Tree Cookies

They're round. They're full of fiber. But unless you're a termite, you can't eat tree cookies! Tree cookies are cross sections of tree trunks that foresters and teachers use to illustrate how trees grow. Tree cookies reveal the many different layers that make up a tree. And each layer can tell us something about the tree's life and the climate in which it grew. Item 1 is called the cambium. It is a layer or zone of cells, just one cell thick, inside the inner bark. The cambium produces both the xylem and phloem cells. This is where diameter growth occurs, and where rings and inner bark are formed. Item 2 is the phloem or inner bark. This layer carries sugar made in the leaves or needles down to the branches trunks and roots, where it is converted into the food the tree needs for growth. Item 3 is the xylem or sapwood. This layer carries the sap (water plus nitrogen and mineral nutrients) back up from the roots to the leaves. Sapwood gives a tree its strength. Item 4 is a growth ring. The lighter portion is called the "early wood" (because it grows in the spring), and the darker portion the "late wood" (which grows in the summer). Together, they represent one year of growth. (You can count the rings to see how old a tree is!) Item 5 is the heartwood. Heartwood develops as a tree gets older. It is old sapwood that no longer carries sap, and gives the trunk support and stiffness. In many kinds of trees, heartwood is a darker color than sapwood, since its water-carrying tubes get clogged up. The tree cookie at right, like many of its fellow young pines, has not developed heartwood yet. Layer 6 is the outer bark. This layer protects a tree from insects and disease, excessive heat and cold, and other injuries.

Why Study Tree Rings?  Many scientists think our climate is changing--and changing fast! Studying what climate was like and what changes have happened in the past helps scientists understand things like: How climate works ･ How climate affects plants and animals, and ･ What things can cause climate to change, such as gases in the air. We can use the info to compare what's going on today with what happened ten years ago, hundreds of years ago, or even millions of years ago! The info from the past can be used to help scientists figure out what could happen in the future if the temperature goes up from people putting so many greenhouse gases into the air. It can help them understand how climate could change and what it could do to plants and animals.Things scientists study to learn about climate in the past include:  ･ Ice cores from the Antarctic and Greenland. (They study bubbles of gas trapped in the ice.)  ･ Pollen from the bottom of lakes  ･ Fossil records of insects.  ･ Growth rings of trees.  Many scientists think our climate is changing--and changing fast! Studying what climate was like and what changes have happened in the past helps scientists understand things like: How climate works ･ How climate affects plants and animals, and ･ What things can cause climate to change, such as gases in the air. We can use the info to compare what's going on today with what happened ten years ago, hundreds of years ago, or even millions of years ago! The info from the past can be used to help scientists figure out what could happen in the future if the temperature goes up from people putting so many greenhouse gases into the air. It can help them understand how climate could change and what it could do to plants and animals.Things scientists study to learn about climate in the past include:  ･ Ice cores from the Antarctic and Greenland. (They study bubbles of gas trapped in the ice.)  ･ Pollen from the bottom of lakes  ･ Fossil records of insects.  ･ Growth rings of trees.

Reading Tree Cookies The rings of a tree give us a lot of information about the age of the tree, its health, and the climate conditions during each year of its growth. Just for fun, predict the number of rings on each of the tree cookies on this page. They are about the same size, but are they of the same age? Count the rings and find out. Hmmmm. How might you account for the differences? (HINT: Think about all the things a tree needs in order to grow.) Here are some explanations to help you think about it: The tree cookie on the first slide has a small number of wide rings, indicating that it came from a young tree that grew in an area where it had little competition for the things a tree needs to grow -- such as sunlight, water, and nutrients. This cookie has many tight rings. It is from an older tree that grew with more competition. The fact that the center rings are offset indicates that the tree either grew on a slope or had to grow around some sort of obstruction. If you were a forest manager, how could you use your knowledge about tree growth to manage a forest for wood production? For wildlife habitat? For water quality? Or for all three? (Most forest managers manage for all these benefits -- and more! What an interesting and challenging job!

How we do it: The tree's age can be figured out by counting the pairs of light and dark rings. It's easier to see the dark rings so they are usually the ones used for counting. Start with the first dark ring in the centre and counts out to the last dark ring before the bark. To help figure out what climate the tree grew in and what the environment was like, look at each ring: Thickness: How wide a ring is can tell you if the environment was good or bad for the tree to grow in. In years when the amount of rain and temperature were good a tree's rings are wider. In bad years a tree's rings are thinner. Shape: If rings start to become thinner on one side than the other it probably means the tree is leaning over to one side. High winds or a big storm can cause a tree to lean. Also look for strange marks, like scars, and other "pieces of evidence." Scars can be left by insects or disease. A forest fire can leave burnt marks. Reading Trees

This tree shows a fast growth pattern. It is only 5 years old, but it has grown a lot each year. This tree was injured when it was only four years old! It has a scar that is healing over. The tree might have been injured by a fire. What happened to this tree? The off-centered rings show that at one time it started growing at an angle. It could be growing on a slope, or may have been pushed on by another tree, or a roc k.

This tree had slow growth during the first years of its life. It was suppressed. Then something happened, and the tree started growing faster. Was there a drought for many years, and then a lot of rain? Or perhaps the tree was in crowded conditions, and then the trees around it were trimmed or cut down. This would give it more water, sun, and nutrients to grow. This tree has had very slow growth its entire life. It is 16 years old, but is the same size as the tree above. It may not be getting enough water, nutrients, or sunlight due to an overcrowded fores t.

Internet Sites How Do Trees Record Time site that has 10 pages that describe ways to use tree cookies to identify past events. pter_no=investigation Reading Tree Cookies Very basic description of how to read a tree cookie. Two sample worksheets where students can look at and identify causes in the changes to the cross section Dr. Fir’s Labratory Interactive Site describing tree ring analysis. Also, analyze a tree cookie on the site.

The Cookie of My Life Now, make your own “tree cookie” describing the events of your own life. Think about the past events that have had an impact on you. Start your ring with the center “your heartwood”, and draw a ring to represent each year you have been alive. Years you have grown a lot, make the rings wider. Years you didn’t seem to get much taller, make skinnier. If you have a traumatic event in your past, record that in your ring by making the ring off center, crooked, or with a scar.