When winter approaches our part of earth receives less sunlight, and the air grows colder. when this changes happen, trees prepare for winter . trees that drop their leaves seal the spots , where the leaves are attached then the fluids cannot flow in and out from the leaves , which change colour and fall off. 
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any chemicals can play the impartant role
2 Pigments that contribute to other colours 2.1 Carotenoids
2 Anthocyanins