| Girth
Measurements |
Edward
Frank |
| Feb
13, 2005 17:38 PST |
Girth Measurements
There have been several questions or comments dealing with girth
measurements in the past couple of months. Will Blozan has
address many of
them in his excellent section on measuring tree girth in his
document -
"Tree Measuring Guidelines of the Eastern Native Tree
Society." I want to
specifically state how to deal with a couple of unusual
situations that
have been mention in some of these posts. There is a need to
have a
clearcut policy in these situations, even though the solutions
seem obvious.
"Tree Measuring Guidelines of the Eastern Native Tree
Society."
http://www.nativetreesociety.org/measure/tree_measuring_guidelines.htm
"Girth is a dimension taken at a point 4.5 feet (BH) above
average
soillevel (A). This measurement is called circumference at
breast height
(CBH). If a burl or other atypical growth formation is
encountered at this
point the least distorted girth below this point is used (B);
otherwise
above BH. When a tree is growing on a slope the girth is taken
at a point
that is the
average of the highest point and the lowest point the tree trunk
appears to
contact the soil (Mid-slope-C). This mid-slope rule is used to
follow the
American Forests guidelines for measuring champion trees. In all
cases the
girth is taken perpendicular to the axis of the trunk at BH, not
parallel
to the soil. Measured girth is the best approximation of size,
since it is
a real number, not a calculation based on fictional premises.
Even girth
has its limitations, as a sinewy or contorted trunk will have
lots of
hollows and ridges that are not accounted for in the
measurement. Diameters
calculated from such trees will be overstated (diameter= CBH/
3.142). For
volume measurements, "footprint" maps must be obtained
to calculate the
"functional" diameter and girth. The functional
diameter is always smaller
than the calculated diameter."
What about trees where multiple stems are fused together?:
"Some ENTS members use the "slice test".
Basically, if the tree was cut at
4.5 feet above the ground, would the tree hold together? I have
trouble
with this, as a tree that would fail one year would pass the
next. I think
of it more as a "pith test". If the tree has more than
one pith at ground
level it is a multiple-stemmed tree. Note I did not say 4.5 feet
above the
ground. This is because the 4.5 foot height is a forestry
standard and is
an arbitrary and convenient place for most people to measure a
tree. Some
trees, like flowering dogwood or rhododendrons, may branch well
below 4.5
feet but have a single pith at ground level. In the case of such
trees, I
would measure the narrowest point below the lowest fork. All
trees do not
conform to our set standards, but we can always set new
ones!"
-------------------------------------
1) What about trees that have large buttresses? (Bruce
Allen wrote Jan
05, 2005): "I measure dbh at 1.37 meter unless the tree has
a buttress or
fluting, then I measure 1 meter above visible buttress to get an
accurate
estimate of basal area. For Cypress and Tupelo, that is often 3
or 4 (or
more) meters up the tree. Every picture I see
of a person measuring in
the middle of a buttress makes me wonder about there
methods."
Answer: The tree should be measured both at a height breast
height, 4.5
feet or 1.37 meters, to maintain consistency with methodologies
used by
other groups and at a more reasonable height as suggested above
at 1 meter,
or 1 yard, above the obvious fluting. The height of this second
measurement
above the ground should be noted along with the girth. It is
important to
do the standard measurements as previously defined, so that we
retain the
capability to compare our results with older datasets, while at
the same
time we should be striving to use better refined techniques for
incorporating data into the ENTS database.
2) There also are the giant sequoias in the west that often have
root
flair/buttressing up to substantial heights above breast height.
Where
should the circumference of these trees be measured?
Answer: They should be measured as stated above. For trees with
giant
bases like the sequoias, a better technique involves detailed
mapping of
the footprint of the tree. This methodology is used by Dr.
Robert Van Pelt
in his work with these giant trees. A more detailed description
of this
methodology is available in his book "Forest Giants of the
Pacific Northwest."
3) There are many trees in which multiple trunks have become
fused together
at their base. This fusing may extend above 4.5 feet (1.37
meters). Where
should the circumference of these trees be measured?
Answer: Several measurements should be made. The tree's girth
should
first be measured at breast height encompassing both fused stems
to obtain
a "standard historical" measurement for comparison
purposes. The height of
the fusion should be noted, and then the circumference of each
branch of
the fusion should be measured, perpendicular to their trunk
direction. The
height of each of these measurements above the ground should be
noted.
The circumference of each stem, if they are equal in size should
be
approximately 2/3 of the circumference of the fused stem. This
approximate
ratio can be refined as we gather additional measurements.
Ideally to
create a supplemental dataset dealing with multi-trunked trees
additional
descriptions an measurement might be appropriate. A sketch or
photograph
of the multi-trunked tree would be useful.
3) Will Blozan discussed measuring Rhododendrons in his
measuring
guidelines. He stated they are to be measured at the narrowest
point below
the lowest branching, if that branching is at less than 4.5
feet.
Answer: I would suggest an alternative, in the case of these
species, and
in the case of dwarfed trees such as on top of Mount Everett, a
better
measurement position would be above the obvious root flair or at
1 foot
whichever is higher. The rationale is that these specimens are
essentially
scaled down size trees, therefore the height of cbh measurements
should
also be measured at a scaled down height appropriate for their
size.
The narrowest point below the first branching seems to be a much
more
variable criteria than the alternative I suggest, and would make
comparisons between these small species and specimens
problematic. If
measuring at the narrowest point below the first branching, you
would not
be measuring at the same position on the tree profile on each
tree.
Anyway, everyone is welcome give me your thoughts and input on
these
questions.
Ed Frank
There also are a couple of other documents on the website
discussing
measuring girth:
Correctly Measuring Girth, Colby Rucker, Aug 2002.
http://www.nativetreesociety.org/measure/measuring_girth.htm
Tree Girth Measurements, Edward Frank, Nov. 2003.
http://www.nativetreesociety.org/measure/tree_girth_measurements.htm
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