Why Leaves change colour, How do trees know its time to change colour?

As the days grow shorter and the nights longer, nature puts on a spectacular display of colours. But have you ever wondered how trees know when it’s time to change their leaves’ colour?

Trees, in preparation for the cold winter days, undergo a transformation that results in a vibrant explosion of colours. This change, however, is not primarily triggered by the drop in temperature. Instead, it’s the light, specifically red and far-red light, that cues the change.

• Light: In the fall, the sun is lower in the sky and there is less sunlight.
• Temperature: Temperatures drop, and plants stop producing food through photosynthesis.

Plants have special receptors called phytochromes that can sense differences in daylight. During daylight, which contains red light, these receptors are active, promoting plant growth. As the day transitions into night, the receptors fall into an inactive state, and the plant halts growth.

As the days get shorter and there is less sunlight, leaves stop making chlorophyll.  Chlorophyll is a green pigment that absorbs light and transfers energy to energy-storing molecules.  When chlorophyll levels decrease, other pigments become visible, such as reds, oranges, and yellows.  These pigments come from chemicals called carotenoids and anthocyanin. 

How does this relate to leaf coloration?

Molecules called pigments color leaves. Chlorophyll, the pigment, causes the leaves to be green. Plants need Chlorophyll to make food using sunlight. In spring and summer, when there is plenty of sunlight, plants produce a lot of chlorophyll.

In autumn, as the weather starts to turn cold, some plants cease the production of chlorophyll. Instead, these plants initiate the breakdown of chlorophyll into smaller molecules. As the plants remove chlorophyll, other pigments begin to reveal their colours. This process is the reason why leaves turn yellow or red in the fall.

Why leaves change colour in the fall?

The vibrant colours of autumn leaves are not only a visual delight but also play a crucial role in the survival of trees.

Trees that shed their leaves in the fall, known as deciduous trees, usually have large, broad leaves. These leaves are green for most of the year due to the presence of chlorophyll, which is essential for photosynthesis. During this process, the leaves absorb energy from sunlight and convert it into sugars to nourish the tree.

However, as autumn approaches, the days become shorter and temperatures drop, leading to less direct sunlight. This causes the chlorophyll in the leaves to break down.

Thus, the breathtaking display of autumn colours is a complex biological process that helps trees conserve resources and survive through the winter months.

The location on the globe varies how much and how fast leaves transform. Dry, sunny and cool weather produces the best colours. Cloudy, damp or warm places do not experience the same degree of color change.

Then, of course, the leaves fall. As the weather turns, trees begin to build a protective seal between their leaves and branches. They absorb as many nutrients as possible from the leaves. However, the leaves wouldn't be able to survive the winter, making the trees vulnerable to damage if they remain. The leaves separate and drop to the ground when they get cut off from the fluid in the branches.

Trigger for autumn colour

The onset of autumn colours is influenced by a blend of day length and night temperature. As the days become shorter, the sugar produced by photosynthesis decreases, prompting hormones in the plant to initiate the process of leaf sealing and shedding. This process is hastened by cold nights, which also expedite the breakdown of compounds left in the leaf.

The ideal conditions for vibrant autumn colours are a warm, dry ‘Indian summer’ followed by colder nights. This keeps the leaves active for longer and shortens the leaf drop process, allowing more compounds to remain in the leaf and less time for them to break down to brown before the leaves fall.

In contrast, during a mild, wet autumn, the entire process slows down, often resulting in leaves turning completely brown before they fall. Additionally, individual trees and even single leaves may change color at different times, diminishing the overall spectacle.

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