Why Leaves Change Colors
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The crisp air, the smell of damp leaves and wood smoke and of course the brilliant yellow, orange, and red leaves swirling to the ground, these are the simple pleasures of fall. As the weather turns cooler and the days shorter we can observe the plants and animals preparing for the colder days ahead. While we pull out our sweaters and hats and turn on our heaters the trees are also getting ready for winter.
Deciduous trees drop their leaves in the fall and regrow them in the spring while conifers or evergreens keep their needles all year round. During the summer when the leaves are full and green they are acting as food factories, turning energy from the sun, carbon dioxide and water into sugars through photosynthesis. Photosynthesis literally means “putting together with light." The chlorophyll in the leaves enables photosynthesis and gives the leaves their green color.
With limited sunlight and water photosynthesis is difficult in the winter. These thin delicate leaves would freeze during the winter. To ensure its survival the tree drops its leaves. The woody parts of the trees are hardier and can survive freezing temperatures. As the tree prepares to shed its leaves the veins that carry the water in and sugars out of the leaves gradually close off. At the base of each leaf is a small layer of cells called the abscission or separation layer. This layer swells and eventually severs the water and sugar transport lines. The sugar or glucose cannot leave the leaf and the leaf receives no fresh water. As the chlorophyll disappears so does the green color.
This loss of chlorophyll is what enables us to see the brilliant fall colors. The orange we see comes from carotene, the same pigment that gives carrots their color. The yellow pigment is xanthopyll and is also found in bananas. Small amounts of these pigments are contained in the leaves all year, but only become visible when the chlorophyll disappears. The bright reds and purples come from anthocyanin, a potent antioxidant that we also find in red apples, beets and red wine. Maple trees can produce these pigments during fall from the trapped glucose in its leaves giving it the flaming red color. Brown colors come from tannin, which is actually a waste product. The combinations of these pigments give us the wide array of beautiful colors we see in the autumn. Most trees will now lose these colorful leaves, but oak trees are the exception, keeping their brown leaves throughout the winter.
For the leaves to drop they have to become separated from the stems. Eventually some of the cells in the abscission form a seal between the tree and the no longer green leaf. The rest of the cells in the separation layer begin to disintegrate causing a week line. A gentle breeze will cause these tenuous attachments to break and the leaf spirals to the ground. This accumulating leaf litter is important for forest health. While we may see fallen leaves as a nuisance to be raked up they provide an important role in the ecosystem.
Invertebrates (small animals without backbones) find habitat in the leaf litter and woody debris on the forest floor. Many of them feed on the rotting organic material helping to decompose it and return the nutrients to the soil. Because of this abundance of insects and arachnids, ground foraging birds will find food amongst the litter. Plants also rely on these returned nutrients and the insulation leaf litter provides. Reptiles find this layer crucial for shelter, camouflage, insulation and food. Decomposing leaves keep moisture from evaporating and provide a nice moist environment for our less obvious decomposers like fungus and bacteria.
Fall is a great time to get out and admire these changes first hand. As we look out our windows, walk through town or hike on nearby trails we can watch the progression of the fall colors. When the last leaves have fallen from the trees outside our windows we can take heart and appreciate the increased winter light in our houses. When you go out into the forest gently push aside a bit of leaf litter and you will surely find something making use of it. To further appreciate the smaller living organisms amongst the leaves carry a hand lens, magnifying glass or even your binoculars turned upside down. Lie down and participate in a little belly biology, check out what’s going on amongst the leaves. Enjoy all the simple pleasures of fall and don’t forget your sweater.
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