Walking around campus these days, we can admire a beautiful autumnal landscape. As we walk through the main gate and into the Parliament Square, we find two Erman’s birch (Betula ermanii), with light yellow hues. Moving towards the front of the Graduates Memorial Building, we find one of the university’s iconic trees, the oriental plane (Platanus orientalis). With a height of 20 metres, its leaves are red, amber and yellow at this time of the year. Walking along the college green we can then find a wide range of colours, highlighting the red tones of the Norway maple (Acer platanoides).
“These beautiful colour replacements are due to complex chemical changes in their leaves as a result of the temperature and weather conditions of the autumn season.”
Our trees don’t show the bright green colours of summer anymore and have changed to warmer tones of oranges, yellows and browns. How does this change occur? These beautiful colour replacements are due to complex chemical changes in their leaves as a result of the temperature and weather conditions of the autumn season. The trees that show these stunning changes in their leaves are known as deciduous trees. These groups of trees are known for losing their leaves for part of the year in order to save energy and nutrients in the cold seasons.
“This gradual disappearance allows the colours of autumn to be revealed to us.
”
There are four types of pigment found to be involved in the colouring of leaves: chlorophylls, carotenoids, anthocyanins and tannins. During spring and summer, chlorophyll gives a bright green colour to the leaves. Chlorophyll is a molecule essential to the process of photosynthesis for its role in trapping the light energy required to create glucose. Other pigments are also present in the leaves, but can only be seen when chlorophyll levels are low. As temperatures drop, sunlight availability diminishes and the days become shorter, nutrient transport to the leaves is blocked. Therefore, the supply and production of chlorophyll slows down to an eventual stop. This gradual disappearance allows the colours of autumn to be revealed to us.
Carotenoids are the yellow and orange group of pigments. They serve as accessory pigments in photosynthesis by passing solar energy to chlorophyll, and function as a photo protector for plants by preventing damage from sunlight. As well as chlorophyll, carotenoids production also slows down during autumn, but at a slower rate, allowing us to see their shades before the leaves fall off. The two types of carotenoids responsible for the different hues are Xanthophylls,responsible for the yellow, and Beta carotenes for the orange. These shades can be seen in practically all trees around campus, such as the wych elm (Ulmus glabra) in college park or the Mongolian lime (Tilia mongolica) in front of house 22.
The third pigment responsible for autumn colours are the anthocyanins. These pigments are responsible for the red and purple tones. Anthocyanins are not present in all trees, as specific conditions are required for their synthesis. As nutrient transport slows down, a high concentration of sugar is trapped in the leaf, leading to anthocyanin production. These tones are more difficult to see, but we can appreciate them in red oaks and some maples around college park.
Before finally falling off the tree, leaves show a brown colour. Tannins are the pigments in charge of this tone and are a waste product of tree metabolism. They are always present, but are only visible after chlorophyll and carotenoids have disappeared and the leaf is about to die.
“The order of appearance of these pigments is not always the same, and it is determined by the weather.”
The order of appearance of these pigments is not always the same, and it is determined by the weather. For example, freezing temperatures can induce the mechanism to produce anthocyanins and can cause the death of the leaf even before chlorophyll and carotenoids are gone. Strong winds can cause leaves to fall off earlier and the lack of sunny autumn days can lead to less intense colouring.
Thanks to these chemical processes, our campus is annually hued with all these special colours and we can enjoy the incredible autumn view that melts onto it. So next time you walk by the trees, you know exactly what’s happening up there and how something as complex as chemistry can give us something as beautiful as the colour palette of an autumn landscape.
