Tree Girth Measurements    Ed Frank    Nov 23, 2003 21:53 PST  ENTS: 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 ½ 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 2003) 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   abi-@u.washington.edu   Dec 05, 2003 05:22 PST  ENTS, 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. Cheers, - BVP RE: Tree measurements   Robert Leverett   Dec 05, 2003 06:20 PST  Bob: 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. Bob Re: Tree measurements   Colby Rucker   Dec 05, 2003 11:10 PST  Bob, 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? Colby   Home