Why do Leaves Change in the Autumn?
There are actually several reasons leaves change color and function in the autumn. One that is sometimes overlooked is that it is very difficult for trees in northern climates that have snowy winters to bear the weight of snow on their leaves. Losing their leaves in the winter means less snow stays on the trees and they don't need to be so strong to survive until the warm weather in spring.
More often, people focus on the beautiful color changes in leaves that take place in the autumn in many places around the world. Why do those color changes happen? And how?
As the days grow shorter and colder, there is no longer enough light and water for the leaves to use (along with carbon dioxide from the air) to produce the food plants need for survival. The plants switch to stored food and when they shut down their food production by chlorophyll and photosynthesis, the green-colored chlorophyll begins to disappear from the leaves. We often are left to see the colors that are left behind. Many times these colors are simply dull browns and tans. However, some types of trees leave behind bright yellow and orange colors that we can see in the autumn. And some other plants actually start to produce other colors, like purples and reds, in the fall when the chlorophyll goes away. Eventually even these colors fade away as the leaves quit working and sever their connections to the trees. Then some wind or rain comes along and they "fall" off the trees to make room for new leaves in the spring.
You can read a little more detail on this explanation at this website.
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Updated: Nov. 1, 2010