Dartbeat Investigates: Why Do Leaves Change Color in the Fall?

By Melanie Kos | 10/23/16 12:08pm

Foliage season is here, in case you haven’t noticed the millions of leaf pictures on social media or looked out a window. But have you ever stopped to think about why the leaves change color in the fall? Hint: It’s not for the Instagram likes. Dartbeat sat down with Biological Sciences professor Matthew Ayres to answer this question and more.

So, how do trees know when it’s time to pack up shop and become dormant for the winter?

MA: The photoperiod is the amount of sunlight during the day that a plant receives. As the seasons change, that photoperiod shortens and induces hormonal changes in plants, triggering different things, including the breakdown of chlorophyll, which is the pigment that makes leaves appear to be green. 

What happens to the chlorophyll and the leaf?

MA: Chlorophyll also contains a lot of nitrogen, which is essential for plants. Nitrogen is not readily found in nature, so it is crucial they recover invested nitrogen, which is why they break down chlorophyll. The nitrogen is then stored elsewhere in the tree during the winter. 

Where do the colors come in?

MA: Without chlorophyll, the other pigments that are usually hidden are shown, with the yellow-reflecting carotenoids being most frequent. If a leaf has little to no carotenoids, it will appear brown. Anthocyanins reflect red, making leaves appear red. This compound is only created when there is sugar present in the leaf. Sugar is still created in the leaf by compounds such as carotenoids, though they are not as productive as chlorophyll. Leaves can have various combinations of all of these colors. We are lucky to have maples here, as they tend to turn spectacularly red.

Is fall foliage the same around the world?

MA: The forests in North America are prettier in the fall than the forests of Europe are. There, very few trees turn red, and that’s likely due to the Ice Age. Mountains in Europe run east to west, whereas mountains in North America run north to south. 50,000 years ago, when the poles began to freeze and ice began to cover the planet, trees continually moved southward to escape this. However, trees in Europe soon ran into mountain ranges, which they could not maneuver around. This caused the extinction of many tree species, many of which likely had anthocyanins. In North America, the north to south mountains allowed for the trees to continue moving towards the equator and survive this, which is why we see red trees in North America, but mostly yellow trees in Europe. 

Melanie Kos