During the spring and summer leaves serve as factories where most of the foods necessary for the tree’s growth are manufactured. The process, known as photosynthesis; takes place in the leaf in numerous cells containing chlorophyll, which gives the leaf its green colour. Along with the green pigment are yellow to orange pigments; carotenes and xanthophyll, however most of the year these colours are masked by great amounts of chlorophyll.
In autumn, because of the changes in daylight hours and changes in temperature the leaves stop photosynthesising and hence the chlorophyll breaks down. The green colour disappears, and the yellow and orange colours become visible. At the same time other chemical changes may occur, which form additional colours through the development of red anthocyanin pigments.
Eventually the tree sheds the leaves to save nutrients and prevent loss of water, the leaves then rot and return to the soil as organic matter. Via Earth Story