Why do leaves change color in autumn?
Trees stop making green chlorophyll, unmasking yellows and oranges that were there all along.
Green is a busy season, not a true color
All summer, leaves are crowded with chlorophyll, the pigment that runs photosynthesis. It absorbs red and blue light and reflects green, which is why a healthy leaf looks the way it does. The catch is that chlorophyll is fragile and has to be rebuilt constantly to keep that green going.
Shorter days pull the plug
As nights lengthen and cool, the tree reads the change as a signal that winter is coming. It is not worth feeding leaves that will freeze, so the tree begins to shut them down and reclaim their nutrients.
Where the reds come from
Yellows and oranges are uncovered, but the fiery reds are newly made. As the leaf seals itself off at the stem, trapped sugars fuel the production of anthocyanins. Bright, dry, cool days tend to make these reds most vivid.
Finally a thin layer of cells called the abscission zone severs the leaf from the twig. Once pigments are spent, only brown tannins remain, and the leaf lets go.