nearly all the trees that turn red in autumn in Britain were introduced from North America or Asia. Examples are the Red Maple, Sugar Maple, Red Oak, Scarlet Oak, Pin Oak and Sweet Gum from North America and Japanese Maple varieties such as ‘Ozakasuki’, Sargent’s Cherry, Persian Ironwood, Snowy Mespil and Pillar Apple from Asia. A few European trees, such as the Wild Service Tree and the Bird Cherry turn orange/red and some, such as the Norway Maple and the Common Rowan, may turn orange/red under stressed conditions. Some varieties of European trees with red autumn leaves include the Sycamore ‘brilliantissimum’ and the Claret Ash ‘Raywood’. Here are 8 examples of trees that turn red in autumn.
Two main theories
A leaf is the main photosynthetic organ of a tree. This is a process in which carbon dioxide from the air is combined with water in the presence of light to produce sugars and oxygen. The molecule that carries this out is called chlorophyll. It absorbs red and blue wavelengths of light and reflects green so that the leaf appears green to us. It is a complex molecule with a ring of nitrogen at its centre surrounding an atom of magnesium. Shorter days and lower temperatures trigger leaf fall but this is a multi-step controlled shutdown process. Some trees just discard green leaves but in other trees the chlorophyll and proteins in the leaf are broken down and essential nutrients, such as nitrogen, are re-adsorbed and stored in the shoots and roots until spring.
As the chlorophyll breaks down, the leaf loses its green colour and other pigments can be seen. Carotenoids are yellow and orange and are already present. Anthocyanins, which give the leaf a red colour, are newly made. Carotenoids are needed to keep the cells going during the re-absorption stage so most trees that change colour have yellow leaves in autumn but 14% have red leaves. Why, then do some trees go to the expense of making Anthocyanins before the leaves fall? There are two main theories.
Anthocyanins protect the leaf from light damage during the period of re-absorption. This is the basis for the photoprotection hypothesis – it extends the leaf life during shut-down and enables it to send more nutrients back to the tree before the leaf drops. If this is true, trees with yellow leaves should drop their leaves earlier.
Alternatively the red coloration may be a signal to parasites, such as aphids, that have a strong preference for green leaves, to not lay their eggs on red leaves in autumn. This avoids future damage and is the basis for the co-evolution hypothesis. Red colour may be correlated with the level of herbivore defence in the tree, and therefore plants investing more in defences show more autumn colours. If insects adapt to avoid red leaves in autumn, this will lead to a co-evolutionary process in which both preference for green in aphids and intensity (or duration) of red in trees increase.