Leaf Foliage Lesson
“Winter is an etching, spring a watercolor, summer an oil painting and autumn a mosaic of them all.” ~Stanley Horowitz
Autumn is a spectacular time of color in many areas, and the Appalachians are no exception. From radiant reds to mustard yellow, the vibrant display of color is short-lived, earning leaf-peeping peak priority.
What causes such brilliant displays of leaf color this time of season, anyway? Think you already know? Well, we thought so too; but we were curious and learned a few things. Here is what we found –
Fall foliage is a byproduct of chemical changes when deciduous trees prepare to go dormant for winter. The difference in day length (photoperiod) triggers biochemical changes in the trees starting around June 21st, the summer solstice, or longest day of the year when the sun begins to move south and the days become shorter. As the days become shorter, trees gradually stop producing food to prepare for dormancy.
A tree’s roots and branches are able to endure freezing temperatures, but the thin and tender leaves of a broadleaf tree – such as a birch or maple – will freeze and die. Any plant tissue unable to survive the cold months of winter is shed to ensure the tree’s survival.
Now, we’ll explore why these colors exist when they do.
A tree leaf contains three primary color pigments: green, yellow and orange. During the warm summer months, tree leaves appear green from the presence of chlorophyll. The chlorophyll in the leaves is responsible for providing the tree with energy during the growing season. Chlorophyll is the primary ingredient for the process of photosynthesis, capturing sunlight, water, and carbon dioxide and turning it into glucose, or tree food…and in the case of maples, pancake syrup!
The orange and yellow carotenoids are also present in the leaf cells during the summer, but the chlorophyll’s green color masks the other pigments present in the leaf. As the daylight shortens, a layer of cells grow over the water tubes in the leaves, so no more water can get to the leaf. Without water, the tree can no longer perform photosynthesis, and the leaves quickly lose their green color. The fading of green allows the other pigments in the leaf to now be visible.
What causes red or purple leaves?
When cells block the tubes in the leaf’s stem to prevent water from entering the leaf, sometimes sap gets trapped inside of the leaf. The sugar may cause the sap to turn red or purple. This is common with maple trees.
What causes brown leaves?
When the leaves no longer have access to water, they can’t produce food. This causes them to die. The green chlorophyll dies first, followed by yellow and orange. The leaf is now dead and brown. They become dry, fall to the ground, and crunch underneath your feet. The leaves decompose and replenish the soil with nutrients, and become part of the spongy humus layer on the forest floor. The surrounding trees and plants can now prosper during the growing season from their own waste!
- Leaves contain the same amount of yellow and orange pigment in the summer (when they are green) as they do in autumn (when they are yellow / orange). The green pigment (chlorophyll) dominates and masks the other colors during the summer. Orange colored leaves come from carotene and yellow from xanthophylls.
- Red color, common to maples, is from anthocyanin pigments, which occur from trapped glucose. Not all trees can make anthocyanin.
- Brown leaves are dead leaves and are caused by the waste product tannin. Organisms break down the dead matter, providing nutrition for the growing season.
- You can help replace nutrients in your own soil by composting fallen leaves.