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  Nature Elective

Lesson Plan 32
Redwood Trees: How Old?
First posted May 6, 2004 Last updated July 5, 2004

This Lesson Plan shows how to estimate
the age of a redwood tree
( Grade Level 9-12)

Remember these points from the Lesson Plan Homepage:

(1) These lesson plans are not rigid requirements, but a starting point for the Nature Counselor's plan for teaching a particular day's experience.
(2) The activity should be fun and emphasize active learning on the student's part: ask a question, don't just state a fact.
(3) You should employ hands-on as much as possible.
(4) Plan each session to also allow time for making entries in the Nature Journal.

Prior to the session

(1) Review this section and the section on the Redwood Forest and the age of redwood trees.
(2) Think about time and our experience of it. Time is a hard concept for younger students to understand and appreciate. Things that occurred in history prior to their birth do not mean much. Be aware of this as you try to teach them about the age of the redwoods. They evolved along with the dinosaurs and were alive before the US was created!
(3) Be sure to have the yardstick, string, and either the sample redwood section or a saw, sandpaper, and branch.
(4) Scout out the log sections at the campfire, select some that will be good on which to count annular rings.


(1) Start with walking around camp, observing the height of the various trees that we have. Identify the species.

*What species of trees are at Cazadero? How can you identify them? Leaves, bark, size, shape?
*Rank the tree species in terms of overall size. Does size always correlate with age?
* Talk about the ages of things, such as the students' ages, their parents' ages, grandparents'; the age of Cazadero, what was going on 100 years ago, 200 years ago, 1000 years ago. A timeline in their Journals may help to explain age in a graphic way.

(2) Redwoods are obviously old.

* How old would you guess a plant or tree is? (Flowers often annual, oaks to about 300 years, redwoods to 2000 years).

(3) It might be fun to know how old our oldest tree is.

* How could you measure the age of the oldest tree? How you could make a rough guess? Each student should independently estimate the age and write their answer in their Journal.
* Discuss the estimates. Do you want to change yours? Why?

(4) Walk to the campfire, examine a log section. Discuss annular rings (students usually grasp this quickly).

* Can you now make a better guess as to the age of the redwoods? (Count the annular rings in the log sections at the campfire. Remember, these are usually pines, which are faster growing than redwoods.) Each student should write their answer in their Journal.
* Discuss this second estimate. Do you want to change yours? Why?
* How can you measure it more accurately? The tools you have are a piece of string, a yardstick, a saw (too small to cut down the tree), and a branch of the redwood tree.

(4) Divide the students into groups of about three or four. Each group should independently try this step.

Solution: the branch probably grows at the same approximate rate as the tree. Cut the branch, count the rings, measure the diameter of the branch.

(5) Now all you need is to know the diameter of the tree. How can you measure it? (yardstick) How can you measure it more accurately? (use the string, determine the circumference of the tree, measure the length of the string, use diameter = ¶ r2 (where ¶ = 3.1416 and r = radius). Now use algebra to prorate the growth rate of the branch to the diameter of the tree. How old do you calculate it to be?

(6) How accurate was your first estimate? Second? The final one? How do you determine or estimate the accuracy of any measurement? Does the use of the measurement help to define how accurate it needs to be? What have you learned about successive approximation?

(7) Discuss the science of dendrochronology.



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