After a hot, muggy summer, nothing feels better than sensing a cool bite in the air, sipping a pumpkin spice latte, and hiking through a forest of brilliant crimson, orange, and gold foliage. The beautiful tones of the autumn woods conceal a complex chemical process that protects each tree from winter temperatures and allows leaves to grow again in spring. Here’s a look at the behind-the-scenes science of fall foliage.
Three Factors Make Leaves Change Color
Three main factors work to change leaves’ color: the amount of sunlight the tree receives, the weather, and the pigments in the leaves themselves. The most important factor is the decrease in daylight and increase in darkness that occurs in September and October in the Northern Hemisphere. The longer nights and cooler temperatures jump-start the chemical processes that take place in the leaves, resulting in their change from green to red, yellow, purple, orange, or tawny brown.
Weather conditions are also in play. The most vibrant colors emerge after an early fall marked by cold and sunny weather. Unusual rain or heat, conversely, may impede the timing and intensity of fall hues. In 2019, for example, mild temperatures and moderate rainfall delayed fall foliage in the Northeast United States. Summer droughts can shift the timing of the color change earlier or later, too. A bad drought in New England in 2016 made trees change earlier than usual and turn brown rather than red or yellow. But at the same time in the South, a milder drought caused leaves to turn about a week later than normal.
Scientists are still learning about why the lack of rain can affect colors in different ways, and climate change promises more uncertainty in the timing and brightness of fall foliage.
How Fall Colors Emerge
Throughout spring and summer, a tree’s leaves undergo photosynthesis. Chlorophyll, the green pigment in their cells, absorbs sunlight and uses that energy to break down carbon dioxide and water. That process creates glucose, which feeds the tree, and oxygen, which the tree expels through its leaves.
As days get shorter, photosynthesis slows down, and the tree redirects its energy away from the leaves. Special cells begin to form a barrier, called the abscission layer, that blocks off each leaf from the rest of the tree. This layer protects the nub of the branch where the leaf will eventually fall off, but it also prevents the renewal of chlorophyll in the leaves. As a result, the green pigment fades away, and other pigments in the leaves come to the fore.
The Proportion of Pigments Determines Color
In addition to green chlorophyll, the leaves of deciduous trees can contain two other types of pigments: carotenoids and anthocyanins. Carotenoids emerge as the chlorophyll fades and appear as gold, orange, or tan hues (it’s the same stuff in carrots, corn, and bananas). Some trees also produce anthocyanins during the color-turning process. These compounds are antioxidants that produce red, blue, and purple tones in leaves as well as blueberries and grapes. The formation and ratio of pigments in a tree’s leaves depend on the season’s temperature, amount of drought or rain, and available sunlight, so each fall brings a slightly different look.
As a leaf ages, the carotenoids and anthocyanins break down. Brown-toned tannins — a chemical in cell membranes — then become visible.
Color Palettes of the Northern Hardwood Forests
Obviously, evergreens such as pine, fir, and spruce trees remain green throughout the year, but each species of deciduous tree has its own basic colors. Trees that turn dazzling shades of red include tupelo, dogwood, sweetgum, sumac, some oak, red maple, and sugar maple. In the yellow and gold category are hickories, aspens, tulip trees, and black maples. Beech and some other oaks don’t have much pigment and turn varying shades of tan and brown.
Depending on which species of trees are present, a forest’s color can be more yellow, more red, or more dark green. The most variegated colors appear in mixed hardwood forests, such as those in New York’s Adirondacks and New Hampshire’s White Mountains, where the species include red spruce, balsam fir, red and sugar maples, yellow and white birch, red oak, and American beech. The southern Appalachians are also known for their eye-popping colors, thanks to their northern hardwoods, plus basswood, dogwood, and magnolia trees.