Why do leaves change colour in fall?

By photka/Shutterstock

Most cottagers know that it’s the fall’s dynamic duo of shorter days and cooler temperatures that cause our green summer forests to erupt into an amazing show of reds, oranges, yellows, browns, and even purples.

You might be wondering why trees go through this whole rigamarole every year and it turns out there are a few theories out there. One reasonably new idea was published in 2001 by researchers at Oxford University, Samuel Brown and the late William Hamilton. They put forward the theory that tree species that have the most dramatic change of colour (we see you, maple!) are the ones most bothered by insects that lay their eggs on trees in the fall. One such pest is the aphid, which is likely to be important in the tree-insect arms race because, Hamilton and Brown wrote, they are “choosy, damaging, and use colour cues in host selection.” Scientists are still trying to understand all the possible explanations for why the trees’ costume change may help fend off the little insects (Brown and Hamilton posited that when a tree pulls a biologically expensive stunt like this that might tell insects that it is healthy, and will be able to fight back, so to look for a weaker tree).

As for which factor — temperature or day length — is more important in causing the change, research done by now-retired Research Scientist Tom Noland at the Ontario Forest Research Institute suggests that the best formula for bringing on the leaf colours is cool nights (without a hard frost) and short days. However, when taken on their own, temperature change is actually a more important factor than the day length. Which makes sense: if it was predictable old day length alone that mattered, we would all know exactly when catch the fall spectacle every year and the annual speculation across cottage country would be over.


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