BY FORREST M. MIMS III
How to Study Tree Rings
The purpose of this column is to provide projects that will encourage readers to begin doing science. Whether you’re a student looking for a good science fair project or an adult wanting to start personal science study, I hope you’ll find this or a future project worthy of pursuing.
In temperate and arctic regions, most trees are
dormant during winter. When spring arrives, a sudden burst of growth expands trunks and branches
with new wood formed from large cells and known
as early wood (or spring wood).
As the growing season peaks, the growth slows,
and the late wood (or summer wood) that’s formed
has cells with thicker walls. This late wood may
appear much darker than the early wood when it
contains more tannin.
Each year, this process forms a new growth ring,
just beneath the bark of the trunk and branches
of a tree.
Not all trees produce annual growth rings. Trees
in the tropics that grow year round may have very
suppressed annual rings or none at all. I learned
this firsthand while sampling trees in Brazil during
a field trip sponsored by NASA to measure the
impact of severe biomass smoke on the atmosphere
Where I live in Texas, most trees are dormant
during winter. But only some of these trees produce
sharply defined rings. These include the red oak,
hackberry, and all pine and baldcypress trees. The
live oak keeps its leaves during the winter, and its
rings can be difficult to count.
The Science of Tree Rings
Astronomer A. E. Douglass established tree ring
science when he postulated that tree growth was
influenced by climate changes caused by the solar
cycle. He developed and taught classes at the
University of Arizona on dendrochronology, the
science of dating trees by studying their rings. In
1937 he established the university’s Laboratory
of Tree-Ring Research.
Douglass showed that archaeologists could use
growth rings to date the timbers used to build
ancient structures. Annual growth rings in trees
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also provide valuable information about past precipitation, climate, major volcano eruptions, and forest
fires. They permit long-ago floods and landslides
to be dated.
How to Obtain and Prepare Tree Ring “Cookies”
You can learn to do tree ring studies by using
slices, or “cookies,” cut from trunks and branches.
Christmas trees are an excellent source, as are
building and road construction sites. Professional
tree trimmers and landscape crews may be willing
to provide you with samples. Firewood may also
provide good sample material. Another way to find
samples is to keep an eye out for piles of recently
cut and discarded branches.
You can even use cross-sections sawn from
lumber, although these will not form round cookies.
If you use this method, try to find dated lumber that
includes the outer edge of the original trunk so you
can determine the age of the rings.
If you have trees or access to trees where you live,
you can cut branches or collect cores from trunks.
If you’re not the landowner, be sure to get permission first. This is especially important if you want to
obtain samples from trees on private land or land
owned or managed by cities, states, or the federal
When possible, use a sharp, fine-toothed wood
saw to slice cookies from branches and trunks.
Living wood should be allowed to dry for a day or
two before smoothing it with sandpaper. I usually
begin with 100-grit sandpaper followed by 220 grit.
The final polish is made with 400 or 600 grit.
Samples cut with a chainsaw can be used, but
they’ll require much more surface preparation. If
possible, smaller samples cut with a chainsaw should
be recut with a handsaw. You can use a power sander
to smooth the rough faces of these samples.
For small samples, I prefer to use a handheld plane
such as the Stanley 21-399 6-Inch Surform Pocket