Why do leaves change color in autumn before falling?

As days get shorter and temperatures drop, trees prepare for winter by forming a layer of cells at the base of each leaf stem that gradually cuts off water and nutrient flow. Chlorophyll, the green pigment responsible for photosynthesis, breaks down first since it requires constant replenishment. This reveals yellow and orange pigments called carotenoids that were always present in the leaf but masked by the dominant green. The red and purple colors come from anthocyanins, which are actually produced fresh in autumn from sugars trapped in the leaf after the flow is cut off. Bright sunny days combined with cool nights encourage more anthocyanin production, which is why some autumns produce more vivid reds than others.