As the leaf gets detached from the plant, gradually the level of cellulose present in the leaf decreases and thus the colour of the leaf turns to yellow.

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trees have hormonal changes in their vascular system which is driven by the DNA of the tree. The DNA dictates what the tree is to do. During changes of season, The DNA of the Tree instructs it to slow growth and production during fall and have all the leaves fall die. in the process the leaves wither and turn from a lively green to amber and brown color. The green part of the leaf is chlorophyll (solar energy capturing part of the leaf). During winter the tree needs to protect itself from the harsh temperatures. So it drops its leaves, in by doing so reduces the metabolic demands of the tree and goes into a low energy stasis all winter. sort of like hibernation.