Why Leaves Change Color - It’s All about Energy

It’s been a beautiful fall in Wisconsin. The weather has been pleasant and the color has been fantastic, although it’s always too short-lived. But why is it that this season of color even exists?

Leaves reveal color as much as they change color. The green color that we see throughout the summer is from the chlorophyll in the leaves. Chlorophyll is the essential compound that captures sunlight during photosynthesis. Photosynthesis is the process that creates sugar/food for the tree (and most of the rest of life on Earth). Chlorophyll is a fragile compound continuously needing to be created by the tree (it’s capturing sunlight after all, a high intensity job!). As the amount of sunlight decreases in the fall, the tree makes an energy decision: is it getting enough energy (food) from photosynthesis to continue to create chlorophyll for a decreasing amount of sunlight or does it makes sense to save the energy and not make chlorophyll because not enough energy would be produced. While photosynthesis is happening, the tree also needs to create compounds that protect the leaves from insects and disease. Photosynthesis requires an energy investment from the trees. At the point that the tree doesn’t get enough energy out of the process, it needs to stop investing energy or it risks its health.

At that tipping point of the tree ceasing the creation of chlorophyll, the leaves start to change color as the chlorophyll breaks down and the yellows and oranges are revealed. These colors come from carotenoids that are always in the leaves. They act something like sunscreen for the leaves, protecting the leaves from high energy sunlight. Some trees also begin, then, to produce anthocyanins which produce the red colors in leaves. We’re not sure exactly why trees produce these compounds, but one idea is that they act as additional sun protection while the tree is extracting nutrients and compounds from the leaves to store and use again the following year. 

So, we see the colors of the leaves in the fall when the tree decides it’s not worth photosynthesizing anymore, that is, when the energy return isn’t worth the energy investment. The value of return on time and energy investment can also be applied to our personal lives and societal values as a whole. What types of activities are we investing our time and energy towards that aren’t providing the types of returns we’d like to see? Is it time to slow down or simply stop doing things that don’t provide enough benefit?