In Autumn, you may notice many differences in the weather, including the changing color of leaves. The leaves on many trees in temperate regions (regions that experience wide temperature ranges) start to turn yellow and orange before falling to the ground. What causes this?
Let’s begin with the three different types of chemicals that produce different pigments in leaves. The first chemical is chlorophyll which produces a dominant green pigment. Chlorophyll is present throughout the growing season. The second type of chemical is the carotenoids which are responsible for the yellow, orange, and some brown colors in leaves. The third type of chemical is the anthocyanins which are responsible for red, purple, and bluish colors. We’ll talk more about these later.
In the summer, the leaves of deciduous trees (trees that lose their leaves) are full of chlorophyll. So full, in fact, that the green in the chlorophyll overpowers the other colors present in the leaves, such as yellow and orange. Chlorophyll is essential for plants. It turns light into energy, producing sugars and starches as food for the tree. This allows the tree to grow during the summer.
When the days start getting shorter due to the earth’s tilt and orbit around the sun, the temperature starts to drop and there are now less hours of sunlight in a day. The lower temperature and fewer hours of sunlight signal the tree to go into storage mode for the winter. The chlorophyll now starts to break down, causing the green in the leaves to disappear. Carotenoids, the orange and yellow colors, which have been present throughout the entire growing season, can now be seen in the leaves. Most of the sugars and starches produced by the chlorophyll in the leaves are brought back into the tree.
Anthocyanins, which are very water soluble, are now present in the watery portions of the leaf cells. The presence of anthocyanins (which is responsible for the red, blue, and purplish colors) is due to the excess sugars that are left behind in the leaf cells after the chlorophyll breaks down. Bright light also helps to bring out this chemical in the leaves. The brown color in the leaves, while partially due to the carotenoids present in the leaves, is more often the result of waste from the tree’s life processes. An abscission layer of cells builds up where the leaf is connected to the branch of the tree, blocking the transport of sugars to the tree. After a while, the wind or even the weight of the leaves themselves will cause the leaves to fall from the tree. The fallen leaves are also high in calcium and potassium. This layer of fallen leaves decomposes, returning these nutrients back to the soil, making the top soil fertile for more plants to grow in the next growing season.