Why do leaves change color in autumn each year?

As days get shorter and temperatures drop in autumn, trees prepare for winter by stopping chlorophyll production. Chlorophyll is the green pigment that dominates leaves during spring and summer, masking other pigments already present in the leaves. As chlorophyll breaks down, yellow and orange pigments called carotenoids become visible, which were there all along. The red and purple colors come from anthocyanins, which are actually newly produced in autumn as sugars become trapped in the leaves when trees seal off their vascular connections. Bright sunny days and cool nights encourage more sugar trapping and thus more vivid reds. This whole process is triggered annually by the reliable seasonal change in day length, which trees detect through light-sensitive proteins.