Why do leaves change colour in the autumn anyway?

One of my friends, an intelligent, employed, forty something, father of 2, quietly asked me this question, and it occurred to me that others may not know why…

Do we all take time to enjoy the colours of autumn leaves or just moan about the messy fallen leaves? The changing foliage never fails to enchant some of us, especially visitors to our shores. Artists are obliged to use the transitions of autumn and take to conception of creativity. Blake and Keats took to poetry, Vivaldi composed and lots of singers sang Johnny Mercer’s song “Autumn Leaves”. Try searching for the works of Levitan or the painting by Van Gogh entitled “Autumn Landscape with four trees”.

What is it that inspires?

Described by someone more profound than I , as “The beauty of the change of colour before the ultimate “death” and fall to the ground?” Perhaps… Who knows what?

But here we have the how and why an autumn leaf changes colour?

Why a maple leaf turns bright red? Or a laurel’s leaves go yellow? Why do some plants of the same species go different colours at different speeds? Where do these reds, yellows and oranges come from? Why do some trees shed leaves and others don’t? To answer those questions, we take ourselves back to our childhood, open eyes in wonder of it all, ready? Now, first have to understand what leaves are and what they do for plants and trees.

Leaves are nature’s food factories.

Plants take water from the ground through their roots. They take a gas, carbon dioxide from the air. (This is why it is generally good for the planet to plant trees and, broadly, bad to cut them down.) Plants use sunlight to turn water and carbon dioxide into oxygen and glucose. (This is the same sugar that humans get from energy drinks and such.)

Plants use glucose as food for energy and as a building block for growing. The way plants turn water and carbon dioxide into oxygen and sugar is called photosynthesis. This literally means “putting together with light.” A chemical called chlorophyll helps make photosynthesis happen.

Chlorophyll is what gives plants their green colour.

As summer ends and autumn comes, the days get shorter and shorter. During winter, there is not enough light or water for photosynthesis. Some trees stay green unless they are dying (evergreen) but other trees will rest, and live off the food they stored during the summer, a bit like hibernating. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves. As the bright green fades away, we begin to see yellow and orange colours. Small amounts of these colours have been in the leaves all along.

We just can’t see them in the summer, because they are covered up by the green chlorophyll. The bright reds and purples we see in leaves are made mostly in the autumn. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves turn this glucose into a red colour.

The brown colour of trees like oaks is made from wastes left, effectively rotting, in the leaves. Each individual leaf will have its own individual level of glucose or waste and so they will individually change at their own rate.

It is the combination of all these things that make the beautiful autumnal foliage colours we enjoy each year. In most cases the chlorophyll does not revert back and re-energise the leaf, so it becomes no use to the main plant and it falls to make way for new growth.

 

This is a simplified explanation which may shed light on that question you never got round to asking.

For more detail feel free to ask about specific cases and I will help where I can.