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Knowing Nature . . .

Why Leaves Change Colour

October 30, 2007
Jocie Ingram

Big-leaved Maple photo © Dave Ingram

Fall is all about leaves. Leaves are in motion, drifting down from the trees and scuttling in the wind. Blazing yellows and crimson reds announce the season boldly, amidst a subtle backdrop of plums, rusts and browns. The leaves make a mosaic of colours and patterns under our feet as we walk through the woods.

This happens every year, of course, so most of us take it for granted. But while admiring all the splendour of fall, one may wonder just why and how the leaves turn colour and fall from the trees each year.

To understand this complicated subject, we have to rewind to the spring. Throughout the spring and summer months, plants are busy growing. They suck water up through their roots and take carbon dioxide from the air. Sunlight is used to transform water and carbon dioxide into glucose, the plant’s food source. Oxygen is released as byproduct. This process of turning sunlight into food is called photosynthesis, which means, “putting together with light”. Excess glucose produced by the leaves is stored as starch.

Cottonwood photo © Dave Ingram Red-osier Dogwood photo © Dave Ingram

Chlorophyll, the green pigment in leaves, is necessary for photosynthesis to occur. Chlorophyll masks other pigments already present in the leaves, such as yellows and oranges. These pigments are called carotenoids, and are the same pigments found in foods including carrots, corn and bananas, and flowers like daffodils.

At the base of each leaf, there is a specialized layer of cells called the “abscission”, or “separation” layer. Through the growing season, veins transport water into the leaf, and food out of the leaf. In the fall, shorter days, and cooler, longer nights, trigger the abscission layer of cells to swell and become cork-like. Eventually, the flow of water and food in and out of the leaf ceases. Without water, photosynthesis can’t occur, and the green chlorophyll begins to break down. As the chlorophyll disappears, the yellow and orange carotenoid pigments become visible, giving the leaves their characteristic autumn colour.

Unlike carotenoid pigments, which are present in the leaf from the beginning, red and purple pigments, known as anthocyanins, are produced only in the fall. Anthocyanins are powerful antioxidants, and colour foods such as cranberries, grapes, blueberries and plums. After photosynthesis stops, sunlight and cool nights cause glucose trapped in the leaves to turn red or purple. Wastes left in the leaves produce brown coloured tannins.

As a final step, the cells at the abscission layer separate the leaf from the branch, and the leaf descends to the ground. The food that the leaves produced is stored in the tree, enabling it to make it through the cold winter months.

Shortening day length is the key factor in triggering the leaves to change colour. However, factors such as the weather do affect the brilliance of the display. Generally, sunny days and cool nights (not freezing) make for the best displays. A wet and cloudy fall will produce a drabber outcome.

In our area there are many leaves to enjoy and fall is a good time to study them. Most obvious are the piles of large maple leaves produced by the big-leaf maple tree (Acer macrophyllum). A smaller maple, called Douglas maple (Acer douglasii) also puts on a nice display. Our western maples are not as spectacular as the maples of Eastern Canada, which turn more brilliant shades of red.

Dogwood turns a pretty purple colour, and black cottonwood and trembling aspen turn bright yellow. More disappointing are red alder and Garry oak, which turn from green to brown without putting on an autumn show.

Fallen leaves aren’t seen as a thing of beauty to everyone. Considered messy, there are frantic efforts to rake fallen leaves, which are then bagged and disposed of. Noisy leaf-blowers are out in force. Fallen leaves, however, do have a higher purpose than clogging gutters and covering driveways.

As fallen leaves decay, they replenish nutrients, contributing to spongy, humous-rich soil. They help to hold and absorb rainwater, and provide food to countless organisms. For human purposes, ground-up leaves create excellent garden mulch, and can be layered as is into the compost bin.

The beauty of autumn is synonymous with the changing colour of leaves. While enjoying the visual display, we should also appreciate the important role of leaves ecologically. If possible, it is best to let the leaves lie, or put them to good use in the garden.