The science behind why leaves change colour in the Autumn

Autumn is marked out by spectacular changes in leaf colour as the greens of summer change into the yellows and reds of autumn. In parts of North American whole tourist industries are based on this change, but why do leaves turn these bright colours before falling off the trees?

New work by Dr Dave Wilkinson (an ecologist in the School of Biological and Earth Sciences at Liverpool John Moores University) and his colleague Martin Schaefer (University of Freiburg, Germany), published in Trends in Ecology and Evolution, has added new twists to this autumnal story.

Most biology textbooks, if they mention autumn colour at all, are likely to say that it is the accidental by-product of the death of the leaves. For over one hundred years some biologists have wondered if there may be more to it than accident, but until recently the ‘accidental’ explanation has gone relatively unchallenged.

The autumn leaf story was reinvigorated by the late WD Hamilton (one of the greatest evolutionary theorists of the twentieth century) and two of his former students. They suggested that autumn colour was actually a signal of tree health designed to tell insect pests that they would be better off going elsewhere to attack a less healthy tree. Their idea was that only a healthy tree would have really bright autumn colours. Over the last five years several scientists (including Dr Wilkinson) have published research articles discussing the merits of this new idea.

In their new paper Drs Wilkinson and Schaefer review many recent studies on the chemistry of autumn leaves which strongly suggest that Hamilton’s imaginative idea is wrong.

There is now good evidence to suggest that these colours have evolved to help plants remove important chemicals from their leaves, for reuse next year. The autumn pigments do this by helping the plant continue to use the sun’s energy during the period at the end of the leaf’s life, so providing the energy needed to extract chemical nutrients before leaf fall.

Dr Wilkinson explained: “Contrary to what many people assume, photosynthesis does not stop once leaves change from green to red, and in the autumn, plants can be subjected to a potentially destructive combination of low temperatures and high light levels. The red and yellow pigments act like sunscreen, protecting the plants from the effects of chemicals produced by light acting on the contents of the dying leaf and may actually help plants photosynthesise better at lower temperatures.” Although Dr Wilkinson thinks Hamilton’s idea is wrong he points out that that doesn’t make it a failure.

He continued: “One of the important roles of new theories in science is to force people to think in new ways and to draw attention to overlooked phenomena in need of explanation. Like many biologists, before Hamilton’s theory it had never occurred to me to think hard about autumn leaf colour. The idea that these brief annual shows of colour may have good biochemical explanations, rather than being just an accident, makes them even more extraordinary to look at”.