Seasonal Autumn ForestGreen is a working color
During spring and summer, leaves are full of chlorophyll, the green pigment that captures sunlight for photosynthesis. Chlorophyll breaks down in sunlight and trees constantly replace it, so leaves stay green as long as the tree keeps producing it.
Shorter days change the plan
As days get shorter and nights cooler, deciduous trees prepare for winter. They stop making new chlorophyll and begin pulling valuable nutrients, like nitrogen, back from the leaves into the branches and roots. At the same time, a layer of cells called the abscission layer forms where the leaf stalk meets the twig. It eventually seals the connection, and the leaf falls.
Yellow and orange: revealed
Leaves also contain carotenoids, yellow and orange pigments that help with photosynthesis. They're present all summer but masked by chlorophyll. When the green fades, carotenoids show through. That's why birch, aspen, ginkgo and many others turn yellow fairly reliably every year.
Red and purple: made fresh
Reds and purples come from anthocyanins. Unlike carotenoids, many trees produce these pigments in autumn, from sugars that are still in the leaf. Scientists think anthocyanins may help protect leaves from excess light while the tree recovers nutrients, though research on their exact role continues. Maples, sweetgums and some oaks are known for strong reds.
Brown: what's left
Once pigments break down, leaves turn brown from tannins, compounds that remain in the dead tissue. Oaks and beeches often show these warm browns and coppers.
Why some autumns are more colorful
Weather in late summer and autumn affects how bright the reds are. In general:
- Sunny days and cool (but not freezing) nights favor bright reds, because sugars build up in the leaves and more anthocyanin is made.
- A hard early frost can kill leaves before they color fully.
- Drought can cause leaves to brown and drop early.
- Warm, cloudy, rainy autumns often give duller color.
Yellows depend less on weather, since those pigments were already in the leaf.
Evergreens and the gold exception
Most conifers keep their needles through winter. The larch is the well-known exception: its needles turn gold and fall each autumn, which makes it easy to spot on a hillside of dark green pines and spruces.
More reading: all articles.