Tree Girth Measurements    Ed Frank
   Nov 23, 2003 21:53 PST 


If we are going to use girth measurements to form a new index then the
methodology of measuring girth should be clearly specified in one
document. There are several relevant posts on the ENTS site by Colby
Rucker... The Rule of 73 and Measuring tree girth. The relevant info
is copied below. Please make any additions, elaborations,
disagreements, etc. to the material and I will compile a protocol
incorporating these materials.

Measurement Height:

Trees are often compared by measurements of the trunk circumference
taken at breast height (4 ˝ feet, or 54 inches above grade). National
and state big-tree registries use this measurement, often known as cbh,
by adding one point for each inch of cbh to points for height and
average spread in their periodic lists of champion trees. (Rucker, July

Base point for measurement:

All height measurements start from the same place - "where the acorn
sprouted." If you stand at mid-slope and sight down the centerline of
the trunk, that line meets the soil at the central basal contour. You
can't determine the contour by averaging the elevation on the high and
low sides of the base; roots enter the soil abruptly on the high side,
but run all over the hillside on the lower. The central basal contour is
a constant elevation; it extends level from your side of the trunk,
through the spot where the tree first sprouted, and out the other side.
Like all contour lines, it may curve, but it's at a constant elevation.
If you measure upwards with the lean of the tree 4.5 feet above that
contour, that point is breast height on either side of the tree.
(Rucker, Aug 2002)

Circumference Measurements:

Circumference is measured at right angles to the lean of the trunk. If
you temporarily mark breast height on both sides of a large tree with a
scrap of paper and a thumb tack you can be sure the tape is even. You
can carry some thumb tacks pressed in a wine cork. Of course, if erosion
has left the tree standing on its taproot, you'll have to estimate the
elevation where the tree first started. If floodplain deposition has
raised the grade, you'll have to accept the existing grade level. We
don't dig 'em out.

On a very large tree, a tape set at breast height may still be below
grade on the high side. In that case, raise the tape to a higher
elevation and attach that elevation to your circumference record.
Actually, a variety of measurements is very useful, including several
taken with the slope, starting at grade. This will allow others to model
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' 8"
tuliptree on a slope above a precipitous drop at the head of a ravine.
(Rucker, Aug 2003)

Burls and Knots:

When a fork, burl or low-branching habit causes the trunk to have its
smallest girth at some point below breast height, many of the big-tree
registries have allowed that lower girth to be entered as cbh. This has
given such trees an unfair advantage over competitors that have a
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 trees
with a low “waist,” and those that taper in a typical fashion. An old
list of national champions provided the elevation of 35 circumferences
of trees measured below breast height. I found that those trees
averaged eight feet in circumference. I then measured actual mature
trees of ten species, which also averaged eight feet in girth at the
same height. After taking circumferences at numerous elevations, I
devised a formula to fit the actual flaring contours of the trunks
measured. That formula, which I call “The Rule of 73,” is as follows:

Measure the smallest trunk circumference at or below 54 inches. Add
one-half the elevation (in inches) of the circumference to 73. Apply
the sum as a percentage of the measured girth. The product is the
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 ground be
listed as an addendum.]

There also is an essay on the website by Colby on how to measure the
trunk diameter using a laser range finder.

Other Questions:

I am wondering about how the cbh or equivalent should done for plants
like Rhododendron or Mountain Laurel. The formula works for trees, but
their major branching is above the height measures. These shrubs are an
important part of the picture of the forest, but in most cases there is
significant branching below this height. Is it reasonable to measure
one of these branches as the diameter of the trunk? I don't think so...
I would suggest that shrub-like plants be measured at a height of 6
inches above grade to provide a fairer measure. The girth index is a
ranking and this should not adversely the conclusions.

Comments, additions...

Ed Frank

Tree measurements
  Dec 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 is irrelevant.

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 measurement.

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.

RE: Tree measurements   Robert Leverett
  Dec 05, 2003 06:20 PST 


Thanks for weighing in. Most of us look to you for the real advances
in measuring methodology. One point that you appropriately make is that
methods that apply to small trees (though large to us) have to be
rethought when applied to really big trees such as those you work with.

Re: Tree measurements   Colby Rucker
  Dec 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 unchanging reference
point throughout the life of the tree. Whether its tip is two feet above
that or two hundred, that's the actual starting point. Of course, locating
that inaccessible point can be difficult. By placing it on the "central
basal contour," a contour line, sometimes curving, like a watermark, that
extends out both sides of the tree, the elevation becomes more accessible,
allowing us to measure up 4.5 feet on both sides. And, like high water, we
can see the point as part of a horizontal plane, like the one passing
through the highest leaf or twig.

The real challenge is where to assign a working elevation to conceptual
levels. Trees grow. I started keeping careful records of trees in 1958,
and many have doubled their girth. To get a fairly accurate idea of growth
rates, I must measure over the same spot, the same flakes of bark, whether
the tree is on a slope, or continues to lean more and more. On a slope, the
high side continues to build to an ever-higher elevation. On the low side,
the change is greater; the roots run all over the hillside. I can't measure
from either.

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 the high
and low sides, as you recommend. In figure 3, the difference in elevation
is influenced by a single root. The central basal contour (as shown in
figure 2) is ignored in figure 3, and the measurer has abandoned CBH, which
actually lies slightly above the tape.

I can appreciate the difficulty in relocating an exact point on very large
western conifers. Their longevity suggests that the basal profile remains
relatively unchanged for our purposes, and an arbitrary starting point
between high and low sides remains relatively fixed despite some deposition
of new wood. That elevation may not be where the acorn sprouted, and may
vary from tree to tree, but if it can be relocated, it's the next best
alternative, and I understand your position.

In the east, the central basal contour is more obvious. Some say they can't
sight the centerline of the trunk, and I reply that if they can split a
frankfurter, they can sight a centerline. It may not be perfect, but
figures 2 and 3 suggest that it's more accurate than taking an elevation
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 cuts standing
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" advantage, for
competitive purposes?