Why do leaves change color in the fall?
Within temperate and boreal forests, trees and shrubs that drop their leaves are called deciduous plants; they include groups such as aspens, cottonwoods, maples and oaks. On the other end of the spectrum are plants that don't shed their leaves — for example, the pines, spruces, cedars and firs that make up the conifers, or evergreens.
Both types of trees produce less energy in the winter, but conifers have adaptations, such as a waxy coating to minimize water loss, that help them keep their needles year-round.
In early fall, the weather conditions start to change: It gets cooler — with crisp, but not freezing, nights — and the duration of daylight is reduced.
Collectively, there's less sunlight for plants to harvest using the chlorophyll in their leaves, so deciduous trees begin to pull back their investment in keeping them alive. By the time the first freezes hit, the trees are well on their way to dormancy.
While chlorophyll is the most common pigment that plants use to harvest light — green leaves look that way because chlorophyll absorbs red and blue light and reflects green light — plants often have a variety of secondary pigments, too.
As the chlorophyll in a tree's leaves begins to wane, these underlying pigments become visible. This is what we're seeing when fall foliage peaks, Kristina Bezanson, an arboriculturist at the University of Massachusetts Amherst, told Live Science in an email.
Red and purple hues stem from secondary pigments called anthocyanins, while carotenoids and xanthophylls yield oranges and yellows, respectively. Within a single group, such as the maples, different species have evolved their own suite of pigments.
Red maples, for example, turn a brilliant scarlet in the fall, while black maples become yellow. And while these pigments make the trees beautiful, they also serve an important purpose.
"Remember that trees are autotrophs; they make their own food through photosynthesis, and the leaves are often called the 'food factories' for the tree," Bezanson said.
Having a range of pigments that can target different wavelengths of light allows plants to harvest more energy during photosynthesis. It can be expensive to invest in so many pigments, so not all trees pursue this strategy. But those that do are often relatively fast growers.