What happens when autumn leaves turn yellow

The green in a leaf is the pigment used to make food from light. When days get shorter and nights cooler, the tree stops making that green. The green that remains breaks down bit by bit.

As green thins, yellow that was hidden becomes visible. That color did not appear in autumn. It was in the leaf all summer, just masked by abundant green. The U.S. Forest Service explains yellow and orange the same way: pigments that were present since summer finally show.

Pigments that look yellow and orange are already in the leaf. The orange of a carrot is related. You do not need the names.

In summer, thick green covers them. When green pulls back in autumn, the undercolor comes out. It is not a new coat of paint. It is color that was hidden. That is why yellow tones stay fairly steady year to year, while reds swing more with the weather.

Unlike the yellow of ginkgo leaves, leaves that turn red often add a new pigment in autumn. Sugar made during the day stays in the leaf, and cool nights help that pigment form.

A stretch of clear days and cool nights tends to make reds sharper. The idea that sunlight burns leaves into new colors is wrong. Color change is not a scorch mark. Green declines, and other pigments show or form. An early frost alone does not guarantee pretty color.

Before dropping a leaf, the tree moves usable nutrients back into the branch. Breaking down green is part of that prep. A seal forms near the leaf stem, water and nutrient paths nearly close, and the leaf is ready to fall.

That is why streets turn yellow and footsteps crunch. The sound comes after the color shift. Yellow leaves are not a new dye job. They are what remains after green is taken back. Color also does not appear only when a leaf hits the ground.