Credit: David Rodway / WTML
Why Leaves Really Fall Off Trees
You think you know why leaves fall off trees. Well, you’re wrong. It’s not the wind. It’s not the cold.
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It’s because trees use “scissors” to cut their leaves off.
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We call this season the “fall” because all around us right now (if you live near leaf-dropping trees in a temporal zone), leaves are turning yellow and looking a little dry and crusty. So when a stiff breeze comes along, those leaves seem to “fall” off, thus justifying the name “fall.”
Sounds reasonable, no?
But the truth is much more interesting.
According to Peter Raven, president of the Missouri Botanical Garden and a renowned botanist, the wind doesn’t gently pull leaves off trees. Trees are more proactive than that. They throw their leaves off. Instead of calling this season “The Fall,” if trees could talk they’d call this the “Get Off Me” season.
Around this time of year in the Northern Hemisphere, as the days grow shorter and colder, those changes trigger a hormone in leaf-dropping trees that sends a chemical message to every leaf that says, in essence, “Time to go! Let’s part company!”
Once the message is received, says Raven, little cells appear at the place where the leaf stem meets the branch. They are called “abscission” cells. They have the same root as the word scissors, meaning they are designed, like scissors, to make a cut.
The scissor cells are stained red and mark the boundary between the branch (left) and the leaf stalk. University of Wisconsin Plant Image Teaching Collection hide caption
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University of Wisconsin Plant Image Teaching Collection
The scissor cells are stained red and mark the boundary between the branch (left) and the leaf stalk.
University of Wisconsin Plant Image Teaching Collection
And within a few days or weeks, every leaf on these deciduous trees develops a thin bumpy line of cells that push the leaf, bit by bit, away from the stem. You can’t see this without a microscope, but if you looked through one, you’d see those scissors cells lined right up.
That’s where the tree gives each leaf a push, leaving it increasingly dangling. “So with that very slender connection, they’re sort of ready to be kicked off,” says Raven, and then a breeze comes along and finishes the job.
So the truth is, the wind isn’t making the leaves fall. It’s the tree.
The tree is deeply programmed by eons of evolution to insist that the leaves drop away. Why? Why not let the leaves stick around? Why drop?
Raven explains that leaves are basically the kitchen staff of a tree. During the spring, summer and early fall they make the food that helps the tree grow and thrive and reproduce. When the days get short and cold, food production slows down, giving the tree an option: It can keep the kitchen staff or it can let it go.
If trees kept their leaves permanently they wouldn’t have to grow new ones, but leaves are not the brightest of bulbs (sorry!). Every so often, when the winter weather has a break and the days turn warm, Raven says leaves will start photosynthesizing. “They get some water up and they start operating and making food and then it freezes again.”
When the cold snap’s back on, the leaves will be caught with water in their veins, freeze and die. So instead of a food staff that’s resting, the tree is stuck with a food staff that’s dead. And when spring comes, the permanent help will be no help. The tree will die.
That’s why every fall, deciduous trees in many parts of North America get rid of their leaves and grow new ones in the spring. It’s safer that way.
Why do leaves change colour?
Leaf colour comes from pigments. These are natural substances produced by leaf cells to help them obtain food. The three pigments that colour leaves are chlorophyll (green), carotenes (yellow) and anthocyanins (reds and pinks).
Certain weather conditions lead to more intensely coloured autumn leaves.
Credit: Laurie Campbell / WTML
Why is autumn colour better some years?
The depth of colour is influenced by the blend of chemical processes and weather conditions.
- Cold nights: low temperatures destroy chlorophyll so the green leaf fades to yellow, but if temperatures stay above freezing, anthocyanin production is enhanced and the leaves take on a red colour.
- Dry weather: sugars become concentrated in the leaves, more anthocyanin is produced and consequently leaves are redder.
- Bright sunny days: although the production of new chlorophyll stops in autumn, photosynthesis can still occur on sunny autumn days, using the remaining chlorophyll. Sugar concentration increases, more anthocyanin is produced and the leaves are redder.
Why do trees lose their leaves?
The beginnings of leaf drop, also known as abscission , start when a layer of cells is formed between where the leaf stalk joins the stem. This layer, known as the abscission layer, is formed in the spring during active new growth of the leaf.
In autumn, hormones within trees begin to change. The most notable is auxin. During the active growing season, production rates of auxin in the leaves are consistent with the rest of the tree. As long as these rates are steady, the cells of the abscission layer remain connected, which in turn, keeps leaves attached.
But as days shorten and temperatures cool, auxin production in leaves starts to decrease. This triggers cellular elongation within the abscission layer. The elongation of these cells creates fractures, allowing the leaf to break away from the plant. The leaf is finally blown off by the wind or falls from its own weight.
Leaves are finally blown off by the wind or fall from their own weight.
Credit: Michael Cooper / WTML
Why do leaves change color in the fall?
Autumnal leaves in vibrant hues are a beautiful part of the season, but those leaves are also a vital part of keeping trees alive.
Trees that have leaves that change color in fall are deciduous. (Evergreen trees with needles, which stay green to continue the photosynthesis process through the winter, are coniferous.) Deciduous trees usually have large, broad leaves.
Most of the year, these leaves are green because of the chlorophyll they use to absorb energy from sunlight during photosynthesis. The leaves convert the energy into sugars to feed the tree.
As the season changes, temperatures drop and days get shorter. Trees get less direct sunlight, and the chlorophyll in the leaves breaks down.
The lack of chlorophyll reveals yellow and orange pigments that were already in the leaves but masked during the warmer months. Darker red leaves are the result of a chemical change: Sugars that can get trapped in the leaves produce new pigments (called anthocyanins) that weren’t part of the leaf in the growing season. Some trees, like oaks and dogwoods, are likely to produce red leaves.
How much and how fast leaves transform varies by location on the globe. The best colors are produced when the weather is dry, sunny and cool. Places that are cloudy, damp or warm won’t see the same degree of changing color.
Then, of course, the leaves fall. Trees start building a protective seal between leaves and their branches as the weather turns. They take in as many nutrients as possible from the leaves, but leaves wouldn’t survive the winter and would make trees vulnerable to damage if they remained. When the leaves are cut off from the fluid in the branches, they separate and drop to the ground.
Learn more about fall tree changes from the Smithsonian Science Education Center, which brings science to K–12 teachers and students.
