Trees are generally divided into two great groups – broad leaved species and the conifers. Broad leaved trees have leaves that are flat and wide. Conifers on the other hand have slender, needle like leaves. As days grow short and cold weather approaches, broad leaved trees shed their leaves all together.
The shedding of leaves in autumn is because of the combination of shortening hours of daylight and cooling temperatures of autumn. A broad leaved tree responds to seasonal change by creating a barrier of special cells which seals off the leaves from the stem. Deprived of nutrients and moisture, the leaves cannot form new chlorophyll. The old chlorophyll breaks down and hence green color disappears.
As the chlorophyll production stops, the yellow and orange pigment (present all along but masked by chlorophyll in summer) becomes dominant. Reds and purples may also appear, produced during the series of chemical reactions involving sugars that build up in the leaf as nights grow cold.
The principal reason for this annual shedding is not to stay warm but to conserve moisture. Broad leaves with their large surface areas, give off huge quantities of moisture through evaporation. In winter, the freezing of the soil cuts off the supply of moisture to the roots, and water conservation becomes very important. So if the leaves are retained, the tree may eventually dry out. Because of needle like leaves, conifers have much smaller surface area and can survive without dropping their leaves.
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