What’s the most striking thing about trees in autumn? It 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 red.
After reading recently published papers on the subject it became clear that, although it has been discussed for many years, it is still a source of debate. At least 10 hypotheses were reported in a paper published in 2009. Eventually I found that there are now two main evolutionary explanations – autumn colours could have evolved in plants to protect them against the physical damage induced by intense light at low temperatures (photoprotection hypothesis) or to avoid parasites by signalling the defensive commitment of the tree (coevolution hypothesis).
A leaf is the main photosynthetic organ of a tree. Photosynthesis 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. Instead of the green leaves just being discarded, the chlorophyll and proteins in the leaf are broken down and essential nutrients, such as nitrogen, re-absorbed and stored in the shoots and roots until spring. Plants generally re-absorb half their total leaf nitrogen.
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 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? This is where the 2 main hypotheses compete.
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.
Another idea is that 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.
I have no idea which theory is correct but when you go out this autumn look out for yellow leaves and red leaves, admire their beauty and think how complicated life can be!
Here are some trees to watch out for in Milton Keynes.
First, two species that go yellow in autumn
1: Norway Maple (photo Alan Birkett)
Photo 1 The Norway Maple Acer platanoides is native to Europe, from Scandinavia to the Caucasus. It was introduced to Britain in 1683 and is now commonly found in gardens, streets and parks. It is one of the first trees to look green in spring, when its green flowers open before the leaves. It has the 5-lobed leaf typical of the Maple family but differs from the Field Maple and Sycamore in that its lobes and teeth have finely pointed tips. It has a winged fruit like all maples but the wings hang down at an angle whereas on the Field Maple they are flat. This tree is at the south end of Furzton Lake in Milton Keynes
2: Aspen (photo Alan Birkett)
Photo 2 The Aspen Populus tremula is a Poplar that tolerates cold conditions. It is a smaller tree than most Poplars. It is a species that grows in cool regions across the whole of Europe and west Asia. (The American Aspen is a different species). It is more likely to be found in the north and west of Britain and is common in the Scottish Highlands. It is typically found in oak or birch woodland. It can spread by sending suckers up from its roots. Male and female flowers are on separate trees. Flowers are in the form of catkins. Aspens are quite common in Milton Keynes; these are on the east side of Furzton Lake.
Here are 2 trees with red leaves in autumn
3: Persian Ironwood (photo Alan Birkett)
Photo 3 The Persian Ironwood Parrotia persica is a small deciduous tree native to northern Iran. It was introduced to Britain in 1841. It is related to the Witch-Hazel. Its wood is extremely hard, hence the name ironwood. It has red flowers which appear before the leaves in late winter and the leaves turn bright red in autumn. There is a huge tree in the Cambridge Botanical Garden and a small tree on the east side of Furzton Lake in Milton Keynes.
4: Sweet Gum (photo Alan Birkett)
Photo 4 The Sweet Gum (Liquidambar styraciflua) is a deciduous tree native to south-eastern USA and the cloud-forest mountains of Mexico and Central America. It was introduced to Britain in 1681. It is an ornamental tree planted in many parks and gardens in warmer areas. It has red autumn foliage and unusual fruit, similar to that of the London Plane. In its native habitat the tree was grown commercially for its aromatic gum, originally known as ‘liquid amber’, hence its scientific name. This tree is in the Emerson Valley of Milton Keynes.
Other trees you may come across, that have red leaves in autumn, include the Sugar Maple, Red Maple and Red Oak from Eastern North America and the Japanese Maple variety ‘Osakazuki’ which is spectacular in autumn in the Cambridge Botanic Garden.
To find out more about trees and how to identify them go to my website: https://www.treeguideuk.co.uk/
and if you have any comments or observations about trees my e-mail is alan@treeguideuk.co.uk.
Alan Birkett
4 September 2020