BUFFALO, N.Y. (WKBW) - Leaves are the food factories of trees, all summer long they are busy taking energy from the sun to turn carbon dioxide and water into oxygen and glucose. Glucose is a form of sugar. This process is called photosynthesis.
The tree uses the sugar as energy and to help it grow, the by-product oxygen helps us breathe..
As for the leaves being green, photosynthesis wouldn't be able to occur without the chemical chlorophyll. Chlorophyll is green in color and is stored in a trees leaves, hence the green color.
When the days get shorter and cooler the tree realizes it is time to shut down for the winter. There isn't enough daylight to continue getting energy so the leaves stop the process of creating food for the tree.
The chlorophyll levels decrease, taking away the green color from the leaves. You then are able to see the other colors that were there all along. Like the reds you get in maple trees or the browns you get in oak trees. The red color comes from glucose left in the maple leaves and the brown color comes from some of the wastes left in the leaf.