As we approach the end of summer and the days grow shorter, trees begin to prepare for the winter season by shedding their leaves. Before they fall, however, the leaves undergo a beautiful transformation in their color, marking the start of the fall season! Fall begins around September 22nd 2024, so you might have already noticed some leaves changing color to beautiful shades of orange, red, and yellow! Have you ever heard the crunchy sound under your feet while walking? It is the fall leaves!
When I first came to Canada, I was fascinated by the color changes of the leaves. In my home country, which is mostly desert, there are trees and plants that remain green all year round! Therefore, it was such a mesmerizing experience seeing the leaves change into vibrant colors! I remember playing ‘the floor is lava’ with my sister on our way to school. We would step on the leaves to avoid ‘lava’ and enjoyed the satisfying crunch of the leaves under our feet!
In this blog, I will explore the science behind why and how leaves change color in the fall season! There are three main reasons that influence this color change: leaf pigments, length of the night, and weather. We will go through each section to deepen our understanding! Make sure you also read the previous blog since the photosynthesis process was discussed in greater detail there!
Leaf Pigments
Leaves contain three main pigments: chlorophyll, carotenoids, and anthocyanins. Chlorophyll, which is the green pigment, absorbs the energy from sunlight that helps convert carbon dioxide and water into oxygen and glucose in a process known as photosynthesis. The plant uses the glucose as its source of food. However, as the fall season begins, leaves stop making food (glucose) and the chlorophyll begins to break down. This causes the leaves to become faded and brittle, which is why you hear that crunchy sound when stepping on them! As chlorophyll fades, other pigments such carotenoids, and anthocyanin begin to appear!
The beautiful orange and yellow color we see comes from carotenoids. These pigments have always been present in the leaves all along, but were hidden by the dominant green chlorophyll. So once the chlorophyll breaks down, yellow and orange colors start to appear due to carotenoids!
Anthocyanins, on the other hand, are responsible for the red color we see in leaves! This pigment appears when the sugars accumulate in the leaves as they begin to prepare for winter, resulting in the red color. Unlike carotenoids, anthocyanins are not present during the growing season of the leaves, but appear as the fall season begins.
Length of the Night
As the fall season progresses, the days get shorter and the nights get longer. You can feel this impact in December when you return home from school and it is already very dark outside! Since the leaves require sunlight for the proper production of chlorophyll, its production begins to significantly slow down and eventually stops. As chlorophyll fades, carotenoids and anthocyanins take over the leaves, leading to the vibrant red color we see!
Weather
So why do trees shed their leaves during fall? Trees want to survive! During the winter season, water becomes less available due to the cold temperatures and nutrient absorption by the roots decreases. By shedding their leaves, trees lose less water, which helps the trees conserve and protect their resources during the harsh winter season. So the next time you take a walk around your school campus or neighborhood, think about the significant changes all the trees around you go through every year! And do not forget to enjoy the beautiful colors of the leaves during your stroll!
Sources
https://www.esf.edu/eis/eis-leaves-color-change.php https://www.fs.usda.gov/visit/fall-colors/science-of-fall-colors https://silvotherapy.co.uk/articles/why-trees-loose-their-leaves-in-autumn
