Every year, the long days of sunshine of summer make room for a different setting — but one that’s also spectacular. It’s autumn, which means the leaves on the trees first turn into different colors, before eventually falling off. While we might take this for granted and not think that much about it, it’s actually a very important process that allows deciduous trees to survive the winter.
Why leaves fall down
French author Albert Camus once said “autumn is a second spring when every leaf is a flower.” Leaves do create whole hillsides or mountainsides of beautiful colors — but nature often combines the sublime with the practical, and the stunning reds, oranges, and yellows of the autumn trees are a great example of this convergence.
Autumn is also known as “fall” because, well, it’s the season when leaves fall down. All around us leaves are turning yellow and looking a bit dry and crusty. So when a stiff breeze comes along, those leaves seem to “fall” off. It sounds reasonable and simple but there’s actually much more to it.
The wind doesn’t gently pull leaves off trees. Trees are more proactive than that: they throw their leaves off. So, instead of calling this season “fall,” trees would probably call Autumn the “get off me” season.
When the days get shorter and colder days, it trigger a hormone in trees, sending a chemical message for the leaves to the leave. Once they get the message, small cells appear at the place where the leaf stem meets the branch. They are known as “abscission” cells. The word shares the same root as the word scissors (from the Vulgar Latin cīsōria), as they are designed to make a cut. Within a few days or weeks, leaves develop a line of cells that push the leaf away bit by bit.
So while this may seem like leaves falling down, a group of substances (such as ethylene, abscisic acid, and the growth hormone auxin) direct the abscission process at the cellular level.
This process can only be observed with a microscope. If you used one, you would see the scissors cells lined up close to each other. The tree is essentially giving each leaf a push, leaving them increasingly hanging. They are ready to be kicked off. Then, a breeze comes along to finish the job.
The role of the leaves
Trees are programed by long years of evolution to push the leave to drop away. During the spring, summer and early fall, leaves make the food that helps the tree to grow, develop and reproduce. But when it gets colder and days are shorter food production slows down. Trees can then keep the leaves or let them go.
If they would keep the leaves permanently, they wouldn’t need to grow new ones. But it’s not that simple. If the winter has a break, leaves would start photosynthesizing. But then, when cold snap’s ack on, leaves would be caught with water in their veins, freeze and die. The tree would be stuck with food staff that is dead and could die.
A leaf has the important task of turning sunlight into food for the tree. To do so, the leaf needs water, which it gets from the soil and is sucked up all the way to the leaves. If there’s not sufficient water, the leaf can be damaged and stop working. The tree takes the nutrients from the leaf back into the stems and roots, recycling them.
Some trees lose their leaves every year. These trees are called deciduous trees, and they lose their leaves in response to the seasons. Deciduous trees mostly come from places where winter gets cold and snowy. When it’s very cold, the water in the tree can freeze and the leaves stop working.
These trees know to prepare for this and start taking nutrients out of the leaves when the days get shorter in autumn. This is when we can see them changing color. But there are also deciduous trees in tropical places, with a dry winter. When the rainy season ends, trees know they won’t have much water and let go of the leaves.
When a tree has no leaves, it can’t produce food. But this doesn’t mean it gets hungry. Instead, it rests, like a bear going into hibernation. Trees take a long sleep until the water in the pies starts moving again. This can happen in spring, or when it starts to rain again. Then, they wake up and put out new leaves to produce food.
The evergreen trees
Some trees hold onto their leaves all year long. These trees are called evergreens because they stay green all the time and the leaves don’t fall off. The reason is that the trees have a thick coating called resin that protects the leaves. This means they don’t freeze and rupture like the leaves of other trees when it gets cold.
Nevertheless, some do fall off sporadically throughout the year, especially when they are old or damaged. Leaves don’t work very well after they’ve been munched on by an animal. Some trees called semi-evergreens only drop their leaves when it’s cold enough. If the winter months are mild, they remain green no matter what.
The greater leaf longevity of evergreens means they can survive in environments that just don’t work for the deciduous trees. At higher latitudes and elevations, shorter and cooler growing seasons can limit photosynthetic activity. In these harsher conditions, a year may not be long enough for a leaf to produce enough energy to pay back its growth costs to the tree. For deciduous trees at least, it just becomes an unsustainable investment.
Meanwhile, evergreen trees have strong, needle-like leaves that conserve water better and can still carry out photosynthesis without expending too much energy. This is why their leaves stay ‘on’, even as their deciduous cousins shed their leaves.
This may explain why evergreen conifers dominate mountaintops and the boreal forests that stretch across high latitudes in Alaska, Canada and Northern Europe. Deciduous trees largely drop out of such habitats, as they can’t balance their accounts with respect to investments in leaves and leaves’ photosynthetic return in a single season.
The role of pigments
When leaves are green they are working as organs for the tree. The green color comes from a part of the cell called chlorophyll, which processes sunlight into sugars that the tree can ‘eat’. During the growing season, trees create as much chlorophyll as they can so leave stay green. But that doesn’t last forever.
As the weather gets colder and there is less sunlight for them to munch on, trees slow the production of chlorophyll until, finally, it stops. Producing more would be a waste of energy because, as temperatures near the freezing point, the process of photosynthesis slows to impractical levels.
While the green pigment retreats from the leaf, other pigments hidden in the greenery during warm months start to appear. Carotenoids, which produce the yellow, orange, and brown colors in the flowers of daffodils and the roots of carrots, are present in leaf cells throughout the growing season, but they’re masked by the green pigment — they’re revealed in the charming familiar colors of autumn.
Trees produce another pigment group, the anthocyanins, primarily in autumn. These pigments give red and purple to such things as blueberries, red apples, and autumn leaves. The role of these pigments is not perfectly understood. Possibly, their presence helps to lower the leaf’s freezing point, giving it some protection from cold and allowing leaves to remain in place longer.