Basal Area - What?
Article #314 September 2020
By Bill Cook
Basal area is a forest measurement for the amount of light passing into a woodland. It is a critical piece of information for the management of some forest types, and not so important for others.
If you were to cut all the trees on an acre, at a height of 4.5 feet, the cumulative area of those stump tops would be the basal area. In the Lake States, the metric is usually square feet per acre. This is a good indicator of stand density.
If the average tree diameter is large, then fewer trees are needed for an optimum stand density. Inversely, a greater number of smaller trees would be needed for that same optimum stand density. Basal area is an important measure, but it’s not the whole story.
The number of trees and diameter of the trees determine the number of square feet of basal area per acre. The leafy canopy of the forest filters light. It’s that amount of light that’s so important to trees and all the other plants of a forest, as well as the wildlife that live in a forest. Photosynthesis rules! Those trees that do more photosynthesis live healthier and longer lives.
Most mature forest stands will have basal areas that run in the 80 to 180 square feet per acre range. Basal area can run as high as over 400 square feet per acre, which is a Lake States extreme that I’ve seen only in certain cedar stands. Cedar has some peculiar life strategies that allow such high stand densities.
In Michigan, two-thirds of the forest area exceeds 80 square feet per acre, a third over 120 square feet. That suggests that much of our forest is mature and might benefit from a thinning or harvest. Hemlock, cedar, red and white pine, northern hardwood (sugar maple dominated), and oak stands tend to have the highest stand densities.
Forest research and management experience taught us that different forest types do better at different stand densities, measured by basal area. A northern hardwood stand grows best, for timber production, at around 90 square feet of basal area per acre. So do red pine poletimber stands. However, red pine sawtimber stands do better at 120 square feet.
So, foresters will recommend thinning practices when basal areas are too high. It’s vaguely like weeding a garden, but so much more.
On the other hand, basal area is not so important of a measure for all forest types. For example, aspen stands will thin themselves naturally (and quickly), as they are very intolerant of shade. Those that grow more slowly are doomed. There is no need to thin aspen. So, there is no need to know the basal area. For aspen, stand age is a more important measure of stand status, among other factors, such as soil type and site conditions.
OK, but how does one measure basal area?
A one-acre circular plot has a radius of 117.5 feet. All the trees over five inches in diameter could be measured for area at a height of 4.5 feet. However, that’s rather tedious and only one plot. Better to use a sampling technique.
Foresters have special tools to measure basal area that make the job quick and easy. A plot center point is determined, a series of them, actually. These tools are used to include and exclude trees as one rotates around the plot center. The tools function using applied geometry.
In the Lake States, our tools are typically calibrated at what is called a “10-factor”. This means that after counting all the “in” trees at a plot, that number is multiplied by ten. That’s the basal area of that plot. Multiple plots are taken to arrive at an average basal area for a given forest stand.
A wedge of glass, called a “prism”, is a common basal area measurement tool. The prism if held directly over plot center and the person rotates around the prism, counting the “in” trees.
An angle gauge is another common tool that comes in various forms. For this tool, the person stands at plot center and turns in a circle, counting the “in” trees. An inexpensive version of an angle gauge involves a penny held at 24.75 inches from your eye. A calibrated string through the penny helps keep that distance consistent and the measurement accurate.
As one might expect, there’s a set of forest inventory skills and techniques needed to obtain accurate and representative basal area estimates. Field foresters have a pretty good “feel” for this after plenty of practice. Knowledge about forest types and their nearly endless nuances is important. Techniques will vary with the level of precision needed and time available.
These abilities might be likened to pulling a tooth. Not a hard concept to understand, but to actually do it properly requires special skills and practices. You probably don’t want a forester pulling your tooth. Neither should you expect a dentist to measure your forest.
If you’re a woodland owner, contact a professional forester when you’re ready to build your future forest.
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|Caption: A boundary line in northern hardwoods. The forest on the right has been thinned to an appropriate basal area level.|
|Caption: Annual rings from a red pine stand that has not been thinned, as shown by more recent close rings on the left, and growth at an earlier age, on the right.|
|Caption: Density distribution for Michigan forest stands. Source: U.S. Forest Service.|
TRAILER- This website was created by a consortium of forestry groups to help streamline information about forestry and coordinate forestry activities designed to benefit the family forest owner and various publics that make up our Michigan citizenry. This website is maintained by Bill Cook, Retired Michigan State Extension Forester/Biologist. Direct comments to firstname.lastname@example.org or 906-786-1575.