Why do leaves turn red in the fall? The science is up for debate

The changing colour of leaves is an impressive annual spectacle, but it can make one wonder, why bother creating something so beautiful just before it withers away?

Well that wonder exists in the scientific community as well, says one McMaster University professor, who said there are two prevailing theories, with no clear cut answer, on why leaves turn from green to red.

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"It comes in waves," said biology professor Susan Dudley on the debate within the science community. "Especially when you take a look at this big picture mystery. Why would you make a new colour in a leaf that you're about to discard? That makes no sense."

Dudley's referring to leaves producing a chemical which changes the colour of leaves from green to red. She said that people often incorrectly assume the change is a loss of chemicals within the leaf. While that's true in yellow leaves, red leaves come from stopping the production of chlorophyll and starting the production of anthocyanin, a chemical also found in raspberries which gives them their red colour.

But the real debate within the science community, Dudley says, is what purpose the colour change serves.

There's two popular theories associated with why leaves turn red — both arguing the process is a form of self defence. One suggests that red leaves are a form of "sunscreen" for a tree, protecting it from sun damage. The other maintains that red leaves keep away insects that could harm the tree.

The sunscreen theory says that as leaves lose chlorophyll in autumn, trees begin to produce anthocyanin, turning the leaves red. The red helps them absorb sunlight that would otherwise damage leaves and trees. Sunlight becomes more damaging in the fall as leaves drop and sun exposure increases on surrounding trees. The trees turn red, in theory, to better absorb the light.

The other theory, referred to as the honest signal theory, says trees turn their leaves red to keep away aphids, which can cause substantial damage to trees. Stopping photosynthesis and producing anthocyanin can be costly to the tree, so only those with strong defences can afford to produce it. Aphids recognize that trees with red leaves have strong defences that could harm the bugs and thus avoid those trees.

Dudley said the scientific community is split on which answer is the correct reason, though the honest signal theory is "a lot harder to prove."

Aphids don't see colours the way humans do, she said, but they've shown that they dislike red leaves and tend to favour green and yellow leaves. That finding might favour the honest signal theory. And given the extensive variety of aphids and trees, further testing to get a definitive, overarching answer proves difficult.

It's also difficult to prove whether aphids are drawn to yellow and green because the leaves are "good for them," or whether they're just seeking out what they enjoy, "like humans with Doritos," she said.

And in some instances red tends to appear on the outside of trees rather than all the leaves. This would follow the logic of the sunscreen theory, where trees would only need protection from the sun.

Dudley, who's given lectures for years on the changing leaves, says she doesn't have a favourite theory.

"My knowledge of trees tells me it's probably both," she said, adding that the unresolved research question is a statement on science as a whole.

"It makes (people) think. We like to think of science as facts, that you learn facts, and, in fact, what we're teaching is how we ask questions, and that in science there are no facts."

And make sure to take in the beautiful fall weather while you can. Hurricane Patricia is going to make it a dreary one for the next little while. Click here to read more on that.