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Estimating BF in standing trees

Guidelines for accurately estimating lumber yields in standing timber. July 26, 2000

Q.
I have measured the DBH (diameter at breast height) of some standing trees, and have estimated the length of the good logs in the trees. Now, how do I figure the board feet (BF)?

I have the equations for figuring BF in logs. The Doyle (and perhaps all of the equations) are based on the small end measurement, not DBH. Do I have to estimate the taper to use the Doyle equation?



The taper you are referring to is known as form class (FC). This factor is derived by dividing the diameter of the top end of a 16-foot log inside the bark (DIB) by the outside diameter of the bark at DBH (4.5 feet above ground, on the high side of the tree). The fractional quotient is multiplied by 100, and expressed as a percentage.

Here in eastern Oklahoma we use an FC of 78 (or 78 percent taper) from the DBH to the DIB at 16 feet. Northeastern Texas uses an FC of 80. So the FC is different in different areas of the country. Call your local forestry folks to find out the FC of your area.

The FC is only used in the bottom 16 feet of log, since the bark thickness decreases as you go up the tree.

With experience, you can estimate the top DIB. I helps me to measure the DBH, then estimate the top DIB.

Bark thickness at DBH can be measured with a pocketknife. Turn the blade sideways to the bark and with your palm pound the knife in, until it goes no further. Then pinch the knife blade and carefully pull the blade out and measure the pinched length.

Be careful not to let the knife fold while pounding the knife blade into the bark. There is a high-dollar tool called a bark gauge that works more safely, but that's just more junk I'd have to carry in the woods. I lost mine a long time ago.



Timber is measured using a DBH and an estimate of the number of 16-foot logs in the bole. This can be further broken down into half-logs.

The FC info given above is on the money. The problem arises when you have trees that have more than one 16-foot log in them. Your upper logs will diminish in size, and that will depend on the diameter and number of logs in the tree.

For that reason, there are tables that can give you the volume of a tree at a given diameter and height. These are available in practically any measuring book, and used to be available from Ben Meadows Company. The forestry book may be available at your local library. Ben Meadows is a forestry supplier in Georgia.



You may also want to consider a Biltmore stick, manufactured in the FC for your area, as opposed to a table. They are available from Ben Meadows, too. If you are accustomed to scaling logs as I am, the tree scale on this stick will give you BF.

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