Why Do Trees Change Colors in the Fall?

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I try (and often fail) to keep my articles topical. It doesn’t make sense to talk about the Nobel’s six months later, or how the full moon works in the middle of a waning gibbous.

This often leads me to have approximately a million almost finished articles in my google docs whose times have passed. This happened to me recently when I wrote about why trees change colors, but didn’t get it out before the first big frost. I thought the opportunity for this one has passed. But as you can see, sometimes everything works out in your favor. As happens rather frequently, Google came to my aid.

Today’s (Friday’s) Google doodle celebrates the 287th birthday of the Dutch scientist who discovered photosynthesis, Jan Ingenhousz. AKA a perfect excuse to pull up this article again. So without further ado, let’s get into it.

Before I talk about why trees change colors in the fall, we have to talk about why evergreen don’t change colors.

Deciduous are most of the big leaf trees that we know and love such as maple, oak, and my personal favorite, aspen. They lose their leaves in the fall and grow them back in the springtime.

The opposite, evergreens, tend to have smaller leaves and are well named because they stay green during the winter. They include all the Christmas style trees, but also holly, live oak, and many rainforest trees. My favorite of these is the Bristlecone Pine, which can live thousands of years.

Confusingly, ginkgo trees are neither of the above or maybe both? They are very old evolutionarily, and seem to bridge the gap between the two.

Back to deciduous. Creating and keeping up large leaves takes a lot of energy for a tree, but they make back more through photosynthesis. As we all remember from elementary school, photosynthesis is how plants gather light from the sun and turn it into food. These kind souls convert sunlight and carbon dioxide into plant food and oxygen, which most living being on earth rely on. Plants are at the very bottom of the food chain, and there would be no cats or pumpkin spice lattes. The next time you see a green plant, make sure to thank it for creating the literal atmosphere you use to breathe.

Most leaves are green because of chlorophyll, which is a molecule that plant use for photosynthesis. There are other molecules in the leaves that are different colors, but we view them as green because there is so much more chlorophyll than any of the other molecules. Big leaves are experts at creating energy from light, but they also take a lot of energy for trees to make. Because there is so much less light in the winter, it’s not worth it for them to use energy to maintain their leaves. So, they fall and regrow in the spring.  

Interestingly, the change in the length of the day determines when leaves start to change their leaves, not the weather. This year Here in Utah, I could walk around in a tee shirt and see the fall colors. By the time it finally snowed, there were almost no leaves left on the trees.

The actual process starts with the trees creating a cork like material between the stem and the leaf. They then literally suck the life out of the leaves, aka sugar and carbohydrates that the tree will use over the course of the winter to stay alive. The cork material hardens, and eventually seals off the leave entirely. The dead leaves are no use to the tree and fall to the ground to be covered in snow, much like my engineering hopes and dreams.

The trees pull out the chlorophyll from the leaves, which is why they lose their green-ness. The other molecules that I mentioned stay in the leaves, turning them red or orange. The fun colored molecules help the leaves from drying out, from getting too cold, from being burned, and other various helpful things that a tree might need. Each kind of tree needs slightly different things, which is why aspens turn yellow but maples turn red.

Trees are way more intelligent than we think and soon I’ll talk about other ways we know they are smart, but I’ll leave this one as a teaser.

About The Author

Just a girl who has a lot of feelings about planetary science

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