A few thoughts came to mind while I was reading a recent thread in the Ask the Experts section.

1. Bark Thickness
When performing drill test boring for decay detection, whether using a hand drill or a Resistograph, you should be careful of how you account for bark thickness. Measure the bark thickness from the point where you measure diameter! IOW, if you use a diameter tape (as I do) the bark thickness should be measured from the outside of the thickest bark ridges to the xylem. If you place your test hole ina bark crevice between ridges, the bark thickness there is NOT representative.

The logic: We want to measure the thickess of sound wood. Since we are measuring from the outside toward the inside, we start with trunk diameter and subtract sound wood and bark to determine where the decay is. If you measure diameter outside the thickest part of the bark, you must use the bark thickness from that point, or you will be overestimating the amount of sound wood.

2. Cavity Width
As I mentioned in the thread in the Ask the Expert section, how you measure cavity width can make a difference in your strength loss estimate. The cavity width used in the formula is actually a ratio of cavity width to total circumference. Therefore, the cavity width should be measured along the line of the circumference. It should NOT be measured as the chord across the cavity face, from one side straight to the other side. Measuring along the circumference will give you a more accurate use of the formula. It will actually increase the percent of strength loss a few percentage points, but that may affect your decision for the tree.

These may seem like minor points, and in many cases may not actually affect your decision. However, when the results are close to the threshold, a few percentage points could be critical. The more accurate your measurements and application of the tools, the greater the confidence level of the results. Get in the habit of doing it right every time, and you won't have to think about it when the time comes.

Reply to post by Russ Carlson, on March 25, 1999 at 12:16:45:

Russ:

Where one chooses to drill holes to determine thickness of holding wood has a bearing on formula results.

Suppose there is a cavity 2 inches under the exterior on one side of the tree.

Method (A) I drill three holes into the cavity and two holes elsewhere, and average my measurements.

Method (B) I drill five holes equally spaced around the circumference of the tree, one of which enters the cavity, and average my results.

Using method (A) would skew the results.

Reply to post by Ed Milhous, on March 25, 1999 at 12:16:45:

Correct, Ed. Where the holes areplaced will skew the results dramatically. Experience and common sense should dictate where to place the holes, and how many are measured.

I usuall place on hole a few inches either side of a cavity or defect, then at least 2 other holes evenly spaced around the remainder. The more holes, the more accurate the estimate (bearing in mind the invasive nature and potential for harm to the tree with more holes.) So if I elect to drill test a tree with a cavity, I make at least 4 holes maybe 5 or 6. Likewise, if I drill a tree with no open cavity, but detect decay, I will often add another hole or two to the list. This helps fill in the mental image of where the decay is inside the tree.

I simple training technique is to take a few hollow chunks of wood, and cover the ends with paper, so you can't see the defect (get someone else to do it, if you can). Then drill just a few inches from the cuts, and record as usual. Sketch what you think the defect looks like, then remove the paper and compare. Amazing how much you learn doing this, and all you need is an old pile of logs.

Reply to post by Russ Carlson, on March 25, 1999 at 12:16:45:


One question:

try to imagine an oak tree 100 feet high with a crown area of 180 sqm and DBH of 4 feet.

There is another oak with a height of 75 feet and a DBH of 4 feet as well.

How hollow may the first tree be and how hollow may the other one be. Let´s say both are standing
in the open countryside.Which residual wall thickness to they need?

Do you have an answer?

please contact my e-mail direct thanks Erk