Why do leaves change color in autumn?

Leaves change color in autumn because trees stop producing chlorophyll, the green pigment used for photosynthesis, as daylight hours shorten and temperatures drop. As chlorophyll breaks down, it reveals yellow and orange pigments called carotenoids that were present in the leaves all along but masked by the dominant green. The red and purple colors are different — they come from anthocyanins, which are actually produced fresh in autumn as sugars become trapped in the leaves when trees seal off their connection to the leaf. This process, called abscission, is how trees conserve water and nutrients before winter. The intensity of fall colors depends on temperature and sunlight, with cool nights and sunny days producing the most vivid reds.