Landowners go to forestry meetings and they keep hearing a term used ... “basal area” and they wonder, “What in the world is Basal Area?”
Basal Area, or BA, is a common term used to describe the average amount of an area, commonly an acre, which is occupied by tree stems. In other words, how much of the area of one acre is taken up in trees.
One way to describe what this looks like is to take a very sharp knife and slice off all the trees at DBH. Oh, there’s another term foresters use. DBH is Diameter at Breast Height, which is a standard measure of 4.5 feet above the ground line. DBH is where you measure the diameter of a tree. Now, with all the trees sliced off at 4.5 feet, you have a lot of saucers or flat surfaces of the trees. The surface of these tree slices have an area in square feet.
One acre has 43,560 square feet (208.71 feet x 208.71 feet). The basal area of the tree stems is the amount of square feet of wood on one acre or say 80 square feet of wood in 43,560 square feet. The area of the tree stems is calculated to determine the square feet of the tree stems. Mathematically it looks like this:
Basal Area = pi x ((DBH)/4) x 144
Basal Area = 0.005454 x (DBH)
The number 0.005454 is known as the “foresters constant,” which converts the measured inches into square feet. Table 1 below illustrates the square feet of basal area per tree for several DBH classes. Table 2 can serve as a guide to assist landowners when estimating trees per acre from certain basal area measurements.
Basal area is a useful tool for understanding forest-wildlife habitat relationships and making timber harvest decisions. The higher the basal area number, the thicker the stand and therefore the thicker the canopy.
Normally, a basal area number higher than 100, would need a thinning operation. Generally, timber stands are thinned back to around BA 60-70 square feet. The chart shows if a stand of pine with an average DBH of 14 inches is thinned to a BA of 60, there would be 56 trees per acre.
The canopy of a stand with a basal area of 60 would be fairly open and the sunlight would be reaching the ground which encourages and enhances the wildlife habitat for deer, turkey, rabbits, song birds and pollinators.
Decisions can be made for various forestry goals utilizing basal area as one of the control factors. Say for example, you were looking at a silvipasture scenario. The stand would need to be thinned back to a BA of 45-60, which would allow for grasses to flourish in the more open tree canopy. However, a landowner who is more interested in tall, straight trees for poles, the stand might be thinned back to 75-80 BA, which will keep the stand a little tighter. The trees will force each other to grow upward and would reduce the limb production. The growth rings would be tighter and the diameters would not expand as quickly. The canopy would be closer, therefore, not allowing as much sunlight to reach the forest floor. The lower intensity of sunlight on the forest floor would not be conducive to browse production and wildlife habitat.
Basal area per acre can be determined by measuring the DBH of each tree on one acre, computing each tree’s basal area by multiplying (DBH) x 0.005454 and then adding each tree’s basal area.
Foresters use a prism, also known as an angle gauge. One of the more common prisms is a 10-factor prism. The prism will determine which trees are “in” (these trees are counted) and which trees are “out” (these trees are not counted). You hold the prism over the “point center” and turn around in a circle, making sure to keep the prism over the center. If you count eight trees, then with a 10-factor prism, you have a basal area of 80.
A 10-factor prism generates an angle of 1 inch in 33 inches, which means a 1-inch DBH tree would have to be 33 inches or closer to be “in.” A 2-inch tree would have to be 66 inches or closer to be “in” and so on.
You can make your own “prism” using a penny and a string. The diameter of a penny is 0.75 inches, so if the penny is held 24.75 inches from your eye, you have a 10-factor angle gauge.
You can take a piece of string, tie a knot in one end of the string, measure out 24.75 inches and tie another knot. Hold one knot against your cheek bone and hold the penny at the other knot, stretch the string out tight. Hold the penny over a fixed point and look at a tree. If the sides of the tree appear to be beyond the edges of the penny, then the tree is “in” and it is counted. If the penny covers the tree and the edges of the penny appear to be beyond the sides of the tree, the tree is “out” and is not counted.
An understanding of basal area can be very helpful in making timber management decisions which will impact the quality of the timber stand, as well as the enhancement of the wildlife habitat.
High basal area equals poor wildlife habitat. You want to keep your timber stands in a healthy and good growing condition through timely thinning operations and frequent understory burns.
When the basal area reaches 90-100, the stand should be thinned. The thinning should work to bring the stand back down to an average of 60-70 basal area. The trees will continue to grow and when they again reach a basal area of 90-100, thin the stand again.
The number of thinning operations will depend upon your desired length of rotation. Some managers choose a shorter rotation, maybe only 32 years. I generally go with a longer rotation, 45-50 years.
Whatever plan works best for you with your timber scenario, consider the basal area, it will help you make decisions.
Remember, Forestry Is Fun! Make sure to share with your kids, keep them engaged with the land.
Involve them in the decisions you make for timber management and share your passion of forestry.
If your kids are not involved with the management of the land and environment, the first thing that will occur as soon as your succession is settled will be a sale of the land.
(Tim Holland is a consulting forester for Mudd & Holland Consulting Foresters in Shreveport.)