How To Make Tree Cookies

Ever heard of a tree cookie? Sadly, unless you are a termite, you can't eat them. But you can use them to unlock the past of a tree. From its age to the weather conditions and hazards it faced in its lifetime, tree cookies can be used to better understand trees and their role in the environment.

So what is a tree cookie? Tree cookies are cross-sections of trees that are usually around 1/4 to 1/2 inch in thickness. Teachers and ecologists use them to teach students about the layers that make up a tree and to illustrate to students how trees grow and age. Here's how to make your own tree cookies and use them at home or with your students to learn more about trees.

Making Tree Cookies

Just as with edible cookies, tree cookies are made using a series of steps in a "recipe."

1. Start by selecting a tree with a trunk or thick branches that you can cut to reveal the tree rings. Take note of the type of tree it is and where it came from.
2. Cut a log that is about three to six inches in diameter and three to four feet long. You will cut this down later but it will give you a good section to work with.
3. Slice the log into "Cookies" that are 1/4 to 1/2 inch wide.
4. Dry the cookies. Yes, you will bake these cookies! Drying the cookies will help prevent mold and fungus from decomposing the wood and will preserve your cookie for many years to come. Set them in the driveway in the sun, or on a drying rack in the yard for several days. Airflow is more important than sunlight, but if you can get both, that would be perfect.
5. Sand the cookies lightly.
6. If these cookies will be used in the classroom, cover with a coating of varnish to help them withstand years of handling.

What You Learn From A Tree Cookie

Now that you have your tree cookies, what can you do with them? Here are several ways you can use tree cookies at home or in your classroom to teach students about trees.

• Take a closer look. Start by having your students examine their tree cookies with a hand lens. They can also draw a simple diagram of their cookie, labeling the bark, cambium, phloem, and xylem, tree rings, center, and pith. This image from Britannica Kids provides a good example.
• Count the rings. First, ask your students to take note of the differences between the rings - some are light-colored while others are darker. Light rings indicate fast, spring growth, while dark rings show where the tree grew more slowly in the summertime. Each pair of light and dark rings - called an annual ring - equals one year of growth. Have your students count the pairs to determine the age of the tree. 
• Read your cookie. Now that your students know what they are looking at and what to look for, help them understand what else a tree cookie can reveal to foresters. Does the cookie show wider growth on one side than the other? This could indicate competition from nearby trees, a disturbance on one side of the tree, a windstorm that caused the tree to lean to one side, or simply the presence of sloped ground. Other anomalies that students can look for include scars (from insects, fires, or a machine such as a lawnmower) or narrow and wide rings that may indicate years of drought or insect damage followed by years of recovery.
• Do some math. Ask your students to measure the distance from the center of the tree cookie to the outermost edge of the last summer growth ring. Now ask them to measure the distance from the center to the outermost edge of the tenth summer growth ring. Using this information, ask them to calculate the percent of the tree's growth that took place in its first ten years.
• Play a game. Utah' State University's Forestry Department has a cool interactive online game that students can play to test their tree cookie reading skills. (And teachers, don't worry, the answers are there too if you need a little help!)