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."

"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

Ed Frank

There also are a couple of other documents on the website discussing
measuring girth:

Correctly Measuring Girth, Colby Rucker, Aug 2002.

Tree Girth Measurements, Edward Frank, Nov. 2003.