why do trees change colors in the fall
The vivid yellow and orange colors of fall leaves have actually been there throughout the spring and summer, but we havenБt been able to see them. The deep green color of chlorophyll, which helps plants absorb life-giving sunlight, hides the other colors. In the fall, trees break down the green pigments and nutrients stored in the leaves. The nutrients are shuttled into the roots for reuse in the spring
As leaves lose their chlorophyll, other pigments become visible to the human eye, according to Bryan A. Hanson, professor of chemistry and biochemistry at DePauw University who studies plant pigments. Some tree leaves turn mostly brown, indicating that all pigments are gone. Burgundy and red colors are a different story. Dana A. Dudle is a DePauw professor of biology who researches red pigment in plant flowers, stems and leaves. Dudle said: The red color is actively made in leaves by bright light and cold. The crisp, cold nights in the fall combine with bright, sunny days to spur production of red in leaves Б especially in sugar maple and red maple trees. Burgundy leaves often result from a combination of red pigment and chlorophyll. Autumn seasons with a lot of sunny days and cold nights will have the brightest colors. In some cases, about half of a treeБs leaves are red/orange and the other half green.
Dudle says that results from micro-environmental factors Б such as only half the tree being exposed to sunlight or cold. Hardwoods in the Midwest and on the East Coast are famous for good color selections. Some of the more reliably colorful trees, Hanson notes, are liquid amber trees (also called sweet gum) that turn a variety of colors on the same tree, and sometimes the same leaf. Ash tree leaves often turn a deep burgundy color. Ginkgo trees, although not native to North America, will feature an intense yellow, almost golden, color. The colors are doing something for the plant, or they wouldnБt be there, said Hansen. But what is the colorsБ purpose? Scientists think that with some trees, pigments serve as a kind of sunscreen to filter out sunlight. Hanson said: ItБs an underappreciated fact that plants cannot take an infinite amount of sun. Some leaves, if they get too much sun, will get something equivalent of a sunburn. They get stressed out and die. Another theory is that the color of a plantБs leaves is often related to the ability to warn away pests or attract insect pollinators. Hanson said: In some cases, a plant and insect might have co-evolved. One of the more intriguing scientific theories is that the beautiful leaf colors we see today are indicative of a relationship between a plant and insects that developed millions of years ago.
However, as the EarthБs climate changed over the years, the insects might have gone extinct, but the plant was able to survive for whatever reason. Because plants evolve very slowly, we still see the colors. So leaf color is a fossil memory, something that existed for a reason millions of years ago but that serves no purpose now. Bottom line: Biologists discuss why leaves change color. In some areas of the world, the weather changes in the fall, making the air turn cold. During this time, many leaves also change colors. Why does this happen? First let's think about why some trees drop their leaves before winter. In the winter, it would take a lot of energy and water for plants to keep their leaves healthy. But winter is cold, dry, and usually there isn't much sun (which helps give plants energy). So, instead of trying to keep their leaves, some plants drop their leaves and seal the spots on their branches where the leaves had been attached. How is this related to what makes leaves colorful? Leaves are colored by molecules called pigments. The pigment that causes leaves to be green is chlorophyll. Chlorophyll is important for plants to make food using sunlight. During spring and summer when there is plenty of sunlight, plants make a lot of chlorophyll.
In autumn when it starts to get cold, some plants stop making chlorophyll. Instead, those plants break down chlorophyll into smaller molecules. As chlorophyll goes away, other pigments start to show their colors. This is why leaves turn yellow or red in fall. The color change usually happens before the leaves fall off of the tree. Why might that be? It takes a lot of energy to make chlorophyll. If the plants break down the chlorophyll and move it out of their leaves before the leaves fall, plants save energy. The plants can reabsorb the molecules that make up chlorophyll. Then, when it's warm and sunny enough to grow again, the plants can use those molecules to remake the chlorophyll. That way the plants don't have to make chlorophyll from scratch. There are other pigments in leaves called carotenoids. Carotenoids are yellow and orange. Anthocyanins are other plant pigments that are only made in the fall. These pigments cause red, pink, or purple colors. Anthocyanins also protect leaves from being eaten or getting sun burned. So the different colors in leaves are caused by changes in the pigments. When the weather changes, some plants break down all the green pigment. This lets beautiful yellows, oranges, and reds come through in the fall. To learn more about how plants grow, try this link.
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