Nov. 19, 2010

Science Dad on Why Leaves Change Color in the Fall

by Vince Harriman

Click to enlarge images
Me, Rowan and Beckett headed out with our Specimen Bags

Well, we didn't clean up the oranges and the lamp in the living room until we were on our way out the door to school the next morning. Driving Beckett to school, I asked what topic we should talk about next. Probably because his school is located on the edge of a golf course and surrounded by trees, Beckett asked me: "Why do the leaves change color in the fall?"

"Let's find out," I said, then headed home to do my own homework. One key I've found to teaching anybody anything -- but especially kids -- is to prepare ahead of time, and an important part of the preparation is selecting an activity that is both fun and age appropriate. I decided that we would go on an 'expedition' to gather leaves in the neighborhood, then, the next day, classify the leaves and prepare a poster.

Saturday morning we headed out the door with our specimen bags (any bag will do really, and if you don't have a bag you can carry the leaves carefully in your hands.) On our block we found leaves from a wide variety of trees -- pear, cherry, crepe myrtle, laurel, dogwood, yellow wood, and ginkgo. We decided to focus on trees that actually changed color, but being good scientists, we kept all the specimens we collected.

By the next block we had found two types of maple and two types of oak leaves, as well as magnolia and holly leaves. We made our downtown, collecting the whole way. Although only two and a half, Rowan understood immediately what we were up to and pulled leaves off everything we passed. We also stopped to look at all the bushes, flowers, annuals, perennials, and succulents. We even found a small stand of bamboo and took a sample. Our specimen bags full, we decided to break for lunch and finish our project the next day.

Beckett and Rowan sorting leaves

The next day we sorted our leaves by shape and color then headed to the internet to discover what we had collected. We had already decided to focus our research only on trees that changed color, eliminating several of our specimens, including evergreen trees (which Beckett calls 'nevergreen' trees -- a name he admits is neither scientific nor does it make a lot of sense.)

We went online to a great website by Purdue University that has a gallery you can browse to see pictures of mature trees and their leaves, flowers, fruits, and seeds.

In our online research, Beckett and I found out that most leaves are green because they contain chlorophyll -- the molecule that absorbs light and begins a chemical reaction that changes light energy, carbon dioxide, and water into food for the tree. Leaves also contain other colored molecules (carotenoids, which are yellow and orange; and anthocyanins, which are red.) However, when lots of chlorophyll is present, the other colors are hidden.

As sunlight decreases with the approach of fall and winter, the trees sense this diminishing light and produce less chlorophyll. As a result, the remaining molecules begin to show more strongly. The loss of the green chlorophyll molecules really lets the colors of the other molecules show.

Final Product-Leaf Poster

We had just this week learned about the seasons and how sunlight changes over the course of the year, so I asked Beckett if he remembered the difference between the sunlight that falls in summer and winter. He did, so we then talked about other things we noticed on our walk and tried to develop theories to explain them.

We noticed at one house, there were two trees side by side but one was completely bare and one still had leaves on it. We also noticed that one side of the street generally had fewer leaves left on the trees. We read some more, and discovered that other factors ---the temperature, the amount of rain, the amount of wind, the health of the tree -- influence the color of leaves and the timing of when they fall. We guessed that the differences we observed among the neighborhood trees had to do with the fact that some trees experienced more shade during the day and received less light because of the nearby buildings. We also hypothesized that some trees got less water than others because they were on hills. All good theories.

During this discussion, Rowan had been tracing leaves onto posterboard -- he wasn't so interested in the science, but he made a great poster. Beckett finished up our poster by labeling all the leaves as best as we could identify them.

Overall, I thought it was a successful outing. We had a great time finding and collecting leaves (so much more fun when you call them specimens) and arranging them by shape, color and thickness. Next time we plan on going much much farther on our field trip -- a hint: billions and billions of miles away. So make sure you get out and see the Blue Moon on Nov 21. And don't forget to tell us about your own experiences leaf collecting in the comments below.

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-5 1/2 and Rowan-2 1/2 ask him 'why' approximately 6,341 times a day.

About Vince Harriman

Science Dad, AKA Vince Harriman, is a freelance writer living in Annapolis. His two sons, Beckett-6 and Rowan-2 1/2 ask him 'why' approximately 6,542 times a day.

The views expressed are those of the author and are not necessarily those of Science Friday.

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