In many temperate forests, a green canopy becomes a mixture of yellow, orange, red, and brown during autumn. The transformation is not paint arriving from outside the leaf. It reflects changes in the pigments and processes already associated with living leaves. [1] [2]
Why leaves look green
Leaves contain chlorophyll, a pigment involved in photosynthesis. Through photosynthesis, plants use energy from light to help make sugars. Chlorophyll also gives many leaves their familiar green appearance during the growing season. [1]
A leaf contains more than one kind of pigment. Some colors are less obvious while the green of chlorophyll dominates. That balance changes as the season changes. [1]
As the growing season slows
Shortening days help signal seasonal changes in many deciduous trees. Chlorophyll production slows and existing chlorophyll breaks down. Trees also recover useful materials from leaves before shedding them, rather than keeping the same summer canopy through every season. [1]
Different pigments, different colors
As green fades, yellow and orange pigments can become easier to see. Carotenoids contribute many of these warmer colors. Red and purple colors involve pigments called anthocyanins; they are not simply another version of green chlorophyll. [2]
Different tree species and environmental conditions can produce different displays. An autumn forest is therefore a mixture of leaves changing in different ways, not a single synchronized color switch. [2]
A seasonal pattern, not a universal rule
This classic autumn display describes many temperate deciduous trees. It should not be treated as a rule that every tree everywhere must follow. Even within one landscape, leaves may change at different times and show different colors. [1] [2]
A useful comparison is to photograph the same tree from the same position on several days. Rather than collecting an isolated “red leaf” fact, the photographs show a process: the appearance of a living plant changing over time.
Sources and image credit
1. National Park Service: Why Do Leaves Change Colors in the Fall?
2. NASA Earth Observatory: From Green to Brown
Cover: NPS / Hadley Seymour. Public domain. Resized and cropped or letterboxed to a 16:9 cover. Image source · Usage / license