If you have had interactions with a forester before or read forestry-related articles, you probably have heard the term basal area mentioned at some point. For some reason, basal area has always seemed to be this mystical term that folks struggle with understanding what it is and why it’s important. In this article, we will discuss not only what I just mentioned but also how you, as a landowner, can measure basal area yourself and use it to help communicate with others in the forestry profession.
To understand basal area, one must imagine it as being a cross-sectional measurement of the tree at Diameter at Breast Height (DBH), which is located 4.5’ above the ground on the tree. The measurement is typically measured in square feet. Most foresters use a tool (i.e., wedge prism) to accomplish this measurement. A wedge prism is just a wedge-shaped piece of glass calibrated to a 10-factor measurement. Wedge prisms come in two colors, yellow and clear, which really boils down to preference and can easily be bought online from professional forestry supply stores.
Basal area is often expressed as being on an acre basis. It tells us the square foot volume of standing timber for that acre. This is then used to help gauge the density of growth on the landscape. Typically, land with the primary goal of timber production has always been considered to have an ideal basal area when ranging between the values of 80-120sq ft basal area. Once the basal area gets above 120 sq ft, a stand needs to be assessed to see if it is time to be thinned back to 80. If landowner objectives are more wildlife-minded, it’s not uncommon for them to want a 60-80sq ft basal area, and they thin timber accordingly. Some wildlife species, such as quail management for example, may target an even lower basal area, usually around 40, within a mature pine stand.
So, how do we measure basal area on an acre basis? Let’s start with a wedge prism example. We are going to say that we bought a 10-factor wedge prism. We randomly pick a spot in our forest, and we will try to put the wedge prism over a center point within the spot we chose. We begin walking around the wedge prism while counting ‘In’ trees. An ‘In’ tree is when we look at a tree through the wedge prism, and the refracted image we see inside the wedge prism shows us that the image (bark) touches both the tree’s trunk above the wedge prism and the trunk below toward the stump. Once we have counted all the ‘In’ trees as we rotated a complete circle, we then multiply by our ‘factor’, which in this example is 10 due to the above-mentioned prism we bought. So, 9 trees counted as ‘In’ would mean a basal area of 90 (9 trees multiplied by 10-factor prism) for the acre. You may hear the phrase ‘Out’ trees, and that merely defines trees where the refracted image did not have the tree’s barked trunk touching the tree’s trunk above and below. Think of it as if electricity had to pass through; you are either connected together or you’re not.
As a landowner, you do not have to own a wedge prism to accomplish this measurement process. Years ago, I wrote a fact sheet on how you can make a homemade basal area tool to calculate basal area on your own timber stand. ‘Homemade Devices to Determine Basal Area’ https://blogs.clemson.edu/fnr/2021/08/20/homemade-devices-to-determine-basal-area/. If you are interested in utilizing this method, please visit the provided link to keep this article short for everyone reading. Just remember, when using this homemade device method, your eye is the plot center, and you rotate the object being used around you. This completely differs from a wedge prism because when using a wedge prism, the wedge prism should be the plot center and you walk around it while taking measurements. Also, when using this homemade device method, you are counting the trees that are wider than your object, those are your ‘In’ trees in this process. Trees that are not wider than your object are ‘out’ trees. Tally all your ‘In’ trees and then multiply by 10 due to the calculation. The number is the basal area for that particular spot.
The one situation we’ve yet to discuss when measuring basal area is what is called a ‘borderline’ tree. That is a tree that’s too close to call if it is an ‘In’ tree or an ‘Out’ tree. My rule of thumb (pun intended) is to count the first borderline tree as ‘In’ but do not count the next borderline tree. Once you have omitted the second borderline tree, you can go back to counting the third one as ‘In’. If a scientific determination is needed on your basal area due to you are getting too many borderline trees, please consult a forester for help.
In Tree 
Out Tree 
Borderline Tree
When wanting to determine your stand’s basal area, the more plots (measurement locations) you take, the more you can average, and the better determination you can make about your property. Remember to pick the most random spot in your forest stand that you can. Try not to cherry-pick prime spots only because this can skew your data. Also, try not to pick open spots either, as this too will skew your data. Also, you may have a timber stand where one portion is on poor soil, and the trees are not as many or as big as they are near the creek bottom. Thus, it might be good to delineate them as separate areas. The more information you can gather, the more in-depth conversation you can have with your forestry or wildlife professional. Again, management decisions are often best left to a qualified professional, but that shouldn’t stop you from starting the conversation with good, knowledgeable information (data) and asking the right questions regarding the numbers collected.
Author
Stephen Pohlman, Cooperative Extension, Forestry and Wildlife Agent
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