Why leaves change colour

Oct 06, 2011

by Steve Blight

Living in the Land O’ Lakes, we are lucky to be in one of those parts of the world where nature has one last fling before settling down into winter's sleep. A great Canadian tradition is to get out in the sunny fall weather and enjoy the breathtaking beauty of the colourful fall foliage.

Certain colours are characteristic of particular species. Oaks turn red, brown, or russet; hickories, golden bronze; aspen and other poplars, golden yellow; ash purplish red or yellow; beech, light tan. Maples differ species by species – red maple turns brilliant scarlet; sugar maple, yellowish-orange and occasionally some red. Leaves of some species such as the elms simply shrivel up and fall, exhibiting little colour other than drab brown.

How leaves change colour is actually a complicated process that is still not fully understood. However, we do know enough to explain the basics. Three factors influence autumn leaf colour – the decreasing length of the day, leaf pigments, and the weather.

The timing of colour change and leaf fall are primarily regulated by the calendar. As days grow shorter and nights grow longer and cooler, biochemical processes in the leaf begin to paint the landscape with nature's autumn palette.

A colour palette needs pigments – complex organic chemicals that are present in most plants that give them their colours. There are three types that are involved in autumn colour:

Chlorophyll, which gives leaves their basic green colour. Chlorophyll is necessary for photosynthesis, the chemical reaction that enables plants to use sunlight to produce sugars for their food.

Carotenoids, which produce yellow, orange, and brown colours in leaves as well as such things as corn, carrots, daffodils and bananas.

Anthocyanins, which provide the reds to such familiar things as cranberries, red apples, cherries, strawberries, and plums.

Both chlorophyll and carotenoids are present in the leaves throughout the growing season. However, most anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells.

During the growing season, chlorophyll is continually being produced and broken down and leaves appear green. As day length decreases in the autumn, chlorophyll production gradually stops and eventually all the chlorophyll is broken down within the leaves. The carotenoids present and the anthocyanins that are being produced are then unmasked and show their colours.

The amount and brilliance of the colours that develop in any particular autumn season are related to weather conditions that occur before and during the period in the fall when chlorophyll in the leaves is dwindling. Temperature and moisture are the main influences. Perfect conditions for producing the most brilliant red pigments include a warm moist spring, lots of sunshine and adequate rainfall during the summer growing season, and warm sunny fall days with cool nights. A late spring, a summer drought, or cloudy wet weather in the fall can markedly reduce the amount of reds produced or even delay the onset of fall colour. In addition a very warm period during fall will also lower the production of red pigments.

Warm, sunny fall days lead to the production of lots of sugars in the leaves but the cool nights and the gradual closing of veins trap the sugars in the leaf tissues. These conditions – lots of sugar and lots of light – spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colours tend to remain fairly constant from year to year.

Now that we know the basics, it’s tricky but possible to make broad predictions of how the colours will pan out in the forests. Looking at this year’s weather around Sharbot Lake – a cool wet spring, hot dry summer with lots of sun and lower than average rain fall, and a very warm dry fall with only a few cool evenings – it suggests that the conditions are not ideal for a spectacular show of reds. Instead this year may highlight more of the yellows, oranges and browns.

Nevertheless, even in a year when conditions are not ideal, the trees always put on a great display. And if the colours aren’t perfect, we can always take comfort in another Canadian tradition – blame it on the weather!

Please feel free to report any observations to Lorraine Julien at This email address is being protected from spambots. You need JavaScript enabled to view it.  or Steve Blight at This email address is being protected from spambots. You need JavaScript enabled to view it.