| Tree
rings |
Colby
Rucker |
| Monday,
October 21, 2002 11:17 PM |
Bob,
About a week ago I had a visit from a classmate of mine whom I
last saw in
the late 1940's. He's a retired engineer, well-traveled and well
read, with
an interest in most everything, and has a country place in the
woods in
western Maryland.
He forwarded a xerox of a draft of a chapter from a book that an
associate,
Richard L. Phipps, at the Tree-ring Laboratory at the U. S.
Geologic Survey
is writing, and sent to him for review. The chapter, Crossdating,
Measuring
and Standardization, is most interesting, and emphasizes the
difference
between counting rings and crossdating. The matter of false
rings and
missing rings may make trees in certain stands, or different
situations
within the stand nearly impossible to date. The matter of
selecting useful
trees, utilizing the most reliable group of rings within that
tree, and
correlating that sequence with a resource of reliable material
is essential.
Such determinations often require specialized equipment and
rather complex
mathematical formulas.
Although reliable sequences can be constructed, and numerous
structures have
been accurately dated, certain species are more difficult to
date. Phipps
mentions two in particular; Taxodium distichum and Nyssa sp. He
states
that Nyssa and other diffuse-porous species from certain
habitats can be
nearly impossible to work with. Discontinuous rings and false
rings are
common in Taxodium, and trees in some areas could not be
crossdated at all.
In reading Fowells (1965), I see a reference to a study by
Beaufait &
Nelson. They found that false rings were common in three
second-growth
stands of Taxodium, and "conventional ring counts averaged
1.6 times the
actual age." Phipps states that "Correctly crossdated
material produces
absolute dates for each ring...It is not possible to know how
many missing
or multiple rings will be found in material of a given species
in a given
habitat until a number of collections of material from an area
have been
checked."
Locally, I have found that trees are usually not nearly as old
as people
make them out to be. The immense southern red oak at Cedar Park
was less
than 200 years old, not 350. The Wye Oak was probably 325, not
460. The
Liberty Tree was probably 375, not 600 plus. As a creator of
stumps, I've
counted rings in chestnut oaks to 160, sour gum to 175, and a
mockernut that
was 211. Judging from limb structure and bark contours, I
believe a few
trees in Chase Creek Woods may be older, especially sour gum.
Of course, none of my ring counts were cross dated. It appears
that we
should be aware that simple ring counts can be inaccurate - as
inaccurate as
some of the claims for tree height. For that reason, we may want
to be a
bit more cautious regarding some of the exceptional counts taken
from black
gum, bald cypress, white cedar and other material.
Colby |
|