Why do leaves turn yellow in the fall?

Leaves turn yellow in the fall due to the breakdown of chlorophyll, the green pigment that is responsible for photosynthesis. As days get shorter and temperatures cool, trees stop producing chlorophyll, and other pigments, such as carotenoids, become visible. Carotenoids are responsible for yellow, orange, and red colours in leaves. This process is part of the tree’s preparation for winter, when it conserves energy by shedding its leaves. The loss of chlorophyll allows trees to slow down their metabolic processes and preserve resources for the coming winter months.