For many, the colour change of the leaves come September instigates a feeling of melancholy. Still, nobody denies that the reds, greens, oranges, and yellows are a spectacular sight! But what is behind this display that forebodes the long, bitter winter?
It has long been understood that trees have a year-long internal clock, which is believed to be powered by the environment surrounding it. The cycle consists of a process called photosynthesis, which is responsible for absorbing energy provided by the sunlight, and consuming the water and carbon dioxide from the surrounding environment, to yield sugar and oxygen. In an additional sequence, called cellular respiration, the sugar is then converted into adenosine triphosphate or ATP. This serves as an energy supply and is observed in all living cells, including our own. In short, these systems keep plants alive.
In order to tackle the issue of why leaves change from their familiar green colour to their associated autumn pigment, it seems logical to first question the origin of this green colouring. The answer lies in chloroplasts, which are structures found within the leaves, in what is known as the palisade layer. Inside one of the three membrane layers of the chloroplasts, the ‘thylakoid membrane’, is a network of chlorophyll molecules that absorb light. It is this chlorophyll that makes the leaves appear green by absorbing wavelengths of blue and red light and reflecting those that our eyes perceive as green. Each chlorophyll molecule has a short lifespan, so chlorophyll is continuously being formed using energy provided by the sun. However, as the hours of daylight decrease, the diminished amount of available light energy causes a decrease in the chlorophyll produced. As old chlorophyll molecules continue to decompose, there is a net loss of the pigment. At the same time, a layer of cells begins to form a barrier in the vein of the leaf that supplies the leaf with water from the branches. Without the essential water, the leaf is left with no choice but to stop making food, resulting in its eventual death, followed by the inevitable plummet to the ground and surrender to the merciless rake.
Three other pigments are found in different combinations in leaves, depending on the variety of tree – carotenoids, anthocyanins, and tannins. Carotenoids are present all the time but their colour is obscured by the prevailing chlorophyll in the spring and summer months. As chlorophyll disappears, the bright yellows and oranges of the carotenoids come to the fore. Anthocyanins, the result of sugar breakdown under the bright autumn sunlight, bring out the reds, blues, and purples of the spectrum after most of the chlorophyll has decomposed. Lastly, tannins reveal the brownish shade, characteristic of the very late stages of fall. The half-lives of the tannins are the longest of the three pigments, and this accounts for the bleak, final days of autumn.
We now have an explanation for how the beautiful shades of autumn arise, but why does this phenomenon occur? An interesting evolutionary theory claims that the colour change is a protective mechanism against the invasion of pesky insects, notably aphids, that infest trees to lay their eggs in autumn. This theory, which was first introduced as the ‘handicap principle,’ states that behaviour that endangers the survival of a species may actually turn out to be beneficial. When a tree allows the chlorophyll to decompose, it would seem to be playing with fire. Without photosynthesis, the tree’s nourishment is sacrificed. This is risky business! This surely would be considered a handicap.
While nothing has yet been proven, some theorists hypothesize that the most colourful (red and purple) displays are comparable to the tail of the flaunting peacock that attracts all the female peahens, in that they are used to exhibit the superiority of the tree. In other words, the more spectacular, the ‘more worthy’ the tree is to procreate. Studies have actually shown that aphids have a preference for yellow-green and an aversion to red. In the case of the tree, the brighter tree escapes the hassle of having to accommodate insect freeloaders – which consumes a lot of the tree’s energy. Weaker trees cannot afford to generate such a signal. Ridding themselves of chlorophyll, sacrificing their food supply, and using the remaining energy to produce the anthocyanin pigments are just too costly to a vulnerable tree and the result is that the healthier trees survive. When you think about it, this behaviour is advantageous to both parties: the healthier trees get a better chance to live and produce healthy offspring, whereas the insects benefit from a method that distinguishes and allows them to inhabit trees that are likely too fragile to fend them off.
So next time you look at an autumn tree, you may have a different attitude, now that you know the tree is actually aiming to communicate with the nature surrounding it.
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