Why Do Leaves Change Colour in Autumn?

Walk under a maple on a bright October afternoon and the whole tree seems to be lit from inside. A few weeks ago it was plain green. Now it is yellow, orange and red all at once, and the pavement underneath looks as if somebody has tipped over a paintbox. The tree did not fetch new paint from anywhere. Everything you are looking at was either in the leaf all along or made on the spot.

The short answer

A leaf is green because it is packed with chlorophyll, the pigment that catches sunlight so the tree can make sugar. Chlorophyll does not last. Sunlight itself wears it out, so all through spring and summer the leaf has to keep making more. Tucked in beside it the whole time are carotenoids, the yellow and orange pigments that also colour carrots. You cannot see them in July simply because there is so much more green.

As the days grow shorter and cooler, the tree starts closing its leaves down. A guide written by a professor of tree physiology at Purdue University explains that it slowly pulls nitrogen and phosphorus back out of the leaves, to store in its twigs and branches over winter. Without them no new chlorophyll is made, the old chlorophyll breaks down, and the green fades. The yellow that sat underneath all summer is finally on show. That is the gold of birches, hickories and ashes.

Where the red comes from

Red is a different story, and the more surprising one. The red and purple pigments, called anthocyanins, are usually not in the leaf during summer at all. Harvard Forest, the research forest of Harvard University, describes them as actively made just before the leaves fall, and reports that about 70 percent of the shrubs and trees growing there produce them.

Here is how it happens. At the base of each leaf stalk a thin layer of cells forms, marking the spot where the leaf will eventually let go. According to the Purdue guide, this layer cuts the route that carries sugar out of the leaf, while water can still get in for a while. The leaf goes on making sugar in the sunshine, the sugar is trapped, and some of it is turned into anthocyanin. That is why a single tree can carry red leaves on its sunny branches and yellow ones in the shade. Trees without the right genes never go red: they only manage yellow and brown.

What people often get wrong

The old story says frost does the painting, pinching the leaves red overnight. It is closer to the other way round. The Purdue guide calls the belief that a hard frost is needed for good autumn colour simply untrue: freezing temperatures kill or badly damage the leaves before the pigments have finished developing, and the display comes out duller. The second mistake is thinking that all autumn colour is just green draining away to uncover what was beneath. That is true of the yellow. It is not true of the red, which is new.

Why some autumns are better than others

The yellows and browns turn up every year, more or less the same. The reds are the part that changes, because they depend on the weather in those last weeks. The Purdue guide and the State University of New York's College of Environmental Science and Forestry agree on what helps: cool nights that stay above freezing. Purdue adds warm, sunny days, since more sunshine means more sugar, and more sugar means more red.

As for brown oak leaves, that colour comes from tannins, the same bitter substances that colour tea. Like the yellows, they were in the leaf the whole time, and they show once the other pigments have gone. So an autumn tree is showing you three things at once: what it was hiding, what it has just made, and what is left over. Not bad for something that is on its way to the ground.