As autumn approaches, many trees put on a spectacular display of vibrant colours before shedding their leaves. Making for one of the most beautiful seasonal changes in any garden as deciduous trees turn their colours from green, to orange, red, gold and yellow. It begs the question why do trees drop their leaves?
This beautiful phenomenon is not just for show; it's a complex and vital process that helps trees prepare for the harsh winter months and helps them survive year after year. Understanding why trees change colour and lose their leaves can deepen our understanding of these natural events, making us better gardeners. It also sheds light on the intricate relationship between trees and their environment when we plant them in our garden designs.
You may be wondering why trees don't just drop the leaves when they are green and skip the colour change when they move into autumn. However, there is more magic behind the scenes that trees would love for you to understand Ninjas!
The colour change in tree leaves is primarily due to the breakdown of chlorophyll, the green pigment responsible for photosynthesis. This pigment helps all plants make energy, such as sugar, which allows them to put on growth each year and produce flowers, fruits or nuts.
During the growing season, chlorophyll is abundant and masks other pigments present in the leaf. However, as daylight hours shorten and temperatures drop, trees prepare for winter. They gradually stop producing chlorophyll, allowing other pigments—such as carotenoids and anthocyanins—to become more visible.
These pigments are responsible for the yellow and orange hues in leaves and are present in the leaf throughout the growing season. They become visible once chlorophyll breaks down. Carotenoids are also found in carrots and corn, giving them their distinctive colours.
These pigments produce red, purple, and crimson colours and are not usually present in the leaf. They are formed in late summer in response to bright light and excess sugars trapped in the leaves. Anthocyanins can protect the leaf from sunlight and act as a defence against herbivores.
These compounds cause the brown colour seen in some leaves. Tannins are left behind in the leaf when chlorophyll and other pigments have been depleted. You can see below the brown leaves of an elm and a hawthorn tree.
Trees shed their leaves to conserve water and energy during the winter months. In cold climates, the water in the ground can freeze, making it difficult for trees to absorb it. Losing leaves reduces the surface area through which water is lost, helping the tree survive in dry or freezing conditions. Additionally, the loss of leaves reduces the weight and wind resistance, which can be beneficial during winter storms. It acts as a protective measure during the trickier months when food is in short supply and environmental factors like harsh winds risk the tree from losing too much moisture if still in leaf.
Only deciduous trees lose their leaves. Evergreens do not drop their leaves in winter primarily because their needle-like leaves are adapted to withstand cold and dry conditions.
These leaves have a waxy coating that reduces water loss and are shaped to minimize snow accumulation, preventing damage. Additionally, evergreens can photosynthesize during mild winter days, allowing them to continue producing energy throughout the season. This adaptation helps them conserve resources and survive in environments where conditions can be harsh.
During winter, photosynthesis is limited due to shorter days and less sunlight. By shedding leaves, trees reduce their energy requirements. Deciduous trees, which lose all their leaves in the fall, enter a state of dormancy where their metabolic activity significantly decreases. This is a period of rest and recovery where the tree's transpiration (the method of taking up water and then expelling) reduces significantly, so the tree needs less energy to survive day to day. Think of it as a hibernation state!
Leaves are vulnerable to damage from cold temperatures and frost. Ice can form within the leaf, damaging cell structures and leading to dehydration. By dropping their leaves, trees protect their branches and trunk from potential harm. It also makes the tree less likely to be blown over in strong wintery winds as there is less surface area for storm winds to impact.
The process of shedding leaves is called abscission. A special layer of cells, known as the abscission layer, forms at the base of each leaf stem. This layer gradually severs the connection between the leaf and the tree, causing the leaf to fall off. This is a highly controlled process that helps the tree seal off the point of detachment to prevent water loss and infection.
Trees decide on the timing of leaf abscission through a complex interplay of environmental cues and internal chemical signals. This is in part impacted by the weather and environmental factors which are increasingly in flux due to global warming and climate change.
As daylight decreases and temperatures drop in autumn, trees begin to prepare for winter by reducing their photosynthetic activity. This triggers the production of hormones like ethylene and a reduction in auxin levels, which initiate the formation of the abscission layer—a specialized layer of cells at the base of the leaf stalk.
The timing of leaf colour change and shedding is influenced by environmental factors like temperature, light, and moisture. Trees have evolved to use these cues to prepare for winter. For example, unusually warm fall temperatures can delay the colour change and leaf drop, while drought conditions may cause leaves to fall earlier.
The shortening of days is the most reliable signal for trees to start the process of leaf colour change and shedding. This is also known as photoperiodism.
Generally, the process begins when daylight falls below approximately 12 hours daily. This reduction in light signals the trees to prepare for winter by reducing chlorophyll production, leading to a change in leaf colour and, eventually, the shedding of leaves.
This response to photoperiodism varies among species and geographical locations, but the shortening of days, combined with cooler temperatures, initiates the abscission process. Some trees may start this process with slightly more daylight, while others may wait until daylight is closer to 10-11 hours per day.
Cooler temperatures contribute to the breakdown of chlorophyll, accelerating the colour change process. However, an early frost can damage the leaves, turning them brown before they reach their full color potential. Deciduous trees lose their leaves when temperatures consistently drop below 7°C/ 45°F.
Trees typically begin to show autumn leaf colour when the soil moisture content drops to around 50% of its available capacity. This reduction in moisture stresses the trees, which can trigger earlier and more vibrant displays of autumn colours. When the soil is too dry, trees enter a state of water stress, prompting them to begin shutting down for the winter season.
No, we understand what makes trees change their colour. Let's look at my top five trees for insanely bright autumn displays and leaf colours. If I only had to pick five trees to display the best autumn colour, these would be them!
Let's start with the biggest and, in my opinion, greatest autumnal tree: a mix of yellow, orange, red, and purple. This whopper grows to a huge 30-40m over 50 years!
If you want to grow a Sweetgum, you need space. They prefer moist, well-drained soils. They are tolerant of clay and sandy soils but not drought-tolerant, which will cause them to drop their leaves prematurely.
The most infamous of autumnal colours are the Japanese maples or Acers. Great for gardens of all sizes as they are slow growing and can be pruned to keep them neat and tidy. I've even grown them in container gardens in small urban spaces. Ranges from deep red and burgundy to orange and golden yellow.
They hate the wind, so keep them sheltered, or they will be scorched and drop their leaves mid-season. Acers prefer well-drained, rich, and slightly acidic soils; sensitive to waterlogged conditions.
Another big tree for larger gardens, they get to 18m tall in some cases, but a real stunner and autumnal head turner. Deep red and russet brown, with some leaves turning dark purple.
They thrive in well-drained, acidic to neutral soils and are adaptable to various soil types, including clay and sandy soils.
Ginkgo trees are often referred to as "living fossils," as they have existed for over 200 million years, making them one of the oldest tree species on Earth. This remarkable longevity highlights their resilience and adaptability throughout changing climates and environments. In autumn, they have beautiful bright yellow leaves. This is a truly amazing tree, so please indulge me as I explain the history of the Ginko!
The leaves of the Ginkgo tree are unique and easily recognizable, characterized by their fan-shaped structure. Ginkgo trees are dioecious, meaning there are distinct male and female trees. Only the male trees produce pollen, while the female trees produce seeds enclosed in fleshy, fruit-like structures that emit a strong smell when they fall to the ground. They stink!
These trees are remarkably resilient to urban pollution, pests, and diseases, making them a popular choice for city landscapes. They can thrive in harsh conditions where other species might struggle. In addition to their ornamental value, Ginkgo leaves are used in herbal medicine and are said to improve memory and cognitive function.
Culturally, Ginkgo trees symbolize longevity and endurance, often planted near temples and parks. They are relatively fast-growing, especially during their youth, and can reach impressive heights of 15 to 24 meters. Ginkgo trees are adaptable, thriving in various soil types and conditions, including both acidic and alkaline soils.
Adapts well to a variety of soil types; prefers well-drained, sandy or loamy soils.
One of the only deciduous (leaf dropping) conifer forms, this beauty turns bright yellow before dropping its needles in winter. This unique characteristic allows them to conserve water and energy during the cold months, making them well-adapted to their native wetland habitats in North America. Another tree that has a lot of growth and is fast growing. This tree will easily reach 20m plus over ten years!
Larch or Larix Prefers wet, acidic soils; often found in bogs and wetlands but can adapt to drier sites.
The term "fall" originated in England as a poetic way of describing the season when leaves "fall" from trees. It was used alongside "autumn" during the 16th and 17th centuries. The phrase "fall of the leaf" was a common way to refer to the season, and over time it was shortened to "fall."
While "autumn" continued to be used more widely in England, "fall" became the preferred term in North America, especially as British settlers brought the word to the New World.
This split between the term across the pond happened because linguistic developments in the United States and England started to differ after the American colonies gained independence. "Fall" remained popular in American English, while "autumn" was favoured in British English. Today, "fall" and "autumn" are often used interchangeably in the U.S., but "autumn" remains more common in the U.K.
If autumn is in the air, why not consider getting snug indoors and learning more about how to design your own garden?
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The transformation of leaves and the shedding process is a survival strategy that has evolved over millions of years. It allows trees to protect themselves from the adverse conditions of winter and conserve resources for the coming growing season. Understanding these natural processes helps us appreciate the intricate ways in which trees interact with their environment, showcasing their resilience and adaptability.
I find it fascinating that the changing colours and falling leaves are not just a seasonal spectacle but a testament to the tree's ability to adapt and thrive in a variety of climates.
Happy Gardening!
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