Why Do Leaves Change Colour in Autumn? The Science Behind Fall Foliage

Each fall, we see the lush green trees turn brilliant shades of yellow, orange, red, and even deep purple. It can be stunning to witness, but why does it happen, and what determines the colour the leaves will change to? How do trees know when it’s time to shed their leaves?

The main reasons for these changes are shorter days and cooler temperatures, but other factors also influence the fall foliage . Everything you wanted to know about why leaves change colour is below:

Shorter Days

As summer comes to a close and fall begins, the days become shorter and there is less sunlight. Trees are sensitive to this change and use it as a signal to prepare for winter, triggering a series of physiological processes.

Cooler Temperatures

As temperatures drop, the chemical reactions within the leaves are affected. These lower temperatures, especially at night, act as a cue to begin the process of senescence, which is the gradual aging and deterioration of the leaves.

Chlorophyll Breakdown

Chlorophyll is the green pigment in leaves that plays a key role in photosynthesis, the process by which trees convert sunlight into energy. As the days shorten and temperatures drop, trees receive less direct sunlight, and the chlorophyll in the leaves begins to break down, revealing the other pigments present in the leaves.

Leaf Pigments

In addition to chlorophyll, leaves contain other pigments, such as carotenoids and anthocyanins.

Carotenoids produce yellow and orange colours and are always present in leaves. They are usually masked by green chlorophyll, but as it breaks down, the yellow and orange pigments become more visible.

Anthocyanins are not present during the growing season. They form when sugars become trapped in the leaves. Through a chemical process, these sugars create red and purple pigments, which are present only in certain tree species.

Tree Species

The exact colours and timing of the change can vary depending on the tree species. Each species contains different combinations of pigments and responds differently to environmental cues. For example, maples are known for their vibrant red and orange leaves in the fall, while certain oak species tend to turn yellow or brown. Oaks also retain their leaves much longer than many other tree species and may remain green even as surrounding trees change colour.

Other Environmental Factors

Decreased daylight and cooler temperatures are the primary reasons leaves change colour, but other environmental factors also have an impact. Soil moisture levels, rainfall, and the overall health of the tree all play a role in the intensity of fall colours. Trees under stress, whether from drought or disease, may display less vibrant autumn foliage.