Updated: Dec 20, 2022
Autumn’s role as the transition between summer and fall is expressed magnificently through its bright and vibrant colors. In the summer, the many trees of the forests paint the environment with green, but when autumn comes, the colors change to individual shades of red, orange, brown, and yellow. So why do these colors change? And why do we have these colors at all? Let's get into it.
The colors in plants are the colors that are reflected away from their leaves and to our eyes. So the greens we see are because that is the only form of visible light that is not absorbed for photosynthesis. This green comes from little organelles in the cells of the leaves called chloroplasts. More specifically the chlorophyll that is within them. This is where photosynthesis takes place which is how the plant converts sunlight energy into sugar.
In Autumn, the angle of the earth is positioned in such a way that causes the sunlight to reach us for a shorter amount of time. The sun is also at a lower angle in the sky, dispersing its energy over a greater area. All of this means there is less sunlight to produce sugar, so it is more energy efficient to drop all their leaves. To do this, the tree cuts off the nutrient flow to the leaves and starts to store its excess sugars into its roots which will be used to make new leaves in the spring.
Because of this, the green chlorophyll dies and decays, which reveals the colors that have been in the leaves before and some only produced as the season progresses. Yellow and orange are from pigments in the leaves called carotenoids, which are always in the leaves but masked by the chlorophyll. These pigments are also found in other familiar colorful plants such as corn and carrots. The red anthocyanins, also found in cherries, are produced during the transition to autumn by trapped sugars in the leaves that did not get absorbed into the tree before the connection was cut off. And of course leaves can have different variations of these pigments leading to different colored leaves.
Above Photo by @matthewraifman
The environment can also impact the colors we see. The temperature needs to drop for the red anthocyanins to form but if we have below freezing temperatures too early in the season that blocks their formation. Drought can cause the leaves to drop without changing color because without water trees cannot photosynthesize and the tree starts dropping branches to conserve energy. This is why some years and some locations are more colorful than others.
The colors of autumn are one of the defining characteristics of the season. It is amazing how small pigments can have such a large impact on our experience of nature. The green chlorophyll that photosynthesize during the sunny summer months mask the orange and yellows that are revealed with the reds that are produced as a part of the vibrant transition to the winter months. We are very fortunate to be able to experience this in New England!