why do the leaves fall of the trees in autumn
In temperate forests across the Northern Hemisphere, trees shed their leaves during autumn as cold weather approaches. In tropical and subtropical forests, trees shed their leaves at the onset of the dry season. Many types of trees shed their leaves as a strategy to survive harsh weather conditions. Trees that lose all of their leaves for part of the year are known as
deciduous trees. Those that donБt are called evergreen trees. Common deciduous trees in the Northern Hemisphere include several species of ash, aspen, beech, birch, cherry, elm, hickory, hornbeam, maple, oak, poplar and willow. In tropical and subtropical regions, deciduous trees include several species of acacia, baobab, roble, ceiba, chaca and guanacaste. Most deciduous trees have broad leaves that are susceptible to being damaged during cold or dry weather. In contrast, most evergreen trees either live in warm, wet climates or they have weather-resistant needles for leaves. However, there are exceptions in nature, such as that shed their needles every autumn and live oaks that retain their broad leaves for the entire year even in relatively cool climates. Shedding leaves helps trees to conserve water and energy. As unfavorable weather approaches, hormones in the trees trigger the process of abscission whereby the leaves are actively cut-off of the tree by specialized cells.
The word, scindere, which means Бto cut. Б At the start of the abscission process, trees reabsorb valuable nutrients from their leaves and store them for later use in their roots. Chlorophyll, the pigment that gives leaves their green color, is one of the first molecules to be broken down for its nutrients. This is one of the reasons why trees turn red, orange, and gold colors during the fall. At the end of the abscission process, when the leaves have been shed, a protective layer of cells grows over the exposed area. The shedding of leaves may also come springtime. Without leaves to get in the way, wind-blown pollen can travel longer distances and reach more trees. Bottom line: Many types of trees shed their leaves as a strategy to survive cold or dry weather. 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.
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