What Do Tree Rings Show Us?
Tree rings are patterns formed in the trunks of trees caused by patterns in the annual growth of trees. The rings appear as concentric circles moving outwards from the center of the trunk with repeating light and dark patterns. The wider, lighter portion of a tree ring is referred to as "early wood" and is formed as the tree is growing rapidly, often in the spring or early summer. The darker ring is called "late wood" and is formed later in the growing season when conditions are not as ideal for growth. Because the tree is not growing as rapidly, the ring appears darker and is more dense than the early wood. The pattern of a light and then dark layer signifies a single year of growth and can tell us many things about the tree and it's environment.
Tree rings can provide a great deal of information about the tree itself, as well the environment the tree grew in. The most obvious of course being the age of the tree. The width of the rings can tell us how favorably the growing season was for each particular year in the tree's history giving us a look into the climate of the last few millennia thinner rings signify unfavorable growing conditions such as a colder than usual summer or perhaps a drought while wide rings mean the tree had adequate moisture and a long, warm growing season. An example of how tree rings can help us identify the climate of the past is how scientists have been able to use rings to determine the El Nino cycles that occured before modern records were kept.
- NASA Tree-Ring Study Reveals Long History of El Niño
- The Lamont-Doherty Earth Observatory Tree-ring Lab
The oldest complete tree ring records are from southern Germany and go back over 10,000 years. Because the records are complete, they can be used as reference points for other tree rings that scientists are trying to date. By matching up patterns of thin and wide rings, wood can be dated as when it was originally grown. This technique has many uses such as determining the exact age of a piece of wooden furniture by examining it's rings and matching it against historical records. Because each region has different climate patterns, each region must develop their own tree ring records to be used in dating wood. By examining different region's tree ring patterns, not only can the age of the piece of furniture be determined, but sometimes even the location of where the tree was originally grown.
Perhaps the easiest way to examine tree rings is to find an exposed stump of a tree that was cut down with a saw. This provides as easy to see cross section of the tree revealing all of the rings. Because dendrochronologists (tree ring scientists) can't be cutting down trees everywhere to examine their rings, they've developed specialized tools that bore into trees and remove a thin core. This doesn't greatly harm the tree yet can still provide scientists accurate data about the tree's rings.
- A huge collection of tree ring info collected by Henri Grissino-Mayer of the University of Tennessee
- The University of Arizona's Laboratory of Tree-Ring Research
- University of Arkansas - Department of Geosciences' Tree-Ring Laboratory
Great lessons for students about tree rings can be found at the following places:
- SD Kids Books - A great hour long lesson for younger students explaining the usefulness of tree rings.
- Science Net Links - A lesson plan for middle school students that involves them going to a website and answering questions about tree rings.
- University Corporation for Atmosperic Research - A lesson plan that goes deeper into how tree rings can be used to tell us the climate of the past.
- Nicolet Distance Education Network - A lesson plan designed for 4th graders and gives the students a deeper understanding of tree rings.
- Time & Cycles Dendrochonology - Students will learn how archaeologists used tree rings from log cabins and Native American pueblos to date dwellings found throughout history.
- Climate Change North - A 2 to 3 hour lesson designed for students in 4th to 8th grade.
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