Fall leaves. Photo: Michelle McCrackin.

But why drop leaves?

Before going into the reason for the color change, we should think about why some tree species – deciduous trees – drop their leaves in the first place. The simple answer is to save energy. Winter is cold, and there are fewer resources available – like sunlight and water (in liquid form) – compared to other seasons. Deciduous trees deal with these conditions by dropping their leaves, rather than maintaining them, and going dormant (kind of like why some animals hibernate). Evergreen trees, in contrast, have a different strategy. These trees invest in specialized leaves that can handle winter weather (like pine needles), so they don’t drop their leaves seasonally.

Fall leaves view. Photo: Michelle McCrackin.

The role of pigments

Fall leaves in garden. Photo: Michelle McCrackin.

There are a number pigments that give leaves their color. The pigment that makes leaves green is called chlorophyll. More importantly, chlorophyll allows plants to absorb energy from sunlight into order to make “food” from carbon dioxide and water. Leaves also contain pigments called carotenoids, which can be yellow to orange to brown. (Carotenoids make carrots orange). Most leaves appear green, however, because chlorophyll masks the carotenoids.

There is plenty of sunlight and water during spring and summer, so trees produce a lot of chlorophyll. When summer turns to fall, temperatures start to fall and the amount and intensity of sunlight decreases. In Stockholm, sunset during the summer can be after 10:00 at night; now it sets around 5:30. And before the year is over, sunset will be as early as 3:00 in the afternoon!

The trees respond to these seasonal cues by reducing and ultimately stopping chlorophyll production. It takes energy for the tree to produce chlorophyll, so before the leaves fall, the chlorophyll is broken down into smaller molecules and transferred to the trunks and roots for storage. The loss of chlorophyll in the leaves reveals the underlying yellow/orange/brown color of the carotenoids.

Interestingly, some trees like the North American red maple (Acer rubrum), start producing an additional pigment during late summer. This pigment is called anthocyanin, and it results in the distinctive bright red color in autumn leaves.

Scientists don’t fully understand why these trees invest in producing anthocyanin so late in the year, rather than saving energy to survive winter. Some research suggests that anthocyanin protects the leaves from sun damage after the amount of chlorophyll decreases. Sun protection means that leaves stay on the tree longer, allowing the tree to “reclaim” more of the nutrients in the leaves and survive the winter. The redness of leaves for these trees can vary between years due to weather. For example, if autumn is cloudy, trees may not produce anthocyanin because it is not needed. In this case, leaves will be more orange or yellow.

Regardless of why trees produce anthocyanin, we can still enjoy the color show.

Further reading: Why autumn leaves turn red