why do the leaves fall 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. For a long time, people thought that trees leaves die because the trees enter in a benumbed phase to face the cold season. But in fact, it is not the cold what affects most a tree during the winter but. drought. Even when the trees live in a wet climate, with an abundant snowfall and rainfall during the autumn, they will suffer from thirst.
Cold provokes what is called physiological drought. The absorption forces do not suck enough water to compensate the water loss at the leaf level at cold temperatures. Leaves have as the main function the photosynthesis: using water, carbon dioxide and light they generate organic compounds and oxygen. But in order to induce a suction force, they constantly sweat. Because the leaves have a large surface, they sweat huge water amounts, so the trees must shed them during the winter in order not to get dried, as the stem and branch surfaces are much smaller. Moreover, the thick bark sweats much less. In fact, trees that do not shed the leaves in autumn, like coniferous trees, are extremely adapted to drought. Their leaves are very small (this decreases sweating surface) and covered with wax (which impedes water loss).
Thus, in fact, trees in temperate-cold climate shed their leaves from the same reason as tropical trees during the dry season. But what triggers the mechanism of falling leaves: temperature or an inner clock? Despite the traditional belief, weather plays no role. The trees possess an inner clock which is triggered by the length of the daylight. The shrinking daylight in autumn activates it, and the trees will lose their leaves even in the warmest falls. The opposite is also valid: the warmest winters will not trigger the greening of the trees. Some leaves also seem to have a genetic longevity, being programmed to die after a certain period. Some plants cultivated in contra-season will die in full summer. And greenhouse plants, at constant light and temperature, will fade after a while.
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