The Science of Autumn

Paola Andrade, Staff Writer

Why do leaves change colour in the fall?

125588-Autumn-Leaves-On-StepsHumans pose themselves many seemingly unimportant questions. We instigate countless investigations with no other goal than to satisfy the ceaseless curiosity with which humans have been blessed. Thanks to this, we know that there is a reason why the beautiful earthy colours of the leaves are repainted with a reddish palette every fall. Many factors influence this gentle change between seasons, such as the leaves’ pigments, the weather and the humidity.

In autumn, the days shorten and the nights lengthen. The lack of sunlight greatly affects the production of three pigments in the leaves: chlorophyll, carotenoids, and anthocyanins. As we know, the chemical in charge of photosynthesis (a process in which a plant turns sunlight into sugars) is chlorophyll. When the amount of light is reduced, so is the amount of chlorophyll being produced. In the spring and summer, it is abundant. But the lively green of the chlorophyll fades with fall. Carotenoids also participate in photosynthesis, and once the chlorophyll starts to break down, we are left with the visible yellow carotenoids. Finally, anthocyanins —which reflect red— appear when the production of carotenoids lessens, resulting in stunning scarlet tonalities.

Concerning the “brightness” of leaves, humidity takes the lead. If there’s a lack of it, leaves will turn brown and fall quickly. But cool temperatures overnight lead to a longer period of coloured leaves. If presented with warm and somewhat sunny days, the weather promotes the appearance of purple and red pigments. Under these conditions, the most marvellous colours will show. If winter decides to make his arrival earlier than usual with a frost, the colours will be finished, destroyed by the cold. On the other side, if there is a drought in the spring or summer, the process may speed up to the point where it would not allow the leaves to change colour. Instead, they would fall directly to the ground before changing colours. Rain and heavy wind may also have the same effect.


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