The vibrant colors of autumn leaves are one of the most beloved signs of the changing seasons. But why do green leaves turn yellow, orange, red, and purple before falling from deciduous trees in autumn? The shifting leaf colors are the result of various pigments that exist in leaves and changes that occur as winter approaches.
The Green Pigment Chlorophyll
During the spring and summer, leaves appear green because they contain a high amount of the pigment chlorophyll. Chlorophyll is essential for photosynthesis, the process plants use to convert sunlight into food. Chlorophyll absorbs blue and red light from the sun and reflects green light, causing leaves to look green.
| Pigment | Color Absorbed | Color Reflected |
|---|---|---|
| Chlorophyll | Blue, Red | Green |
Declining Chlorophyll in Autumn
As autumn approaches, deciduous trees like maple, oak, aspen and birch begin preparing for winter dormancy. The amount of chlorophyll starts to decrease in leaves. With less chlorophyll, the green color starts to fade and other pigments that have been masked by chlorophyll begin to show through.
Cooler temperatures and fewer daylight hours in autumn signal trees to stop producing chlorophyll. The chlorophyll in leaves breaks down faster than it can be replaced. Trees also reabsorb nutrients like nitrogen from leaves and transport them to branches, twigs and roots for winter storage. As a result, leaves have less raw material available to produce more chlorophyll.
Carotenoids Create Yellow and Orange
As green fades, the yellow and orange pigments called carotenoids become visible. Carotenoids are present alongside chlorophyll in leaves year-round, but chlorophyll overrides the yellow and orange hues. Xanthophylls and carotenes are the two major types of carotenoids.
Xanthophylls produce yellow colors in leaves. They absorb blue and green light and reflect yellow and orange. Common leaf trees like aspen, birch, cottonwood and elm leaves turn bright golden colors thanks to high amounts of xanthophylls.
Carotenes create more orange shades when chlorophyll is not present. They absorb blue and green light and transmit colors in the yellow to orange spectrum. Trees like maple and gum display more orange leaves because they contain high levels of carotenes.
| Pigment | Color Absorbed | Color Reflected |
|---|---|---|
| Xanthophylls | Blue, Green | Yellow, Orange |
| Carotenes | Blue, Green | Orange |
Anthocyanins Create Red and Purple
While carotenoids unmasked by chlorophyll create yellows and oranges, red and purple autumn leaf colors are produced by anthocyanin pigments. Anthocyanins form as autumn progresses and are not present in leaves year-round like carotenoids.
Anthocyanins develop in leaves in late summer and peak in early to mid-autumn. They act as a sunscreen by blocking harmful solar radiation and protecting leaf cells as nutrients retreat during senescence. The production of anthocyanins is influenced by the amount of sugars trapped in leaves as photosynthesis slows down.
Red maple and dogwood leaves turn vibrant red because they accumulate high amounts of anthocyanins. Other trees and shrubs like sumac, Virginia creeper and black tupelo can also have red leaves.
Purple leaves result when anthocyanins combine with the yellow carotenoids still present. Red maple can display breathtaking purple leaves while Japanese maple varieties are prized for brilliant crimson foliage.
| Pigment | Color Absorbed | Color Reflected |
|---|---|---|
| Anthocyanins | Green, Yellow | Red, Purple |
Why Do Some Trees Turn Color Earlier?
Peak autumn color varies by geographic location and tree species. At higher elevations, leaves turn earlier because colder temperatures hasten the end of chlorophyll production.
The same tree species can display different colors based on climate conditions. For example, red maples produce bright reds in sunny, dry climates and duller orange-reds in wetter areas.
Trees that turn color later in autumn have physical adaptations to protect leaves from early freezing. Sourwood trees have waxy coatings that resist freezing. Tulip poplar leaves have water conducting cells that prevent frost damage.
Some early color changers are non-native trees that are not adapted to prolong keeping their leaves. Bradford pear and silver maple are examples of trees that turn color ahead of most native species.
Why Leaves Fall From Trees in Autumn
After displaying fall foliage, leaves eventually die and fall from deciduous trees. Broad-leaved trees shed their leaves as an adaptation to withstand cold winter weather.
In autumn, the veins that transport water and nutrients to and from leaves gradually close off. This process is triggered by the same environmental cues that cause chlorophyll to break down.
A special layer of cells develops at the base of each leaf stem where it attaches to twigs. These cells weaken the leaf stalk attachment to the branch so leaves can detach and fall.
Fallen leaves on the forest floor return nutrients to the soil as they decompose. This enriches the ground for future tree growth after winter dormancy.
Conversions in Leaf Color by Tree Type
Different species of deciduous trees have characteristic autumn foliage based on their mix of pigments and adaptation strategies.
Maples
Maples contain abundant amounts of all the major fall pigments. They can display outstanding range of yellow, orange, red and purple hues. Red maples and sugar maples are renowned for brilliant autumn displays, especially in areas with sunny fall weather.
Oaks
The high tannin content of oak trees leads to more muted autumn colors in the bronze, red-brown and rust range. But pin oak and scarlet oak can develop vivid red leaves. White oaks turn purple-red while black oaks become orange-brown.
Aspens
Aspen trees are famous for vibrant yellow fall color because they contain lutein, a yellow xanthophyll pigment. Quaking aspen literally quiver in the wind due to their flat, flexible leaves set on long leaf stems. The fluttering movement may help discourage insects.
Birches
Birch foliage turns primarily golden yellow in autumn but can include some orange. Tree species like paper birch, river birch and yellow birch contribute to beautiful golden displays in parks and forests.
Dogwoods
Dogwood leaves are noted for deep burgundy and purple-red hues. Two Eastern US species, Cornus florida (flowering dogwood) and Cornus sericea (red osier dogwood), are especially prized for crimson and purple foliage.
Conclusion
The splendor of autumn leaves results from the interplay of color and chemistry in nature. As spring and summer fade, the green mask of chlorophyll recedes from leaves to uncover the yellows, oranges, reds and purples that add such beauty to the fall landscape. Understanding the science behind seasonal color transformations enhances our connection to the cycles of life on Earth.
