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Autumn Questions


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Why do leaves change color?

Trees and plant leaves contain pigments that give them their color. Three pigments are involved in fall color. Chlorophyll-gives leaves their green color Carotenoids-provide the yellow, orange and brown colors Anthocyanins-give the red and purple colors. In contrast to the other two pigments anthocyanins are produced in autumn, in response to bright light and excess plant sugars in the leaf cells. During the growing season, most tree leaves are green because they are full of chlorophyll. Plants use chlorophyll to capture sunlight for photosynthesis, the process that enables them to manufacture their own food. The amount of chlorophyll is so high during the summer that the green color masks all other pigments present in the leaf. As the days grow shorter in the fall, chlorophyll production slows down and eventually stops. The carotenoids and anthocyanins that are present in the leaf then become visible.

Do different types of trees turn different colors?
Certain colors of leaves are characteristic of particular species of trees. Oaks turn red, brown or russet Dogwood turn purplish red Beech turn light tan Red Maple turns brilliant scarlet Sugar Maple turns orange-red Black Maple turns glowing yellow Aspen, Birch and yellow-Poplar turn golden yellow Some trees, such as elms, have leaves that simply shrivel up and fall off with a brown color. The timing of the leaf color change varies by species. For example, oaks put on their fall color display long after many other trees have lost their leaves. The timing of the color change for certain species appears to be consistent regardless of local weather patterns or changes in latitude and seems to genetically inherited.

Why are some Autumns more colorful?
Temperature and moisture greatly influence autumn color. No two Autumns are alike. A succession of warm, sunny days and cool, but not freezing nights seem to bring about the most spectacular color displays. Since carotenoids are always present in leaves, yellow and gold colors are fairly constant from year to year. In order for the brilliant scarlet, purple and crimson colors to develop, bright sunlight in the early fall is needed. Bright sunny days increases food production in trees and plants. These sugars are trapped in the leaves spurring the production of anthocyanin pigments, providing the red tint to foliage. The amount of moisture in the soil also affects autumn color. A late spring, or a severe summer drought, can delay the onset of fall color by a few weeks. A warm period during fall lowers the intensity of autumn color. Trees defoliated by insects during the growing season may also show less fall color.

Why do leaves fall off?
In early autumn, in response to the shortening days and declining intensity of sunlight, leaves begin the processes leading up to their fall. The veins that carry fluids into and out of the leaf gradually close off as a layer of cells form at the base of each leaf. These clogged veins trap sugars in the leaf and promote production of anthocyanins. Once this separation layer is complete the connecting tissues are sealed off, the leaf is ready to fall. Deciduous trees drop their leaves in order to survive the harsh conditions of winter. Stems, twigs, and buds are equipped to survive extreme cold. Tender leaf tissues, however, would freeze in winter, so plants must either protect their leaves or shed them. Evergreens are able to survive winter because they have developed a way to protect their foliage. Their needle-like or scale-like foliage is covered with a heavy wax coating and the fluid inside their cells contains substances that resist freezing.



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