Unlike herbaceous plants, deciduous trees stand tall all winter, minus some important accessories-leaves. What trigger causes leaves to fall in the autumn? To understand why leaves fall off, it helps to first understand what the job of a leaf actually is. Plants and other photosynthesizing organisms have a very special talent. They can turn sunlight into food. It is a pretty neat trick that only photoautotrophs can do (photo=sun; auto=self; troph=feeder). In order for plants to make food energy, they need water, carbon dioxide (CO2) and sunlight. From this special combination, a plant is able to make its own food, in the form of glucose, a type of sugar. Plants then use the glucose as food energy to live and grow. In order to harvest sunlight energy, plants have a green pigment called chlorophyll. This pigment is what makes a plant s leaves appear green. As winter approaches, the days get shorter and cooler. This change in day length and temperature triggers some trees to go dormant, essentially hibernating for the winter. A tree s woody roots, branches and twigs can endure freezing temperatures, but most leaves are not so tough. It is also very energetically expensive for a tree to run its leafy food factories in the winter, when there is often little sunlight and freezing temperatures make water transport (from the ground into the tree s trunk and leaves) a problem. So it s more energy efficient for a leafy tree to close down operations in the winter and go dormant. A tree is full of vascular cells that transport water and sap throughout, from root to leaf tip. As the amount of sunlight decreases in autumn, the veins that transport sap into and out of a leaf slowly close off.
Then a layer of cells, called the separation or abscission layer, develops at the base of the leaf s stem. When this layer is completely formed, the leaf falls off. This process happens in all deciduous trees (trees that annually shed their foliage), with oak leaves as a notable exception. In oaks, the separation layer doesn t fully allow the oak leaves to detach. That s why most dead oak leaves remain on the tree through winter and even into early spring (much to the perpetual leaf-raking consternation of home owners with oak trees on their property). * Why Don t Evergreens Drop their Leaves? *
Evergreen trees include pines, spruces, cedars and firs. These trees don t lose their leaves, or needles, in winter, because they employ a different survival strategy than do deciduous trees. The needles of evergreens are covered with a heavy wax coating to help prevent moisture loss, and the fluids inside the cells contain substances resistant to freezing, essentially evergreen antifreeze. Evergreen leaves can live for several years, through all four seasons, before they are dropped and replaced by new growth. To learn more about the different growth forms and survival strategies that trees use see the New World Encyclopedia and Backyard Nature. 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. 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. 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.