Green has given way to russet and gold. Leaves flurry, like sepia-toned snowflakes. The beauty of autumn can seem like magic, but like most natural wonders is steeped in scientific reasoning. So, why do leaves fall in autumn?
Many of us would feel lost without the cycles of nature. Day and night, winter and summer entwine with our own circadian rhythms. The changing seasons influence our thoughts, feelings, fashions, and foods. Like us, nature responds accordingly to shifts in temperature and light. Leaves fall, animals hibernate, and some plants retreat into their seeds.
Just like not all animals hibernate, not all trees drop their leaves. Some, like members of the Pinus genus (pine trees), hold onto their leaves through winter. They are tough, slender structures built to withstand wind, rain, cold and snowfall. They can survive months covered in snow due to their bendy branches and small surface area.
Deciduous trees shed their leaves annually in autumn. Before the leaves fall, they put on a dazzling display of colours. After falling, they cover the ground, providing a natural mulch and returning their organic matter to the earth.
Why do leaves fall in autumn?
Like everything in nature, there is a scientific explanation. Processes, growth and behaviours take time and energy. Many millions of years of evolution have calculated and fine-tuned these expenses. Organisms such as trees rarely do things that aren’t in some way profitable to themselves.
When leaves fall in autumn, they remove the need to protect themselves from the cold. The leaves also fall to the ground, creating a thick mulch of organic matter. This helps protect the soil, feed the soil life, and protect the trees’ roots from erosion.
Why do Leaves Change Colour?
Leaves are remarkable structures, perfectly designed to harvest sunlight. The process of photosynthesis takes place within leaves, literally converting gas into plant mass. During the summer, leaves take in carbon dioxide and convert it into oxygen and carbohydrates, using energy from the Sun. Finally, they expel the oxygen, allowing the rest of life on earth to survive, and use the carbohydrates for growth.
Leaves are perfectly designed to capture sunlight. Leaves are green because of a pigment called chlorophyll. This pigment reflects green light, so this is the colour we see. Before leaves fall, they go through a process called senescence. During senescence, chlorophyll is broken down and reabsorbed by the tree to be stored for spring. One of the most valuable and often limited nutrients to trees is Nitrogen, and chlorophyll is rich in Nitrogen. As the chlorophyll breaks down, it reveals other pigments, such as anthocyanins and beta carotenes, that appear red and yellow, respectively.
These secondary pigments are better at collecting the longer wavelength lights we see in autumn and winter. By harvesting this light, rather than breaking down like chlorophyll, they can protect the leaf from sun damage, acting almost like sunscreen and preventing free radicles from building up. Free radicles are unstable atoms that cause ageing in humans and similar damage to plants. Thus we see myriad colours in autumn that were there, within the leaves all year, but not visible over the green.
The primary reason is to protect the tree from cold. Many higher-latitude trees have incorporated this process into their yearly schedule. Firstly, protecting a delicate structure such as a leaf from the cold requires a plant to produce a protein-based antifreeze. This is costly to make. So, where the latitudes are higher, and there is less sunlight and warmth to power photosynthesis, the leaves are not worth keeping.
Once the reabsorption of the essential nutrients has taken place from the leaves, the tree forms a seal between itself and the leaf, where it joins the stem. The tree will either hold onto the leaf during winter – like young Beech trees – or drop them to the ground.
The trunk and branches of the tree are rich in sugars and proteins and enclosed by bark. This makes the woody parts of the tree frostproof. They can ride out winter, bare and dead looking, waiting to burst into life in spring. Whilst they may look like nothing is happening, anyone who has planted a bare-root tree will know that trees concentrate their growth beneath the soil’s surface in their root system during winter.
Some plants choose to retain their leaves through winter. This is also a carefully calculated adaptation. This allows the plant to go on photosynthesising throughout winter. Plants adapted in this way are often native to warmer climates or have large leaves or thicker leaves that are costly for the plant to produce. Sometimes it isn’t immediately obvious why they choose this method, only that it must be the best thing for them.
The other adaptation plants have to the cold is that of annual plants. These plants retreat into seeds to ride out the winter in their embryonic form. They are encased in a hard seed coat and sealed with a tiny amount of energy to start their growth. Seeds are the perfect vessel to ride out winter.
Perennial plants die back in winter, losing their leaves and green stems, withdrawing into the soil where they wait as bulbs, corms, rhizomes or roots. However, evolution has taught plants to seek opportunities and to minimise risk. So they find a way of life that maximises their ability to grow and reproduce.
When Do Leaves Fall?
Leaves fall or begin to senesce when they receive an environmental trigger. This is usually when they sense a shortening in the number of daylight hours. In reality, plants are measuring the night’s length, not the day’s length. Leaves are so sensitive to daylight hours that turning a light on in your greenhouse in the middle of the night for just a few minutes can “trick” plants into thinking the nights are short. This makes the plant respond as though there are more daylight hours, focussing on flower and leaf production (note: I do not recommend tricking your plants!).
As I mentioned earlier, some trees drop their leaves in autumn, allowing a frost and the organisms in the soil to break the leaves down, adding humus to the ground.
Other trees will hold onto their leaves after they have died back. This is called marcescence. We can see this in beech hedges that hand onto their brown leaves through winter, only to dispense them in large numbers in spring. If you have a garden with a beech hedge, you will be familiar with the mess they make. Why this happens is not fully understood. It is hypothesised that it is designed to suppress seedlings from germinating in spring. This theory has not been proven, but it is popular in the scientific world.
What Happens to Fallen Leaves?
We, gardeners, like to make leaf mould with fallen leaves. It is a soft, crumbly texture and makes an excellent seed compost. In addition, fallen leaves are low in nutrients, making them good seed compost.
When leaves fall to the ground, they slowly break down and become part of the soil. Soil is a mixture of sand, silt, clay and organic matter. Leaves add to the organic matter. Another component of the organic matter in the soil is alive. There are many billions of tiny organisms living within the soil. These organisms interact with our plants’ roots and release nutrients from the soil.
The organisms in the soil feed on fallen leaves and other dead organic matter, along with carbohydrates fed into the soil by living plants. In coming to the surface to feed, organisms such as earthworms open up spaces in the soil. Their movements and secretions stick together sediments forming peds and opening up pores.
Soil organisms pull the organic matter down into the soil, meaning that we do not get a vast build-up of leaves year after year. Instead, this organic matter feeds an abundant and essential ecosystem.