Understanding Why Leaves Change Colour in the Fall

The changing colour of the leaves in the fall is just part of a natural process of seasonal change. Scientists have recently come up with an explanation as to why a tree with no red pigments in its leaves during the spring and summer turns red in the fall.

"The recycling of nutrients in the leaves is probably the best theory and has the best support for it," states Tom Noland, a Tree Biochemist at the Ontario Forest Research Institute of the Ministry of Natural Resources in Sault Ste. Marie. "Since not all trees turn red, the ones that do turn red are probably better in recycling their nutrients than the ones that do not."

Noland's personal research deals with pigments in tree leaves. He conducts experiments with all the leaf pigments including the ones that give the yellow, orange, red and purple colours to leaves during the fall. It is one of his specialized areas of study, "because it is often an indicator of the health of the trees." If trees are stressed they often change colour early or more intensely depending on the type of stress they are undergoing.

The causes of tree stress can range from droughts and cool summers to extreme temperatures, pesky insect problems or undetectable disease. In relation to cool summers, Noland refers back to a few years ago when tree leaves were green and red at the same time. "The temperatures were so cool at night that the maples were turning colour even during the summer."

Scientists are quicker to explain how tree leaves change colour than to describe why they do. In the fall, cooler temperatures and shorter days, serve as a trigger to a tree that it needs to start getting rid of its leaves. And once the colour has reached its peak, the leaves start to fall off the tree.

To understand the process of how tree leaves change colour in the fall, one must visualize a leaf with different coloured layers. These layers disintegrate, reveal and/or mix pigments to show off the tree's natural fall foliage colours.

Chlorophyll is the green pigment in trees and plants that absorbs sunlight and helps photosynthesis occur so that a tree can make its own sugar for energy and growth. But since fall brings less sunlight and cooler temperatures, it makes the process of photosynthesis slow down causing the"chlorophyll in the leaves to break down and the tree to recycle the nitrogen in its leaves," illustrates Noland.

The process continues by allowing the carotenoids, and xanthophylls, which are the yellow and orange pigments already found in the leaves to be shown off.

As the chlorophyll is being destroyed, the leaves start turning yellow. Thus, revealing yellow colouring that was previously hidden within the leaf. It is normally masked by the chlorophyll because there is more green pigment than there is yellow in a leaf.

The colours red, and purple, come from anthocyanins. "Anthocyanins are the red , blue or purple pigments that you find in the leaves," defines Noland.

The oranges are a combination of pigments: red anthocyanins, and the yellow cartenoids and xanthophylls.

Therefore, the golden yellows and oranges, dark purples, and scarlet reds that we see each fall seep through the leaves as the original green chlorophyll breaks down. The chlorophyll breaks down due to destruction of chlorophyll.

"If you get a leaf turning red or orange, these anthocyanin pigments act as a kind of sunscreen for the leaf. They keep the intense fall radiation from breaking down the leaf pigments too quickly, allowing the tree more time to get the nitrogen and other nutrients out of the leaf and recycle it back into the woody part of the tree for next year," explains Noland.

Certain species of trees will change colours earlier or later than others. Trees like ashes and some species of maples turn before others. "Depending on the mix of species you will have a certain percentage turning earlier," describes Noland.

Tree colour change occurs worldwide. However, the intense reds, oranges, purples that that can be seen in maples and oak trees throughout Ontario, Quebec, and the northern part of United States are basically a North American phenomenon.

"There are people that come to Ontario from places as far away as Japan just to see the fall colours because they do not have the intense oranges, reds, and purples in Japan like they do in North America," reports Noland. That is why many of Canada's parks and forests are crowded in the fall season with nature lovers who want to be enthralled by the spectacle of Canada's autumn beauty.
Glossary:

Chlorophyll is the pigment in leaves that makes them appear green.

Carotenoids and xanthophylls are yellow and orange pigments in the leaves that make leaves appear yellow or gold in the fall.

Anthocyanins are the red, crimson, or purple pigments found in leaves that make them look orange (in combination with carotenoids and xanthophylls), red, crimson, or purple in the fall. Anthocyanins are usually only found in the leaves in the spring or fall, not in the summer.

Photosynthesis is the process that describes the use of chlorophyll to convert light energy
into sugar, carbohydrates and water.

Contributed by Reena Besa, Public Relations Assistant, Ontario Parks