23, 2003 21:53 PST
If we are going to use girth measurements to form a new index
methodology of measuring girth should be clearly specified in
document. There are several relevant posts on the ENTS site by
Rucker... The Rule of 73 and Measuring tree girth. The relevant
is copied below. Please make any additions, elaborations,
disagreements, etc. to the material and I will compile a
incorporating these materials.
Trees are often compared by measurements of the trunk
taken at breast height (4 ˝ feet, or 54 inches above grade).
and state big-tree registries use this measurement, often known
by adding one point for each inch of cbh to points for height
average spread in their periodic lists of champion trees.
Base point for measurement:
All height measurements start from the same place - "where
sprouted." If you stand at mid-slope and sight down the
the trunk, that line meets the soil at the central basal
can't determine the contour by averaging the elevation on the
low sides of the base; roots enter the soil abruptly on the high
but run all over the hillside on the lower. The central basal
a constant elevation; it extends level from your side of the
through the spot where the tree first sprouted, and out the
Like all contour lines, it may curve, but it's at a constant
If you measure upwards with the lean of the tree 4.5 feet above
contour, that point is breast height on either side of the tree.
(Rucker, Aug 2002)
Circumference is measured at right angles to the lean of the
you temporarily mark breast height on both sides of a large tree
scrap of paper and a thumb tack you can be sure the tape is
can carry some thumb tacks pressed in a wine cork. Of course, if
has left the tree standing on its taproot, you'll have to
elevation where the tree first started. If floodplain deposition
raised the grade, you'll have to accept the existing grade
don't dig 'em out.
On a very large tree, a tape set at breast height may still be
grade on the high side. In that case, raise the tape to a higher
elevation and attach that elevation to your circumference
Actually, a variety of measurements is very useful, including
taken with the slope, starting at grade. This will allow others
the architecture of the base in the future.
Yes, getting the tape even on the downhill side can be
difficult. I had
to use a telescoping aluminum pole to raise the tape on a 22'
tuliptree on a slope above a precipitous drop at the head of a
(Rucker, Aug 2003)
Burls and Knots:
When a fork, burl or low-branching habit causes the trunk to
smallest girth at some point below breast height, many of the
registries have allowed that lower girth to be entered as cbh.
given such trees an unfair advantage over competitors that have
similar lower girth but taper to breast height, where their
circumference is less. (Rucker, Aug 2003)
Rule of 73:
A simple formula was needed to allow a fair comparison between
with a low “waist,” and those that taper in a typical
fashion. An old
list of national champions provided the elevation of 35
of trees measured below breast height. I found that those trees
averaged eight feet in circumference. I then measured actual
trees of ten species, which also averaged eight feet in girth at
same height. After taking circumferences at numerous elevations,
devised a formula to fit the actual flaring contours of the
measured. That formula, which I call “The Rule of 73,” is as
Measure the smallest trunk circumference at or below 54 inches.
one-half the elevation (in inches) of the circumference to 73.
the sum as a percentage of the measured girth. The product is
hypothetical circumference at breast height. (Rucker, Aug 2003)
[I would recommend that if a measurement is converted using this
formula, the actual circumference measurement and height above
listed as an addendum.]
There also is an essay on the website by Colby on how to measure
trunk diameter using a laser range finder.
I am wondering about how the cbh or equivalent should done for
like Rhododendron or Mountain Laurel. The formula works for
their major branching is above the height measures. These shrubs
important part of the picture of the forest, but in most cases
significant branching below this height. Is it reasonable to
one of these branches as the diameter of the trunk? I don't
I would suggest that shrub-like plants be measured at a height
inches above grade to provide a fairer measure. The girth index
ranking and this should not adversely the conclusions.
05, 2003 05:22 PST
I enjoy many of the conversations that flow through the daily
ENTS chat. I often do not have enough time to read each one but
I do read many.
One comment that passed through a month or two ago I wish to
address - the issue of determining ground level for both height
and DBH measurements. I recall the phrases the axis of the
tree and where the acorn sprouted. Both of these,
theoretically, are the same and seemingly the ultimate
objective. Trees on slopes grow faster in the downhill
direction, thus inflating DBH measurements and making the tree
taller by lowering its base. Looking at the axis of the tree
a line passing through the pith, or center of the trunk
would enter the ground above a point that is the average
of the high and low sides of trees on slopes. On flat ground it
This is ideally a great way to approach this problem, but there
are several reasons why I do not use this method.
1. The axis of the tree is very difficult to measure. Perhaps
not with small trees but with giant trees here on the West
Coast, a transit, total station, or similar device is needed.
2. A given measurement is difficult to repeat. The presumed axis
of the tree may appear to be different from different angles.
Giant trees are often not round in the section of the lower bole
thus confounding the problem. Even the same measurer, years
later, would have a hard time repeating their earlier
3. Several species of trees, Picea sitchensis and Tsuga
heterophylla for example, start their lives on logs or stumps.
The phrase where the acorn sprouted is not useful here
because many of these trees germinate several meters above the
ground. Once their seedbed rots away, what was once roots now
becomes stem and is fully functioning as part of the aboveground
portion of the tree in all respects.
No, in this case I must disagree and submit that the simple
method of just averaging the high and low sides, however
illogical, is still the best method. This is how I determine
ground level for both my height and DBH measurements.
This method does indeed tend to make DBH measurements increase,
but as most of you know I do not put much stock in these anyway.
As for heights, the differences are negligible. In all of my
ecological work, DBH is measured 1.37 m above the HIGH POINT of
ground. These measurements are often used in regressions or
other predictive equations so that the influence of the roots is
to be minimized.
This is obviously a superior method and the only reason I have
not adopted this in my Big Tree measurements is because this is
not how it has been done in the past. To make comparisons
possible with past measurements, the same methods must be used.
Anyway, just some thoughts on a seemingly never ending issue.
05, 2003 06:20 PST
Thanks for weighing in. Most of us look to you for the real
in measuring methodology. One point that you appropriately make
methods that apply to small trees (though large to us) have to
rethought when applied to really big trees such as those you
05, 2003 11:10 PST
I always enjoy your constructive ideas. I think we both would
agree that a
fixed point, "where the acorn sprouted, " forms an
point throughout the life of the tree. Whether its tip is two
that or two hundred, that's the actual starting point. Of
that inaccessible point can be difficult. By placing it on the
basal contour," a contour line, sometimes curving, like a
extends out both sides of the tree, the elevation becomes more
allowing us to measure up 4.5 feet on both sides. And, like high
can see the point as part of a horizontal plane, like the one
through the highest leaf or twig.
The real challenge is where to assign a working elevation to
levels. Trees grow. I started keeping careful records of trees
and many have doubled their girth. To get a fairly accurate idea
rates, I must measure over the same spot, the same flakes of
the tree is on a slope, or continues to lean more and more. On a
high side continues to build to an ever-higher elevation. On the
the change is greater; the roots run all over the hillside. I
In figure 2 of "Forest Giants," The tree is measured
at the correct
elevation. The central basal contour coincides with the
centerline of the
trunk, as I suggest. It's not halfway between the elevations of
and low sides, as you recommend. In figure 3, the difference in
is influenced by a single root. The central basal contour (as
figure 2) is ignored in figure 3, and the measurer has abandoned
actually lies slightly above the tape.
I can appreciate the difficulty in relocating an exact point on
western conifers. Their longevity suggests that the basal
relatively unchanged for our purposes, and an arbitrary starting
between high and low sides remains relatively fixed despite some
of new wood. That elevation may not be where the acorn sprouted,
vary from tree to tree, but if it can be relocated, it's the
alternative, and I understand your position.
In the east, the central basal contour is more obvious. Some say
sight the centerline of the trunk, and I reply that if they can
frankfurter, they can sight a centerline. It may not be perfect,
figures 2 and 3 suggest that it's more accurate than taking an
from some wayward root that continues to venture downhill.
Of course, different approaches must be made for floodplain
trees buried in
silt, stilt trees originating on logs, and trees on roadside
on their taproot. Common sense always trumps arbitrary rules.
Incidentally, looking at figure 4, have you considered applying
the Rule of
73 to such low-waisted trees, which have an "unfair"