Plants are very smart, efficient and unforgiving in the energy management. Unlike the animals, who cannot regrow their limbs, plants see to it that inefficient leaves are cast off and replaced by new efficient one’s so that the food making machine continue to run smoothly. The leaves make the food by combining carbon dioxide and water in the presence of chlorophyll which shepherds the energy from sun and store it in the plant. Animals make use of this energy by eating the plant parts. This pigment imparts green color to the leaves and masks the faint colors of other chemicals like carotenoids.
As soon as, the plant know that the a given leaf is underperforming, it gradually withdraws the supply of chlorophyll to it and also reclaims whatever it can of the other components. Gradually the color of leaf begins to become yellowish and it looses it strength and fall off the branch. Older leaves are constantly replenished and plant continues to make its food.
When the winter comes, the Sun is hardly visible because sky is mostly overcast. So the plants plan to shut down the food making factories. Plant leaves have chlorophyll which is green. There are Xanthophylls which are yellow pigments, and carotenoids which give leaves an orange color. As the plants withdraw the chlorophyll from the leaves and like polar bears go into a state of hibernation. Due to deficiency of overpowering chlorophyll, leaves acquire beautiful brown, reddish and yellowish hues as the other pigments now dominate. Trees look even more beautiful due to this as the leaves acquire mixtures of colors due to the varying proportions of yellow and orange pigments. As the winter recedes, Sun comes out, the food making is resumed.