Why Do Leaves Change Color in the Fall?

Which pigments give leaves their green, yellow and red colors?

Leaves change color in autumn because the amounts of the pigments inside them change as the leaves get ready to fall.^[3] Three main kinds of pigment are involved: chlorophyll, carotenoids and anthocyanins.^[2][7]

Chlorophyll makes leaves green and supports photosynthesis, the chemical process plants use to turn sunlight into sugars for food.^[2] Sunlight breaks chlorophyll down quickly, so through the warm summer months a leaf keeps making new chlorophyll to replace what it loses.^[5]

Carotenoids are yellow and orange pigments.^[6][7] They are always in the leaf, but for most of the year the larger amount of green chlorophyll hides them.^[6][7] UConn Extension calls the orange pigments carotenes and the yellow ones xanthophylls.^[5] Carotene is also the pigment that makes carrots orange, and xanthophyll is the one that makes egg yolks yellow.^[1]

Anthocyanins give reds and purples.^[2] Unlike the other two, they are not in the leaf during summer.^[5] The leaf builds them in autumn from sugars that collect in its tissue.^[5] At Harvard Forest, about 70% of shrubs and trees make anthocyanins as their leaves age.^[3]

The pigments also mix: anthocyanins with carotenoids give orange shades, and anthocyanin with chlorophyll gives brownish ones.^[3] Plain brown comes from tannins, a waste product that builds up in the leaf.^[5]

What triggers the color change in autumn?

Day length is the trigger: shorter days signal the tree to stop making food, and it stops making chlorophyll.^[6] The National Park Service adds cooler temperatures to that signal.^[2] As the chlorophyll is lost, the yellow and orange pigments, which are more stable, stay in the leaf and become visible.^[5]

Trees begin preparing before any color shows: in northern latitudes the process starts as early as late July, when days are already about 30 to 45 minutes shorter than at the June solstice.^[1] With less sun on their leaves, trees slow photosynthesis and start drawing nitrogen back out of the leaves.^[1] By the end of September, more than half of a leaf's nitrogen has moved into the tree's woody tissues.^[1]

Cool nights then cause cells at the base of each leaf stem to form a corky barrier between the leaf and the tree, known as the abscission layer.^[1][5] As the layer forms, sugar can no longer move freely out of the leaf, yet the remaining chlorophyll keeps producing it.^[5] When sunny days are followed by cold nights, the trapped sugars are turned into red pigment.^[6]

Why are fall colors brighter in some years?

The National Park Service names three factors with the biggest effect on a foliage season: day length, temperature and precipitation.^[1] Red anthocyanin needs sunlight and sugars to form, and weather plays a key role in how much of it a leaf makes.^[1]

• Temperature. Warm, sunny days followed by cool nights between freezing and 45°F (0 to about 7 °C, by BodaWiki's conversion) favor anthocyanin.^[7] Cool nights limit how much of the stored sugar the leaf uses up.^[1] An early frost weakens red pigment production.^[5] Hard frosts rupture plant cells and can make leaves drop early.^[1]
• Sunlight. Sunny days in early fall maximize photosynthesis, while cloudy, rainy autumns bring fewer reds.^[1]
• Moisture. Soil moisture, along with elevation, direct sunlight and temperature, influences the sugar concentration in leaves, which helps determine both the color of the pigments and how bright they are.^[2] Prolonged drought and extreme heat stress trees, and their leaves can shrivel and turn brown before they color.^[1] Too much rain in the weeks before fall also stresses trees and can reduce the pigments.^[1]

Sources describe the best pattern slightly differently.^[5][7] NC State Extension says the most vivid colors follow a warm, dry summer and early autumn rains that prevent early leaf fall.^[7] UConn Extension says the showiest seasons have dry, sunny days and cool nights.^[5]

Why do different trees turn different colors?

Species differ in the pigments their leaves keep or make.^[4] Trees that turn yellow do so because chlorophyll breaks down and some carotenoids stay behind.^[4] Trees that turn red make anthocyanins, in amounts that vary.^[4] How much red forms depends on the weather, the tree species and the acidity of the leaf.^[5]

Sugar maple leaves contain xanthophyll, carotene and anthocyanin, so they can run through the whole range.^[1] On red maples, color can differ sharply within one tree or even within a single leaf.^[3]

Why do leaves fall, and why do evergreens stay green?

Broadleaf trees get ready for winter cold by losing their leaves so they will not freeze when temperatures drop.^[6] The abscission layer is a weak band of cells that cannot hold the leaf, so wind and heavy rain pull it off.^[6] The tree heals the break, leaving a leaf scar on the twig.^[7] On the ground, the leaves decompose and return elements such as calcium and potassium to the top layers of the soil.^[7] Some trees hold onto their leaves until new ones begin to push out in spring.^[5]

Conifers can get through dry, freezing winters with their needles on.^[8] Penn State's Center for Private Forests gives five reasons: a waxy coating on the needles, stomata (openings in the needles) that close tightly to limit water loss, stomata set in pits or grooves, fewer stomata than deciduous leaves have, and natural "antifreeze" compounds that lower the freezing point of water in the cells.^[8] Keeping their needles also lets these trees make food during warm winter spells above 45°F (about 7 °C, by BodaWiki's conversion) and begin photosynthesizing earlier than deciduous trees.^[8]

Evergreens still shed: all of them drop some of their older leaves or needles every year.^[8] Harvard Forest notes that each autumn, one-third to one-half of a white pine's needles turn yellow-brown and drop.^[4] Not all conifers are evergreen, either.^[8] Larches turn bright yellow and lose all their needles in the fall.^[4]