Using the MSU Basal Area Angle Gauge
Basal area (BA) is commonly used by foresters as a simple and easily measured estimate of the density of a stand of trees. BA is simply a measure of the cross-sectional area of a stand of trees at 4.5 feet aboveground and is expressed in square feet per acre (ft2/ac). In other words, think of BA in the context of slicing all trees on a given acre at 4.5 feet above the ground, then adding the cumulative area of diameters at the point where all trees were cut.
This is the procedure for calculating BA for an individual tree:
Basal area (BA) = 0.005454 x DBH2
(DBH = diameter at breast height, or 4.5 feet above the ground)
To determine basal area on a per-acre basis, you can use an angle gauge of a known basal area factor (BAF). Using a known BAF, each tree measuring “in” on the angle gauge counts as one multiple of a predetermined BA. For example, using a BAF 10 angle gauge results in each “in” tree representing one multiple of 10 ft2/ac of BA. This provides a fast and simple method of BA estimation for foresters and other land managers.
BA estimation on a per-acre basis follows this simple method:
Basal area/acre (BAacre) = (number of “in” trees) x (BAF)
The measured BA can be used as a “trigger” by forest managers in making the decision for thinning a stand of trees. For example, when BA of a pine plantation reaches 120 ft2/ac, foresters know that it is time for a thinning. Mangers can also use target BA as a guide for how many residual trees to leave in thinning operations. For example, many foresters desire to leave a residual post-thinning BA of 70–90 ft2/ac. The angle gauge is the simplest of several tools available to measure BA. If used properly, the instrument can give relatively precise estimates of BA.
Using the MSU Basal Area Angle Gauge
The MSU Basal Area Angle Gauge can be used to obtain BA estimates quickly. Be sure to extend the angle gauge the correct distance, and evaluate trees carefully to avoid incorrect estimations of BA. This is the correct procedure for using the MSU Basal Area Angle Gauge:
- Extend the angle gauge 25 inches from your eye. To maintain the correct distance from your eye, measure string or twine and tie it to the angle gauge (Figure 1). Keep your eye at a central point and extend the angle gauge from that point. One commonly used technique is to place a staff or stick at the center point of your measurement plot, and extend the angle gauge from that point.
- Mark your first tree and pivot in a clockwise direction, evaluating each tree individually. Look at each tree at 4.5 feet above ground level and assess the “in” status using the angle gauge. Decide whether each tree is “in,” “out,” or “borderline” (Figure 2). Each “in” tree counts as 1 unit of the BAF of your angle gauge (BAF of 10 using the MSU Basal Area Angle Gauge), and each “borderline” tree counts as ½ unit.
- “in” tree = diameter of tree at 4.5 feet appears larger than the box width on the angle gauge
- “out” tree = diameter of tree at 4.5 feet appears smaller than the box width on the angle gauge
- “borderline” tree = diameter of tree at 4.5 feet appears exactly the same width as the box on the angle gauge
- Once you have pivoted back to your initial measurement tree (take care to not overlap), multiply your “in” and “borderline” tree count by 10 (the BAF of the angle gauge). For example, if 11.5 trees were counted, you would multiply 11.5 by 10 for a BA of 115 ft2/ac. Multiple BA counts should be averaged throughout a timber stand to obtain a good estimate of stand BA. At this point, the BA estimate can be used in management decision-making processes.
If you would like an MSU Basal Area Angle Gauge, please contact your local MSU Extension Forestry specialist or the Extension agent in your county.
Information Sheet 1995 (POD12-17)
By A. Brady Self, PhD, Assistant Extension Professor, Forestry, and James Floyd, Forestry Instructor, Holmes Community College.