1) What are tree rings?

Wood is made of cells called xylem that mainly carry water from the tree's roots up to its leaves. Xylem cells are actually dead when they carry the water-- just hollow tubes. Every year, the tree produces new xylem cells from an area under its bark (a ring of tissue called the "vascular cambium"). The cambium makes xylem cells all summer long. Early in the summer, when it is warm and the water is plentiful, the xylem cells produced are big. These early cells are called "early wood," and look light-colored in the wood. As the summer goes on, temperatures cool and water becomes scarce (in some places), the xylem cells produced are small. These late cells are called "late wood," and look dark-colored in the wood. Together, the band of light- and dark-colored wood makes up one "annual ring," or one year's growth. Check out any cut spruce tree stump and you can see the annual rings.

(PICTURES: xylem cells, early vs late wood, concentric rings, pics of cores...)

2) What did we do?

Cored trees: screw in borer, pull out core, sand it down, measure/count.

3) What can we learn?

The study of tree rings is called "dendrochronology."

Basic things:

age (estimate b/c not at base and rings are sometimes absent)

growth rate. Lots of things influence a tree's growth rate: temperature, moisture, porcupines! By looking at growth changes over time, we can learn things about how the tree's environment has changed over time.

(caution:) microsite variation like trees dying (new resources) or trees shading out other trees.

 

(PICTURES: tree core up close with ages marked in years, different growth rates/rings)

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Listen to Dr. Andrea Lloyd core a tree. Click here for text.

Click here to download a free RealPlayer so you can listen.

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Since we want an estimate of the tree's age, we screw the increment borer into the base of the tree, which is where the first rings are.	Coring a tree entails twisting the borer (which is like a cross between a drill and a screw) into a tree. The tip of the borer is hollow, so as we twist the borer in, it screws in around a pencil-sized section of tree. When we reach the middle of the tree (think about why we stop there...), we twist the borer backwards and break off the core inside. This is what it looks like as you core.
Once the core is broken off inside, a little "spoon" grabs it and pulls it out. Although sometimes the core breaks into little pieces as it comes out (which is quite frustrating), usually the core is in one piece when we pull it out.	This is what a tree core looks like: a long pencil with tree rings. There are two major things we can learn from coring a tree. One is its age-- and we get a very close estimate of that by counting these rings.
The other thing we can learn is about how much the tree grew in each year of its life: its growth rate over time. To figure this out, we measure each year's ring.	We can compare these ring lengths to weather records to learn how changes in climate affect how trees grow. Think of the trees in a forest as a growing library of information: we can learn about how old all the trees are and what kinds of conditions they've grown in. Because some of the trees are over 500 years old, there are many things to learn!

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Click here to learn more about "dendrochronology"-- the study of tree rings.

How to use tree cores from different parts of the tree-- looking at growth form.

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Coring trees | Measuring seedlings | Collecting dead wood | Measuring Growth form | Planting seedlings | Soil Sampling