Stand Volume and Growth:
Getting the Numbers

1. Key numbers you will generate

By following the procedures we outline, you’ll be able to generate several numbers describing your stand that will be useful in evaluating its performance. These include:

Number of trees per acre.
This is a good start, but the other numbers generated through the use of this publication are more valuable in terms of management decisions.

Number of trees per acre by diameter class.
Also called a stand table, these numbers can be used for planning logging jobs and evaluating the tree size that’s available to merchandize. We’ll also use them as the starting point for projecting future stand growth.

Average stand diameter.
This is another number valuable when making decisions regarding merchantability and the selection of appropriate logging equipment. It is also used to project stand growth. Average stand diameter, along with trees per acre, can provide information useful in making thinning decisions.

Basal area.
This is a measure of the cross-sectional (circular) area of a tree 4.5 feet above the ground, commonly called breast height. The sum of the basal area for all trees in the stand is the total stand basal area, a commonly used measure of stand density.

Tarif number.
A tarif number identifies the taper or shape of your trees and is the key to determining volumes. A tarif number is the cubic foot volume of a tree with a basal area of 1 square foot and a given height. For example, a 13.56 inch diameter tree has a basal area of 1 square foot. If this tree had a volume of 35 cubic feet, its tarif number would be 35. Trees with lots of taper will have low tarif numbers, while trees with high tarif numbers will have a minimum amount of taper.

Stand volumes.
Using the tarif number from your sample trees, you can look up volumes of trees of various diameters in board-foot or cubic foot volume tables. To convert these into per-acre volumes by diameter class, multiply the individual tree volumes by th enumber of trees per acre. Tree volumes are probably the most useful numbers you'll generate. It’s important to remember that these numbers are gross volumes – they don’t consider losses for defects and breakage.

Board-foot volume often is the figure of greatest interest, since most timber is sold at a price per 1,000 board feet (MBF). There are several methods of scaling or measuring board feet, but we’ve used the most standard for the Pacific Northwest, the Scribner Volume Table.

Cubic-foot volume of the tree stem is a basic measure of wood volume that’s independent of how the tree is cut into logs. It’s also useful in determining some of the basic growth relationships for the stand, and for comparison with other stands or species.

By utilizing the above-described numbers along with measuring their ring widths from increment cores, you’ll be able to measure past growth and estimate future growth. This will allow you to determine:

Growth projection factor (GPF). This number can be used in conjunction with board-foot or cubic-foot volumes to determine future stand volumes given current growth rates

Mean annual increment (MAI). This number represents the average volume growth per year over the total life of the stand.

Periodic annual increment (PAI). This number represents the annual volume growth measured over a specified period of time. The PAI is usually calculated for a 5 or 10 year period. We recommend you use the growth over the last five years when calculating PAI.

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