Why do leaves change color?
The short answer
As days become shorter and temperatures fall, green chlorophyll breaks down and other yellow, orange, or red pigments become visible.
How it works
Many trees prepare to shed leaves by forming a separation layer. As chlorophyll decreases, carotenoids reveal yellow and orange colors. Some trees also make red anthocyanin from sugars.
A familiar example
Even trees of the same species can change color at different times because sunlight, temperature, and water conditions differ.