Why do leaves change colour in autumn?

 What opulent bouquets the leaves of autumn do create—whole mountainsides of dazzling colour. But nature has a way of combining the sublime with the practical, and the fiery reds, yellows and oranges of autumn trees stand out as an excellent example of this convergence. Do leaves have some hidden purpose?

Unlikely as it may seem, the vivid yellow and orange colours have actually been there throughout the spring and summer, but we haven’t been able to see them. The deep green colour of chlorophyll, which helps plants absorb life-giving sunlight, hides the other colours. This extraordinary chemical absorbs from sunlight the energy that is used in transforming carbon dioxide and water to carbohydrates, such as sugars and starch. In the fall, trees break down the green pigments and the nutrients stored in the leaves. The nutrients are now shuttled down into the roots for reuse in the spring.

This transformation happens when cooler temperatures arrive. As soon as the temperature starts dropping and the days become shorter, the leaves stop producing chlorophyll. All this happens just to save energy. By stopping the process of photosynthesis, which requires the stronger sunlight of the Spring and Summer months anyway. It is trees’ form of saving up for “hibernation” or their dormant late Fall-Winter period. As the chlorophyll breaks down, the green colour disappears, and the yellow to orange colours become visible and give the leaves part of their fall splendour.

. How pigments play a part

It’s all about chlorophyll—the green pigment that allows plants to absorb sunlight and turn it into food that can be stored for winter dormancy, much as a bear stores fat for hibernation. During the growing season, trees create chlorophyll as fast as they use it up, so leaves stay green. But as daylight declines, trees slow the production of chlorophyll until, finally, it stops. Producing more would be a waste of energy because, as temperatures near the freezing point, the process of photosynthesis slows to impractical levels. While the green pigment ebbs from the leaf, other pigments hidden in the greenery during warm months begin to appear.

Carotenoids—which produce the yellow, orange and brown colours in the flowers of daffodils and the roots of carrots, in the rinds of pumpkins and the peels of bananas—are present in leaf cells throughout the growing season, but they’re masked by the green pigment. Once the chlorophyll disappears, the carotenoids give leaves a burst of colour. Yellow and gold colours vary little from year to year, however, because leaves contain carotenoids at all times. 

Red leaves, however? A completely different story!

Trees produce another pigment group, the anthocyanins, primarily in autumn. These pigments give red and purple to such things as blueberries, cherries, red apples, concord grapes, and plums. And autumn leaves.

Red leaves are the colour they are because of these anthocyanins molecules. As photosynthesis slows or stops, a day with too much sunlight can actually cause damage to the leaf. This is where this red molecule comes into play.

Anthocyanins act as an absorber of UV rays….basically, the “red colour” is a LEAF SUNSCREEN, for those extra-sensitive trees and other plants trying to make it past the late summer/early Autumn. The red colour also seems to pop-up when temperatures are or have been colder.

Possibly, the presence of anthocyanins molecules also helps to lower the leaf’s freezing point, giving it some protection from cold and allowing leaves to remain in place longer, giving trees more time to absorb nutrients.

Burgundy and red colours, therefore, tell a different story.

“The red colour is actively made in leaves by bright light and cold,” says Dana A. Dudle, DePauw professor of biology who researches red pigment in plant flowers, stems and leaves. “The crisp, cold nights in the fall combine with bright, sunny days to spur production of red in leaves – especially in sugar maple and red maple trees. Burgundy leaves often result from a combination of red pigment and chlorophyll. Autumn seasons with a lot of sunny days and cold nights will have the brightest colours.”  

Sun-lit autumn days stimulate leaves to produce sugars, and chill nights close the veins leading into and out of the leaf, locking in the sugars—which in turn lead to the production of anthocyanins and their crimsons and violets.

Weather Affects Colour Intensity

Many people mistakenly believe that weather makes leaves change colour. While this is not true, weather can affect how vibrantly the colours appear. If the weather is too hot or cold, the leaves will not be as bright as they begin to change. The best weather for brilliant autumn foliage is sunny, warm days and cool nights.

Water also plays an important role in autumn leaf colours. If a tree doesn't receive enough water, the leaves will die faster and fall to the ground. If there is too much rain, the tree won't receive enough sunlight, and the leaves will not be brightly coloured.

Temperature, light, and water supply have an influence on the degree and the duration of fall colour. Low temperatures above freezing will favour anthocyanin formation producing bright reds in maples. However, early frost will weaken the brilliant red colour. The best time to enjoy the autumn colour would be on a clear, dry, and cool (not freezing) day.

Some of the more reliably colourful trees are liquid amber trees (also called sweet gum) that turn a variety of colours on the same tree, and sometimes the same leaf. Ash tree leaves often turn a deep burgundy colour.

Leaves can also get sunburn

 With some trees, pigments serve as a kind of sunscreen to filter out sunlight. According to scientists, it is an underappreciated fact that plants cannot take an infinite amount of sun. Some leaves, if they get too much sun, will get something equivalent of a sunburn. They get stressed out and die. Pigments serve as a kind of sunscreen to filter out sunlight.

More fiction than theory

Another theory is that the colour of a plant’s leaves is often related to the ability to warn away pests or attract insect pollinators. In some cases, a plant and insect might have co-evolved. One of the more intriguing scientific theories is that the beautiful leaf colours we see today are indicative of a relationship between a plant and insects that developed millions of years ago. However, as the Earth’s climate changed over the years, the insects might have gone extinct, but the plant was able to survive for whatever reason. 

Because plants evolve very slowly, we still see the colours. So leaf colour is a fossil memory, something that existed for a reason millions of years ago but that serves no purpose now. 

French author Albert Camus tells us, “Autumn is a second spring when every leaf is a flower.”

Enjoy these bursts of vibrant  colour, it only occurs for a brief period each fall