Autumn is a spectacular season in the temperate parts of the world. Maples, Oaks, Sourwoods, Cherries, Dogwoods, Birches, Alders, Sycamores, and other trees are ablaze with fiery red, orange, and yellow leaves. But why is it that these leaves change color and drop from deciduous trees every fall?
Why some tree leaves fall and others don’t.
The foliage of evergreens like pines, firs, rhododendrons, junipers, yews, boxwoods, arborvitae, and many hollies, are coated with a kind of wax that helps protect their leaves from cold and freezing temperatures. Hence their classification as “evergreen” – their foliage doesn’t fall from the tree or plant based on the season.
Not so for deciduous trees, which shed their leaves and become dormant in winter. The tissues of deciduous leaves are tender and the fluids that flow through the leaves are very susceptible to freezing. In order to save resources during winter, deciduous trees activate a sort of self-destruct mechanism for shedding their foliage as autumn draws near. Throughout the tree are chemicals that are sensitive to light, called cryptochrome and phytochrome. These chemicals are so sensitive that trees can register day length changes of as little as 30 minutes. It is these chemicals and others that trigger changes in the trees that produce autumn colors and falling leaves.
Why leaves change colors.
Chlorophyll masks every other pigment
Chlorophyll is the dominant pigment in a leaf. It’s this chemical that imparts green color to leaves during the growing season. There is so much chlorophyll in a leaf that all other colors are masked, even though they’re always present in the leaf.
Chlorophyll is vital to the tree’s existence – it’s necessary for photosynthesis, the process by which the plant captures energy from sunlight to manufacture food for the tree. Chlorophyll is continually being used up during the growing season, but the tree replenishes it rapidly so the supply remains as high as needed. Thus, the leaves remain green.
But once day length begins to shorten in autumn, the tree produces less and less chlorophyll. Slowly, the other colors which have been hiding in the leaves – red, yellow, or orange – are revealed. As autumn progresses, all of this pigment slowly deteriorates until only the brown tannins remain. Then the leaf dries and crumbles.
What creates yellow, orange, and red leaves?
Other pigments are revealed once chlorophyll decreases
Xanthophyll and Carotenoids are responsible for the bright yellows and oranges of autumn leaves. These are the same substances that make a carrot orange and corn yellow. Carotenoids and Xanthophyll are present in leaves at all times during the growing season (they help absorb sunlight) but are hidden by the green pigment from chlorophyll. As chlorophyll diminishes in autumn, the carotenoid and xanthophyll pigments are revealed, resulting in brighter and brighter yellow or orange leaves.
At the same time, some tree species like Maples, Dogwoods, and Oaks produce anthocyanins in the sap of their leaves. These are the same pigments that make cherries red. Bright light ramps up anthocyanins, which is why the leaves on a Red Maple tree on a clear, sunny day in early November appear brighter than on a cloudy day.
The amount of carotenoids, xanthophyll, and anthocyanins in the leaves are dependent on temperature, moisture, and sunlight throughout the growing season. That’s why the intensity and hue of foliage can change each year due to the balance of these chemicals inside the leaf. During years with drought and heatwaves, you may see tree leaves in fall that have less intense color than the year before.
Why leaves fall from trees.
Chemicals in the tree recognize that days are getting shorter
Trees contain highly sensitive chemical light receptors which signal when the days are getting shorter. These chemicals are so sensitive that trees can register day length changes of as little as 30 minutes. Phytochrome detects changes in red wavelengths and cryptochrome detects changes in blue wavelengths. It is these chemicals that trigger the chemical and physical changes in the trees that produce autumn colors and begin the process of shedding leaves.
Trees create zones that “scissor” the leaf from the tree
Shedding leaves starts when the tree forms an “abscission zone” where the leaf attaches to the twig or branch. The abscission layer acts like a scissors that cuts the leaf off the tree. The tree also creates structurally weak areas between the leaf and branch where wind or any other force can aid in removing the leaves.
The thin layer of cells that make up the abscission layer slowly swells, limiting the flow of water and food into the leaves. This restriction causes the amount of chlorophyll in the leaf to decrease, reducing the green pigment and drying the leaf. As autumn progresses, the abscission layer gets thicker and thicker, eventually pushing the leaf away from the stem completely, allowing it to be pulled off by environmental forces (gravity, wind, rain, etc). The open wound is then closed by other cells that block and protect the wound, forming a scar. This layer of cells defends the tree from pests and the environment.
Isn’t it amazing to realize that all of this happens when a tree recognizes that the days have gotten shorter?
Sources: School of Forest Resources, University Of Georgia (pdf), Loyola Center for Environmental Communication, National Wildlife Federation, Wikipedia.
What a great read, lots of great information. A really cool lesson to have with your kids, too!