Why do leaves change colour in autumn?

Leaves change color in autumn primarily because of the shifting balance of chemicals within the leaf as days grow shorter and temperatures drop.

Throughout the spring and summer, leaves are green because they are filled with chlorophyll, the pigment that captures sunlight for photosynthesis. This process is highly active during the long, sunny days.

As autumn approaches, the decreasing daylight and cooler temperatures signal the tree to prepare for winter. The tree begins to shut down its food-making process and seals off the veins that carry sap to the leaves.

Here is what happens next:

• Chlorophyll breaks down: As the tree stops producing new chlorophyll, the green color fades.
• Hidden pigments emerge: Other pigments, which were present in the leaf all along but hidden by the intense green of the chlorophyll, finally become visible. Carotenoids produce yellow, orange, and brown colors.
• Anthocyanins form: Some trees, like maples, produce a different set of pigments called anthocyanins in response to the trapped sugars in the leaves as the veins close. These create the vibrant reds and purples we see.

Eventually, the connection between the leaf and the branch weakens completely, the leaf falls, and the tree enters a dormant state to conserve energy for the coming spring.