What Causes Leaves to Change Color in Autumn?

What Causes Leaves to Change Color in Autumn?

Updated Oct 6, 2019

When it comes to seasonal scenery, Autumn provides some of the best views. As the trees’ leaves change from green, we are given one last treat from nature before another frigid winter takes center stage. What is it about the leaves that generate these color changes?

Earth’s Tilt

Earth orbits the sun once every 365.25 days, and does so with a 23.5° tilt relative to the orbital plane. This means that different parts of the planet get more or less sunlight during a 24-hour cycle (i.e. longer or shorter days) depending on where the Earth is in its orbit. The image below shows how the Earth’s tilt changes the seasons in the Northern hemisphere over the course of a full rotation. In the Southern hemisphere, the seasons are reversed.

An image showing how the amount of sunlight hitting parts of the Earth changes over the course of an orbitSeasons in the Northern Hemisphere (source)

Colors of a Leaf

During Spring and Summer seasons, the days are long and the trees receive plenty of sunlight. Leaves take in sunlight energy through chlorophyll pigments and it convert into chemical energy through photosynthesis, a process that keeps the tree well-fed. The chorophyll in a leaf is what gives it its green color.

Along with chlorophyll, leaves also have carotenoid and xanthophyll pigments which give off yellow to orange hues. Carotenoids and xanthophylls are actually present in the leaves during the Spring and Summer, but the visibility of their colors is drowned out by the shear amount of chlorophyll surrounding them.

As Autumn approaches and the days get shorter, trees begin receiving less light energy. Chlorophyll production, which was previously very active, now slows to a stop. Chlorophyll pigments stop being refreshed, and this allows the other pigments to dominate and become visible to us.

The changing of the seasons also signals production of another pigment called anthocyanin which is believed to protect the leaves from UV damage by acting like a sunscreen. These pigments are responsible for the reds and purples.

All together, these three pigments—carotenoid, xanthophyll and anthocyanin—give rise to the Fall foliage that so many enjoy. Also, other factors like temperature, amount of sunlight and availability of water in the soil can change things like the intensity of certain colors and how long the colors last before leaves break off and fall to the ground.