Determining Tree and Log Volume

By Thembi Borras

Given the tree diameter at breast height (dbh) and the tree height to a merchantable top, there are tools available to help you estimate the volume in a standing tree. Volume tables are one such tool. Volume tables are, in part, specific to species and log rule. Log rules are ways to estimate the number of boards a milled tree will yield. The log rule used regionally is Scribner Decimal C. Volume is most commonly reported in board feet for trees that will be made into lumber. A board foot is 12" wide by 12" long by 1" thick. These are familiar units to anyone who buys lumber.

The volume of a tree may be measured several times before it is made into lumber. When I mark a tree to be cut, I record the dbh and height in logs to a merchantable top. In the local species specific volume table developed by Peter Joos that I use, logs are defined as 16' long and the merchantable top diameter is defined as 6". For example, a 26" dbh redwood with (5) 16' logs to a 6" top is reported in the table to have 620 board feet. As a general rule, height to a 6" top is usually about 30' less than the total height, but can vary greatly depending on the form of the tree. This tally of tree volume helps the landowner, logger and forester plan.

The tree, once it has been cut and bucked into logs, may be measured again by the faller. Henceforth each log is considered individually. The volume of each log is determined by measuring the small end diameter inside the bark and the length of the log. Given these inputs a different volume table is used to estimate the board foot volume. For example, a log 24" in diameter and 20' long with trim has a gross volume of 500 board feet. After the log is skidded, loaded and trucked to the mill, people called scalers will make the same measurements, but will reduce the gross volume of the log by the defect that they identify in it. The resulting figure is called the net volume. Commonly, the landowner is paid by the mill based on the net volume scaled at the mill.

Measuring Tree Diameter and Height

By Thembi Borras

Trees are in part valued on their volume. To determine the volume of a tree the diameter and the height of the tree must be measured.

A flexible measuring tape can be used to measure the diameter. The diameter of a tree is measured at breast height, which is defined as 4.5 feet above the ground on the uphill side of the tree. The circumference of the tree is what you will measure by wrapping the tape around it, making sure there are no kinks in the tape and the tape does not sag. To convert the circumference at breast height to the diameter at breast height (dbh), divide by 3.14. For example if the circumference is 82", dividing by 3.14 will yield a result of 26", therefore the dbh of the tree is 26".

A measuring tape and two sticks of equal length can be used to measure the height of a tree. Find the center point on one of the sticks, hold the other stick perpendicular to it, effectively making the letter "T". Step away from the tree until you can see the top and bottom. Put the base of the "T" between your eyes, then walk away or toward the tree until the top and bottom of the stick line up with the top and bottom of the tree. Make a mark on the ground, the distance between your mark and the tree is the total height of the tree, if you are on flat ground. If you are on sloped ground, the slope distance will need to be converted to horizontal distance.

These are exercises for interested persons that do not have the tools of the forestry trade available to them. Foresters use diameter tapes, biltmore sticks and relaskops to measure diameter and height, or their eye to discern diameter and height more efficiently.