Autumn colour - part 1

 Autumn beech woodland © Don Sutherland

The warm tones of oranges, golds and reds are synonymous with autumn. As well as enjoying the spectacle, questioning why leaves change and fall is just as fascinating.

For as long as our own species has lived in the northern temperate parts of Europe, America and Asia, I am sure that people have been captivated and fascinated by the beauty of autumn colours.

Why do leaves change colour as the season shifts from summer to winter, what’s happening and how do they do it?

Science has, to a large extent, answered the questions of ‘how’ leaves change colour and we understand much of the plant chemistry behind what’s happening.

We know, for example, that a leaf’s colour depends on the presence and relative balance of four main groups of plant pigments: chlorophylls are the primary pigments (the greens); and the secondary pigments, which include carotenoids (the oranges), xanthophylls (the yellows) and anthocyanins (the reds and purples).

The background dull brown colours may be tannins, which can have different functions, and which can be found in many plant cell walls and wood.

Each of these pigment groups has a function in the leaf and many are present throughout the year.

The principle ‘green’ leaf colour is derived from the high concentration of chlorophylls in the spring and summer.

Chlorophylls are a key part of the complex system by which plants capture the energy in sunlight.

Plants use light energy to convert carbon dioxide from the air into sugars – the chemical energy ‘store’ which fuels plant growth and metabolism – and indeed virtually all life on Earth.

As autumn draws in, the sensitive leaf tissues of deciduous plants and trees become vulnerable to frost and poor weather – and, so to avoid losing valuable resources before the leaves are killed, the leaves undergo ‘senescence’.

This is the controlled process in which the plant extracts valuable resources (especially the chlorophyll) from the leaf before it is discarded for storage back in the main plant or in specialised storage organs during the winter.

As chlorophyll is broken down and removed, its green colour disappears to reveal the underlying secondary pigments of yellow, brown and reds.

So effectively during the summer, the green colour of chlorophyll masks the presence of the other colours.

In many cases, we can probably conclude that autumn colours are merely an incidental by-product of a tree’s careful recycling of precious materials and its preparations for winter. There may be no need for a higher explanation to answer the question ‘why’.

In our relatively mild, damp autumns and winters of North West Europe, we observe that the unmasked yellow and orange pigments tend to predominate amongst native species of trees and shrubs – only a few species regularly turn red.

But as always in the natural world, the situation isn’t as simple or complete as this suggests...

Read part 2 of this Autumn colours blog.

Woman playing in autumn leaves in cozy jumper - copyright Tom Marshall

© Tom Marshall

Visit one of these woodland nature reserves to experience the wildlife spectacle of Autumn leaves