Autumn leaves | goesmagazine

Autumn leaves:
not as trivial as THEY sound?

By Juniper Kiss

When the leaves are starting to turn brown and trees shed their leaves, everyone knows that autumn has come, school and university is starting. However, this well-known phenomenon has been fascinating scientists for a long time as it is not clear what is the true purpose of these colours.

In Sir David Attenborough’s BBC series, The private life of plants, he talked about that trees that "withdraw the valuable chlorophyll from their leaves. As the green pigment drains away, waste products that have accumulated over the year are revealed and the leaves change colour. In New England, day after day, whole hillsides of maples and aspens flush yellow, orange and red".

This short description simplifies an extremely complex subject.

Where do the colours come from in the first place?

The healthy looking green leaves’ colour comes from the well-known green chlorophyll pigments in chloroplasts, which plays the main role of photosynthesis. These mask the presence of carotenoids and flavonoids which are also present in the leaves. Xanthophylls, a subclass of carotenoids, are responsible for the yellow colour of the leaves. So when the chlorophyll production decreases in the autumn and the pigments start to degrade, the carotenoids become prevalent, hence the yellowness of the leaves.

Abstract of Hamilton and Brown’s (2001) paper published in the Proceedings of the Royal Society of London:

“Many species of deciduous trees display striking colour changes in autumn. Here, we present a functional hypothesis: bright autumn coloration serves as an honest signal of defensive commitment against autumn colonizing insect pests. According to this hypothesis, individuals within a signalling species show variation in the expression of autumn coloration, with defensively committed trees producing a more intense display. Insects are expected to be averse to the brightest tree individuals and, hence, preferentially colonize the least defensive hosts. We predicted that tree species suffering greater insect damage would, on average, invest more in autumn–colour signalling than less troubled species. Here, we show that autumn coloration is stronger in species facing a high diversity of damaging specialist aphids. Aphids are likely to be an important group of signal receivers because they are choosy, damaging and use colour cues in host selection. In the light of further aspects of insect and tree biology, these results support the notion that bright autumn colours are expensive handicap signals revealing the defensive commitment of individual trees to autumn colonizing insect pests.”

The brown, red and yellow colours are signs of a new term beginning but what is the real function of the intense colours of tree leaves?

Beta-carotene, one of the most common type of carotenoid, absorbs the green and blue lights strongly, reflects the red and yellow, causing the orange colouration. Although carotenoids also start degrading along with the chlorophyll, but at a much slower rate. The red and purple colours are due to anthocyanins, which are vacuolar flavonoids, and are only produced in the autumn. The intensity of red and purple colours varies at different chlorophyll concentrations, pH, presence of metal ions etc. Kevin Gould from University of Auckland titled his article eloquently ‘Nature’s Swiss Army Knife: The Diverse Protective Roles of Anthocyanins in Leaves’ in 2004 – describing the versatility of anthocyanins.

The question remains – why did evolution ‘come up with’ this mechanism and what are the trees adapting to?

The two broad explanations are:

(1) protection/reaction to environmental factors or

(2) induced by animal-plant interactions.

Hamilton’s theory

Many scientific papers are questioning this ‘trivia’ and propose that the red and yellow colours could be a signal to insects about the tree’s ability to defend itself against herbivores.

So if a tree is more brightly coloured, they are better prepared against herbivores.

This is the core idea of Hamilton’s theory (Hamilton and Brown, 2001). The researchers found that there were lots of specialised aphid species on different intensity of particularly yellow leaves. However, a paper by Wilkinson et al. (2002) has thoroughly examined and questioned this theory and by reviewing their main points, we will see that a well-known phenomenon can be extremely complex and debated among scientists.

Problem number 1.

The timing of aphid colonisation does not seem to be perfectly timed to the leaf colour change – it would make sense if the tree increases the brightness of the pigments before the aphids colonise them but in general, aphids colonise trees way before the autumn, around spring time and summer. One group of aphids actually has to lay eggs on tree leaves before the leaf fall so the eggs can safely overwinter. So the authors of this paper ask ‘Why has there not been selection for a better-timed signal?’

Problem number 2

The leaves’ anthocyanin levels greatly differ on a single tree and even on a single branch (Chang et al. 1989). These differences on a tree would confuse aphids about the overall defence capability of the tree itself which would serve no purpose in an evolutionary sense. So the second question the authors ask is ‘Why should the signal from a single individual be so variable if it is primarily designed to convey information about a plant's defensive capability?’

Problem number 3

If you have bought any sticky aphid traps, they are likely to be yellow because researchers have observed that aphids prefer yellow leaves to green ones (Kennedy et al. 1961). But if this is a well-documented behaviour, how come, that according to Hamilton’s theory, aphids ought to be avoiding the bright yellow leaves in the autumn?

Problem number 4.

Another fact is the environment's (e.g. light, resource availability, and weather) effect on anthocyanin, which are synthesized only in the autumn, and their production in plants (Mol et al. 1996). General signs of plant deficiencies can be diagnosed by leaf colouration, for example, yellowing at the leaf tips and edges and yellow or dead patches in younger leaves could be a sign of low potassium. If the lower leaves turn yellow from outside going in while the veins remain green, it could be a sign of low magnesium levels. So colouration can be a ‘sign of weakness’ – which would be an attraction for insects as it will be easier to digest. But according to Hamilton and Brown's (2001) hypothesis – the trees that have the brightest colours will deter insect attacks…. Which is contradictory to the fact that colouration is a sign of weakness!

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the answer is....

After highlighting the problems with Hamilton’s theory and the aphid numbers on autumn trees, the authors of Wilkinson et al.’s (2002) paper and Archetti (2008) offer the following alternative hypotheses.

The autumn leaf colours could be:

Non-adaptive: just by-product of leaf aging (senescence) as Sir David Attenborough simplified it.
Adaptive to abiotic factors: it could act as a sun-screen to protect against harmful effects of light.
Helping with the relocation/reabsorption of important nutrients, such as nitrogen before the leaf fall.
Attracting birds for dispersal by trees whose fruits ripe in the autumn.
Lowering surface reflectivity (leaf albedo) by turning deep red, absorbing more sunlight and therefore warming up the leaves.

Many theories, yet no absolute conclusion.

This subject is a great example how many questions are ‘out there’ for scientists to answer about phenomenona that are thought to be trivial. As Archetti (2008) writes:

It is curious how autumn colours, given that they are such an astonishing and well-known phenomenon, have continuously escaped the attention of evolutionary biologists. […] For the moment, we have many leads to follow, and an expectation of unexplored and fascinating mysteries to unravel.’