Why do trees shed their leaves in the autumn?

21 September 2023

Deciduous trees shed their leaves in the autumn, turning from green to multihued to bare. But how do they do it? And, for that matter, why?

With less food and water on offer and life-threatening low temperatures, the winter is a hard time for most living beings; for many of them it’s just a matter of survival. This is why some animals hibernate in their burrows, others hoard supplies, and almost all of them put on a thick layer of fur. Deciduous trees do it the other way around: they shed ballast – in the form of their leaves. It’s through their foliage that trees evaporate most of their water. But during the dry winter season, their roots can draw less and less of it from the frozen earth. If trees had to keep supplying their leaves with precious H₂O over the winter, they would wither and die.

An average deciduous tree has some 30,000 leaves. An adult European beech produces 800,000, which amounts to 28 kilos of leaves per tree. A horse chestnut puts forth up to 25 kilos’ worth. For people with gardens and urban street cleaning services, this is an enormous amount of organic material that has to be disposed of. Berlin’s BSR refuse collection service gathers up some 36,000 tons of leaves in the capital every autumn; that's three and a half times the weight of the Eiffel Tower.

There’s a simple reason why these leaves are almost always colourful, but complex processes are at work in the background. Trees can’t afford to waste anything, which is why they extract as many nutrients as possible from their leaves in the autumn. They break down sugars and minerals and store them in their branches, trunk and roots as an important reserve for the coming spring. Nor do trees want to forgo their precious chlorophyll. This green pigment in their leaves is essential for photosynthesis. It allows them to absorb the energy of sunlight and convert it into oxygen and glucose using water and carbon dioxide extracted from the air. The trees use this energy-rich glucose to stay alive and grow. Oxygen, in turn, is what allows all of us to breathe.

In spring and summer, when the sun shines brightly for hours on end and rain and thunderstorms keep the soil moist, photosynthesis goes into full swing. In the dark seasons, however, the green branch-based power plants simply can’t produce enough, and at the same time, they become too much of a burden for the trees.

So, the trees shut down their metabolic function and begin to break down and store their chlorophyll. Now it’s time for the other pigments, which have thus far been obscured by the green of the chlorophyll, to take centre stage: these are known as carotenoids, to which carrots, for example, owe their orange appearance. Depending on the concentration of pigments, the leaves tend to turn yellowish, reddish or brownish.

However, if they are going to shed their leaves, the trees must first cut the umbilical cords that connect the leaves to their branches. They do this by sending phytohormones to the petioles. These molecules signal to the stems that it’s time to create specialist cells to cut off the leaves. These cells form a separating layer which causes the leaves to fall off. Only a few deciduous trees retain their dried leaves over the winter. These include native oak species and hornbeam. These trees do not create separating cells; instead, they just clog the nutrient pathways to the leaves with what are known as tyloses. It is due to these that the last leaves are often only blown off the branches by spring storms.

Unlike deciduous trees, almost all conifers remain green all year round. This is made possible by the firm skin of their leaves, which are also covered with a thick layer of wax. This significantly reduces evaporation, as does the small surface area of the needles, which consequently do not offer any purchase for frost. Many conifers can also actively close the tiny stomata in their needles, thereby regulating evaporation on sunny winter days. Because their prickly foliage contains bitter oils or even toxic substances, they are simply inedible for potential predators. The needles remain on the tree for many years until they fall off due to age and are replaced with fresh ones.

If conifers are so much more efficient, why don’t we plant them everywhere? This is what the forestry industry actually did from the beginning of the 19th century, sending fast-growing spruces to nurseries all over Germany. In times of climate change, however, the trend is to revert back to native deciduous trees or mixed forests. This is because beech or oak trees require less water than thirsty spruce. With their deep taproots, deciduous trees can also store more water in the soil and are better equipped to withstand storms. It is precisely because they generously evaporate this water through their leaves in summer that they act as biological air conditioners in overheated cities, helping create a fresher environment. Last but not least, there is much more happening around them on the forest floor than in coniferous woodlands, as their foliage is broken down by microorganisms into valuable humus, which in turn feeds many plant species and the animals which feed on them.

However, the best option – and not only for biodiversity – is forests with different types of trees. This is what an international team of researchers has discovered. Mixed forests are particularly well equipped to withstand extreme weather conditions, which are increasing at an accelerating rate with global warming. Not only that, though: compared to forests with a single tree species, they also bind almost three times more of the CO₂ which is being released into the atmosphere at much too great a rate. So, in woodlands, as elsewhere, diversity will boost the chances of our long-term survival and make everything much more colourful – and that not only in the autumn.