Tree Ring Dating & Measuring a tree’s diameter
Trees provide us with paper, lumber, plywood, pine pitch, dyes, turpentine and many other products. They also
provide us with shade, beauty, oxygen, clean air and water, reduce soil erosion, and fruit. Trees are regarded as an
important renewable natural resource but many take many years to grow and reach maturity. Their renewal
requires careful planning. Once older trees have been harvested, new trees need to be planted in order for this
resource to be renewable.
Tree rings can be used to estimate a tree’s age as well as provide important ecological information. Each ring
represents one year’s growth. Wide rings represent good growing conditions whereas narrow rings represent poor
conditions. Tree rings are formed from the center outwards. The ring towards the center is the oldest and the ring
closest to the bark layer is the youngest. Foresters use an instrument called an increment borer to determine the
age of a tree. The increment borer is used to obtain a core sample that is approximately 1/8 inch in diameter. This
is preferable to cutting down a tree and counting the annual rings in a transverse section.
The following lab activity will give you a basic idea of the principle of tree ring dating by using simulated core
samples – one sample is from tree (A) and another is from a Forest Ranger’s cabin (X).
This exercise was adopted from Dendrochronology Tree Ring Dating Kit Cat. No. 52 by Lab Aids Inc,
Ronkonkoma, NY 11779.
How can you measure the diameter of a tree?
A common method is to use a special measuring tape called diameter tape. This is calibrated tape where every 1
inch marked on the tape corresponds to 3.14 inches or Pi. If you use a regular cloth tape then you will need to
divide the circumference measurement by Pi (3.14) to obtain the diameter.
Procedure: Find some big trees on campus.
1) Determine breast height (dbh) for a given tree. This is about 4 ½ feet from the base.
2) Wrap the diameter tape around the tree at breast height. The reading from the diameter tape is the diameter of
the tree at breast height. Make sure the tape is level and not twisted. The diameter measurement is read where the
tape overlaps with the zero marker.
3) Record this information for five different trees. Usually trees become wider as they get older and older. The
more girth a tree has, the older the tree.
dbh
Tree Species
A.
B.
C.
D.
E.
The Tree Aging Formula
Begin by determining the tree species and taking a diameter measurement (or circumference measurement) using a
tape measure at Diameter Breast Height or 4.5 feet above stump level. If you are using circumference, you will
need to make this calculation to determine the tree diameter: Diameter = Circumference divided by 3.14 (pi)
Then calculate the age of the tree by multiplying the tree's diameter by its growth factor (see below): Diameter X
Growth Factor = Approximate Tree Age. Let's use the hickory above to calculate age. A shagbark hickory's growth
factor has been determined to be 7.5 and its diameter is 10 inches: 10 inch diameter X 7.5 growth factor = 75
years.
Growth Factors by Tree Species
Red Maple Species - 4.5 Growth Factor X diameter
Silver Maple Species - 3.0 Growth Factor X diameter
Sugar Maple Species - 5.0 Growth Factor X diameter
River Birch Species - 3.5 Growth Factor X diameter
White Birch Species - 5.0 Growth Factor X diameter
Shagbark Hickory Species - 7.5 Growth Factor X diameter
Green Ash Species - 4.0 Growth Factor X diameter
Black Walnut Species - 4.5 Growth Factor X diameter
Black Cherry Species - 5.0 Growth Factor X diameter
Red Oak Species - 4.0 Growth Factor X diameter
White Oak Species - 5.0 Growth Factor X diameter
Pin Oak Species - 3.0 Growth Factor X diameter
Basswood Species - 3.0 Growth Factor X diameter
American Elm Species - 4.0 Growth Factor X diameter
Ironwood Species - 7.0 Growth Factor X diameter
Cottonwood Species - 2.0 Growth Factor X diameter
Redbud Species - 7.0 Growth Factor
Dogwood Species - 7.0 Growth Factor X diameter
Aspen Species - 2.0 Growth Factor X diameter
http://files.dnr.state.mn.us/education_safety/education/plt/activity_sheets/howToUseADiameterTape.pdf
http://forestry.about.com/od/silviculture/a/Estimating-A-Trees-Age.htm