why do some trees shed their leaves in winter

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In and,
deciduous, including, and perennials, are those that lose all of their for part of the year. This process is called. In some cases leaf loss coincides with winternamely in or. In other parts of the world, including tropical, subtropical, and arid regions, plants lose their leaves during the or other seasons, depending on variations in. The converse of deciduous is, where foliage is shed on a different schedule from deciduous trees, therefore appearing to remain green year round. Plants that are intermediate may be called ; they lose old foliage as new growth begins. Other plants are semi-evergreen and lose their leaves before the next growing season, retaining some during winter or dry periods. Some trees, including a few species of, have desiccated leaves that remain on the tree through winter; these dry persistent leaves are called leaves and are dropped in the spring as new growth begins. Many deciduous plants during the period when they are leafless, as this increases the effectiveness of. The absence of improves wind transmission of pollen for wind-pollinated plants and increases the visibility of the flowers to in insect-pollinated plants. This strategy is not without risks, as the flowers can be damaged by frost or, in dry season regions, result in water stress on the plant. Nevertheless, there is much less branch and trunk breakage from glaze ice storms when leafless, and plants can reduce water loss due to the reduction in availability of liquid water during cold winter days. Leaf drop or involves complex physiological signals and changes within plants.

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The process of photosynthesis steadily degrades the supply of chlorophylls in foliage; plants normally replenish chlorophylls during the summer months. When arrives and the days are shorter or when plants are drought-stressed, deciduous trees decrease chlorophyll pigment production, allowing other pigments present in the leaf to become apparent, resulting in non-green colored foliage. The brightest leaf colors are produced when days grow short and nights are cool, but remain above freezing. These other pigments include that are yellow, brown, and orange. pigments produce red and purple colors, though they are not always present in the leaves. Rather, they are produced in the foliage in late summer, when sugars are trapped in the leaves after the process of abscission begins. Parts of the world that have showy displays of bright autumn colors are limited to locations where days become short and nights are cool. In other parts of the world, the leaves of deciduous trees simply fall off without turning the bright colors produced from the accumulation of anthocyanin pigments. The beginnings of leaf drop starts when an abscission layer is formed between the leaf and the stem. This layer is formed in the spring during active new growth of the leaf; it consists of layers of cells that can separate from each other. The cells are sensitive to a called that is produced by the leaf and other parts of the plant. When auxin coming from the leaf is produced at a rate consistent with that from the body of the plant, the cells of the abscission layer remain connected; in autumn, or when under stress, the auxin flow from the leaf decreases or stops, triggering cellular elongation within the abscission layer.

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The elongation of these cells break the connection between the different cell layers, allowing the leaf to break away from the plant. It also forms a layer that seals the break, so the plant does not lose sap. A number of deciduous plants remove nitrogen and carbon from the before they are shed and store them in the form of proteins in the vacuoles of cells in the roots and the inner bark. In the spring, these proteins are used as a nitrogen source during the growth of new leaves or flowers. After spending 342 consecutive days onboard the International Space Station from 2015 to 2016, astronaut Scott Kelly now holds the for longest single space mission by an American. But his \”One-Year\” study with NASA was about more than breaking records: Its purpose was to show how prolonged time in orbit would effect Kelly\’s genetic makeup compared to that of his identical twin brother on Earth. Now, following recent evaluations of the two men, it appears that Scott Kelly and his brother Mark are no longer genetically identical, reports. NASA the most recent findings from its ahead of a more comprehensive paper combining the work of multiple teams of researchers that is slated for later in 2018. Like his brother Scott, Mark is also an astronaut, making the pair the only twin astronauts in history. So when NASA was looking for a way to study the long-term effects of space life, the siblings were a perfect fit.

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As Scott was sending and on the ISS, Mark stayed on Earth to serve as the control. Biological samples taken from both subjects before, during, and after the space flight showed some dramatic differences. According to an investigation conducted by Susan Bailey of Colorado State University, Scott\’s telomeres, the protective \”cap\” at the ends of chromosomes that shorten as we age, got longer in space. The telomeres began shrinking back to preflight levels, however, a few days after Scott\’s return to Earth. Scott was subjected to regular exercise and a restricted diet aboard the ISS, so the new lifestyle may explain the sudden telomere boost. Other genetic differences stuck around even months after landing. \”Although 93 percent of genes expression returned to normal post-flight, a subset of several hundred \’space genes\’ were still disrupted after return to Earth,\” acccording to a NASA. About 7 percent of Scott\’s genes may show longer-term changes, included the genes associated with DNA repair, immune health, bone formation, hypoxia (an oxygen deficiency in the tissues) and hypercapnia (excessive carbon dioxide in the bloodstream). A long list of factors, like radiation, caloric restriction, and zero gravity, may have contributed to the results. NASA plans to use these findings to develop countermeasures against these effects, which will be essential if the agency plans to send humans to Mars, a journey that could take three times as long as Scott Kelly\’s ISS mission. [h/t ]

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