North of the equator, in temperate regions, autumn is paving the way for winter. Deciduous trees, such as oak, ash and maple, decorate our towns and cities with every shade of yellow, orange and red. The life-cycle of leaves is so integral to our image of autumn, but have you ever thought about how these colours are formed?

In this seasonal post, we’ll go beyond the aesthetics and look at some of the biological and chemical mechanisms behind the golden shades of autumn.

Autumn, a second spring

Autumnal change has always attracted the attention and reflection of artists and writers. Captivated by this natural spectacle, Albert Camus is quoted as saying:

“L’automne est un deuxième ressort où chaque feuille est une fleur.”

(Autumn is a second spring when every leaf is a flower.)

Maybe you’re reading this on your phone, walking down the street or on a bus. Look around you. Have the leaves begun to change colour? Have they fallen to the ground?

Here’s what autumn looks like in Central Park, New York.


Green beginnings

As you probably know, the green shades we see during the summer months are due to the presence of chlorophyll in leaves. Chlorophyll is a green photosynthetic pigment, which help plants produce sugars, such as glucose.

In fact, the term chlorophyll – chloros (‘green’) and phyllon (‘leaf’) – is quite self-explanatory, assuming you speak Greek, that is!

Why is chlorophyll green? Well, it absorbs blue and red portions of the electromagnetic spectrum, meaning that green light is reflected.

Before continuing with deciduous trees, those that lose their leaves, we should quickly mention evergreens. This term is used in reference to species of tree that don’t lose their leaves or needles and, therefore stay ‘green’ all year round.

Though funnily enough, not all evergreens are strictly green. Maybe I’m being over-scrupulous, but the Colorado blue spruce is definitely blue!

The icy tones of the Colorado blue spruce. By Iryna Imago |
The icy tones of the Colorado blue spruce. By Iryna Imago |

Yellows, oranges and reds

Autumn is characterised by fewer hours of daylight and cooler temperatures. Deciduous trees sense these changes and prepare for the approaching winter. With insufficient energy for photosynthesis to take place, leaves essentially lose their value.

What happens next? In order to conserve resources, the tree begins to shut down the vein system, which usually provides each leaf with the water and nutrients it needs. As a result, the chlorophyll starts to break down, and the green pigment fades away.

The yellow and orange shades that we see are formed by two other pigments, carotenoids and flavonoids. Interestingly, the carotenoid pigments in leaves are the same as those found in carrots, sweet potatoes, papaya, mangos, tomatoes and oranges!

As this great graphic from Compound Interest explains, bright red colours, such as those seen in maple leaves, are caused by another set of pigments: anthocyanins.

Chemistry of Autumn leaves
Chemistry of Autumn leaves

Wildlife amongst the leaves

The days may be drawing in, but a walk in the autumn woodlands is still well worth your while. As you enjoy the rich mosaic of foliage, keep your eyes peeled for wildlife.

With the abundance of berries and nuts, autumn is a good time for you to see squirrels, as well as birds like as jays and thrushes.

If you’re interested in seeing larger mammals, September to November coincides with the rutting season of most European and North American deer species. The male of the red deer, found across Europe and parts of Asia, makes a particularly impressive roaring sound during this period.

Red deer stag in an autumn setting. By Mark Bridger |
Red deer stag in an autumn setting. By Mark Bridger |

Finally, bear in mind that, for many animals, autumn is a time of migration. From geese and butterflies to salmon and whales, a vast range of species are currently on the move.

Wherever you are, make sure you put aside some time to get out into the wild and savour the autumn.

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