A Puzzle for the Autumnal Equinox

Every now and then you come across a scientific hypothesis that is so elegant and powerful in its ability to explain that it just feels right. Yet that doesn’t automatically make it right. Even when an elegant hypothesis gets support from experiments, it’s not time to declare victory. This is especially true in biology, where causes and effects are all gloriously tangled up with one another. It can take a long time to undo the tangle, and hacking away at it, Gordian-style, won’t help get to the answer any faster.

I was reminded of this while reading Andrew Brown’s review of A Reason For Everything by Marek Kohn in the Guardian. The book sounds fascinating. Kohn recounts how a small group of English biologists shaped the course of modern evolutionary biology–in particular, by pondering how adaptation through natural selection could account for just about everything in nature. One of the foremost of these thinkers was William Hamilton, who died a few years ago. Brown writes that "even the colours of the leaves on autumn trees around the grave of Bill Hamilton have been given a meaning by evolution – they are so vivid in order to warn parasites that the tree is healthy enough to repel them."

I wrote about Hamilton’s leaf-signal hypothesis here. It is one of those beautifully elegant hypotheses, and some studies have even supported Hamilton’s idea that the brilliant colors of autumn evolved as a way for trees to tell insects to buzz off. But readers should not have finished reading my post by thinking, "Well, that sews that question up."

Here’s why. H. Martin Schaefer and David M. Wilkinson have written a review of the Hamilton hypothesis which has just gone into press in Trends in Ecology and Evolution. They offer a lot of evidence suggesting that Hamilton may have been wrong–or at least may not have captured the whole picture. They show how a completely different process may be responsible for fall colors. Trees may produce them as they prepare for winter.

When leaves die, their nitrogen, phosphorus, and other nutrients get shipped back into their tree. It’s a crucial, carefully orchestrated stage in a tree’s life; it will survive on these reserves through the winter. In order to pump the nutrients back into the branches, the leaves need a lot of energy, which they have to generate with photosynthesis. That’s where the pigments may come in. Pigments act as a sunscreen for leaves, shielding them from harmful UV rays that can shut down their photosynthetic machinery . What’s more, as the leaves ship their nutrients back to the tree, they may produce harmful free radicals as a byproduct. It just so happens that pigments are veritable magnets for free radicals.

If the authors are right, then the evidence that seems to support Hamilton’s hypothesis might not actually support it at all. For example, researchers have found that birch trees that display brighter leaves grow more vigorously the following year. You could argue that these trees did so well because they could create such strong warning signals, which warded off insects. But perhaps those bright leaves are just a sign that these trees were doing a particularly good job of protecting their leaves as they stored nutrients for the winter–nutrients that made them more vigorous the follwing spring.

Fortunately, evolutionary biologists can do more than just come up with beautiful hypotheses. They can test them. Schaefer and Wilkinson lay out a list of experiments that could discriminate between the leaf-signal hypothesis and the winter-storage hypothesis. It’s even possible that evolution has produced fall foliage in order to both ward off insects and ship nutrients out of the leaf. As beautiful as any one hypothesis may be, it’s the interplay of different ideas and the experiments that put them to the test that’s most beautiful of all. It would not bother Hamilton one bit, I suspect, if it turned out that the leaves that fell on his grave had taken on their autumn colors for an entirely different purpose.

0 thoughts on “A Puzzle for the Autumnal Equinox

  1. I never liked the leaf-signal hypothesis (elegant though it undoubtedly is). It seemed to me that the fact that leaves change colour just before they fall implies that the colour-change is more likely to be a by-product of the leaf-dropping process. Not all we Englishmen are strict adaptationists!

    (Thanks for the link in your sidebar, by the way.)

  2. This doesn’t explain why countries such as New Zealand are green all year round, with no naturally occurring deciduous plants – despite a healthy supply of insects …

  3. Why should a deciduous tree in autumn concern itself about leaf-eating parasites? Those leaves are just about to fall off, anyway.

  4. Always enjoyable posts, Carl. Keep up the good work.

    My horticulture education at a land-grant Uni taught us that the yellow/red/orange pigments are always there.

    The absence of green in the leaf is the translocation of nutrients back into the plant for storage. The grad and PhD students that were studying this issue at our Uni found that there wasn’t necessarily any additional tannins or other bad-tasting goop in the leaf.



  5. Latest hypothesis on why NZ trees are evergreens- soil quality- NZ soil is very young geologically and quite nutrient poor- with mild winters for the most part (its a sub tropical rain forest mostly so cold rain rather than snow to deal with)

    I never bought the parasites hypothesis my self- seemed too much effort for gain


  6. Nice catch. Hamilton was certainly wrong about Aids. But his argument about leaf colours wasn’t that they deterred possible leaf-eating parasites. He thought they deterred the parasites which might eat their way into the trunk (unless I have completely misremembered).

    If the pigments are always present, and only show up when the nutrients are brought back into the trunk, this doesn’t affect Hamilton’s claim that there might be selection for high levels of pigment.

  7. Leaves fall off evergreen trees, too. And it’s a programmed process (I think). They just don’t all fall off at the same time.

    The parasite-warning hypothesis seems to be inapplicable to evergreen trees. (If a parasite sees a lot of yellow or red leaves on an evergreen tree, it will probably think “That tree’s losing all its leaves. It’s sick.”) But the nutrient reclamation hypothesis shoud apply.

    Do leaves on evergreen trees turn bright colours before they fall off? Gee, I’m not sure. I think they turn yellow. I think that on southern beeches (Nothofagus) in New Zealand they turn red.

  8. Research has shown that the biochemical process of chlorophyll breakdown is directly tied to senesence (in Arabidopsis, at least)–pheide a oxygenase, a component of chorophyll breakdown is only active during that time. To me, this suggests that as Richard said above, the color change is a by-product of the leaf-dropping process: as chorophyll is degraded as part of leaf senesence, the pigments that were always there then become visible.

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