All too soon, crisp fall days and frosty nights will give way to winter. In preparation for the seasonal change, trees and shrubs transform their leaves from a sea of green to a riotous combination of vivid reds, yellows, oranges, and purples. No two autumns are alike and it’s exciting to see how the foliage display varies from year to year. As the leaves change color, detach from their branches, and slowly drift to the ground below, we are left wondering about the science (or mystery) behind this annual phenomenon.
Why do trees shed their leaves in autumn?
With some exceptions, most deciduous trees and shrubs are genetically predisposed to shedding their leaves in preparation for winter dormancy. During the growing season, the leaves convert sunlight into energy. We all know this process as photosynthesis. But as days become shorter and temperatures cool down in autumn, complex physiological processes cause the leaves to start shutting down. As photosynthesis slows, vital nutrients (nitrogen and phosphorus) gradually move from the leaves to twigs and branches for storage during winter. Cells within the leaves begin to break down and the vessels that transport water to the leaf and sugars to the rest of the plant close off. This triggers the formation of an abscission layer at the base of each leaf where it attaches to a twig or branch. This is the point where the leaf will shed from the plant. The purpose of the abscission scar is to protect the plant from disease and cold weather once the leaf drops off.
What causes leaves to change color?
The colorful display that occurs before leaves drop off is due to a combination of three factors: day length, weather conditions, and changing levels of pigments in the leaves. Of these three factors, day length is the only one that is relatively consistent from year to year. Shorter days and longer nights are the initial trigger that starts the change in leaf colors and preparations for leaf drop.
Weather conditions can vary widely from hot, dry, and sunny to chilly, overcast and rainy. Hot, dry weather can cause early onset of color changes. Chilly, wet weather can postpone the color display. Meanwhile, the gradually lowering of temperatures is a key trigger that influences the onset of leaf changes.
Pigments within the leaves are responsible for the colors that are revealed in autumn. They are influenced by both day length and weather as to the onset of color changes and the intensity of colors.
Why do some leaves turn red while others turn yellow or some other color?
Three types of pigments are responsible for leaf color – chlorophyll, carotenoids, and anthocyanins. Here’s how they work in determining the various colors of autumn leaves.
- Chlorophyll: This pigment gives leaves their green color and is present throughout the growing season. Essential to the process of photosynthesis, chlorophyll absorbs sunlight and processes carbon dioxide from the air and water to produce carbohydrates (sugars and starch), which plants need for food. Chlorophyll is not a very stable compound and sunlight causes it to decompose with the result that it must be continuously regenerated over the summer months. As the days grow noticeably shorter and temperatures cool in fall, plants use chlorophyll faster than they can replace it. This reduction in chlorophyll causes the green color to fade allowing the other pigments in the leaves to become visible.
- Carotenoids: In addition to chlorophyll, carotenoids contribute to the capture of sunlight for photosynthesis. Carotene contributes to the orange colors in leaves, while xanthophyll contributes to the yellow colors. These pigments are present in leaves all summer, but they are masked by chlorophyll and don’t normally become visible until chlorophyll levels recede in autumn. However, they can become apparent during the growing season if chlorophyll levels are reduced due to nutrient deficiencies, drought, or disease.
- Anthocyanins: While chlorophyll and carotenoids are present in leaf cells throughout the growing season, most anthocyanins are not. They are produced in autumn in the cell sap. When anthocyanins are combined with sugars in the leaf cells, these complex compounds produce a range of pinks, reds, and purples in leaves. The color variances are influenced by cell pH. Reds result from acidic pH conditions whereas purples result from more neutral pH conditions.
The role of anthocyanins isn’t entirely clear. Unlike chlorophyll and carotenoids, anthocyanins don’t contribute to photosynthesis. However, they do appear to serve as a protective sunscreen, which prevents ultraviolet radiation damage to foliage cells. Also, anthocyanins help deter insect feeding and their darker hues make leaves appear less visible to herbivores.
While anthocyanins are typically absent in the leaves of most woody species until fall, there are some exceptions to the rule. Plant species that naturally display deeply hued purple or red foliage all season long owe their color to anthocyanins. A few examples include some varieties or cultivars of beech, smokebush, redbud, and ninebark.
In addition to the three types of pigments described above, tannins, which are common waste products of the metabolism process, also influence leaf color. Like chlorophyll and carotenoids, tannins are always present in leaves, but they don’t become visible until the chlorophyll and carotenoids disappear. In combination with carotenoids, tannins can enhance the deeper golden yellow hues visible in the leaves of some species such as beech or hickory. They are also responsible for the subdued brown color in the leaves of some tree species such as oaks and beeches.
What causes leaves to look beautiful one season and blah the next?
The intensity of fall leaf color varies from year to year depending on the availability of moisture, nutrients, sunlight, length of day, and temperatures. Those factors vary from year to year and explain why fall colors may be spectacular one year and disappointing the next.
The most vivid leaf colors are influenced by warm sunny days followed by cool (below 45°F) but not freezing nights. That’s because more sugars are produced on warm, sunny days, which means the more pigment available to leaves, the more intense the colors. Cool night temperatures cause sugars to remain in leaves further enhancing vivid leaf color, whereas warm night temperatures cause the sugars to move out of the leaves resulting in duller colors.
What are some examples of trees and shrubs that turn yellow, red, orange, or purple?
Some woody plant species are genetically predisposed to turn specific colors in fall. For example, because hickory and ginkgo trees do not develop anthocyanins in their leaves, they reliably turn golden yellow or bright yellow, respectively. Other species that do produce anthocyanins, such as maples, may turn various shades of yellow, orange, and red, depending on the variety and cultivar. Some plants such as Fothergilla and fragrant sumac can display a vibrant mix of colors or hues on the same plant.
A few examples of trees and shrubs that display showy fall color in Virginia include:
- Yellow – Birch, yellow poplar, ginkgo, hickory, clethra, spicebush, Ohio buckeye, sycamore, ash, witch hazel, and winterberry.
- Orange – Some crape myrtles (which can turn yellow, orange, or red, depending on cultivar), Fothergilla, black chokeberry, flameleaf sumac, and sugar maple (which may display yellow, orange, or red leaves).
- Red – Dogwood, redbud, sourwood, sumac, blueberry, chokeberry, some oak varieties such as scarlet oak and pin oak, and some maple species such as red or Japanese.
- Purple or burgundy red – Sweet gum, smoke tree, purple leaf plum, some ninebark varieties, oakleaf hydrangea, Virginia sweetspire, and some viburnum species.
Why do ginkgo trees drop all their leaves at once?
Typically, abscission scars form at different rates and in different parts of trees with the result that leaves fall off individually over a period of several weeks. However, ginkgo trees are unique because they form their abscission scars on their leaf stems simultaneously. A hard frost finishes the abscission process and triggers the leaves to fall all at once. That’s why you see a fully leafed out tree one day and completely bare limbs the next.
Why do some trees hang on to their leaves in winter?
Most deciduous tree species drop their leaves before the onset of winter. American beech, ironwood, musclewood, and some oak species are exceptions to this rule. Their leaves turn brown in fall but continue to cling to their twigs over the winter months due to a physiological process called marcescence. This term refers to the incomplete development of the abscission layer at the base of a leaf’s petiole in fall. The abscission layer doesn’t completely form until spring, at which point the newly emerging leaves push the old leaves off. Marcescence is typically observed on younger, immature trees and in the branches closest to the ground of more mature trees.
Why don’t evergreen or conifer species drop their leaves?
While deciduous plant species shed their leaves in autumn, most evergreen or conifer species in cold climates don’t. That’s because they are capable of withstanding harsh winter conditions. Their leaves or needles have a waxy coating that protects them from cold temperatures. Also, they have the equivalent of antifreeze in their leaf cells. A few needled species such as Larch and bald cypress trees are exceptions to this rule. Their needles both change color and drop to the ground in autumn.
What are some benefits of fallen leaves?
Colorful autumn leaves provide spectacular beauty and wonder to the landscape, but they provide more than just visual interest. As they fall, they carpet the ground where they gradually decompose and nourish the soil. On the forest floor, they become part of the spongy humus layer where they absorb and retain moisture from rain and snow. Early in the decomposition process, they provide shelter for overwintering insects as well as their larvae and eggs. Once they decompose, leaves become food for numerous soil organisms that are vital to our ecosystem. For these reasons, leaf drop is a vital contributor to the health of our forests and landscapes.
In Summary
Leaf drop and the intensity of fall foliage colors depend on a variety of factors: Shorter days, longer nights, cooler temperatures, ground moisture, plant hormones, nutrient levels, genetic predisposition, and more. The science behind how and why leaves change color in fall and are shed is mostly understood, but more remains to be learned about these complex processes and why they matter. In the meantime, enjoy the show.
FEATURED PHOTO: Fallen leaves carpeting the grass in autumn. Photo credit: Anita Breach
SOURCES
Changing Colors of Leaves, University of Tennessee Publication SP 529 by Wayne K. Clatterback, Assistant Professor Forestry, Wildlife and Fisheries
“Leave” them Alone: Lawn Leaf Management, Virginia Cooperative Extension 430-521
Science of Fall Colors, U.S. Department of Agriculture Forest Service
The Chemistry Behind the Color, U.S. Department of Agriculture Forest Service
Why do leaves fall off in autumn?, Purdue University Extension – Forest and Natural Resources
Why Fall Color is Sometimes a Dud, The Purdue Landscape Report, October 2019
Why Leaves Change Color — The Physiological Basis, Purdue University Extension Publication FNR-FAQ-5
Why Some Oak Trees Retain Their Leaves, University of Illinois Extension
