What Is the Basal Area of a Tree?

The basal area of a tree provides a point of reference for measuring tree cover.

When you want to measure the total density of the tree coverage on a property, one way to take this measurement is to first find the basal area of each individual tree. The tree's basal area is a measurement of the horizontal width of the tree at roughly chest height for the average person.

Basal Area Height

Since chest-high obviously varies from person to person, a more specific approximation for measuring the basal area is 4½ feet from the surface of the ground. If you would like to be exact with this measurement, measure up from the base of the tree with a measuring tape and mark the bark with a piece of chalk or tie a string around the trunk of the tree at 4½ feet.

Taking a Basal Area Measurement

Measure the width of your thumb and multiply the measurement by 33. Consider the product the "distance" for your measurement and cut a piece of string to this length. Hold one end of the string up next to your eye and the other end of the string straight out from your body so that your thumb is the length of the string away from your eye. Stand in the center of the trees that you want to measure, point your thumb at the 4½ foot mark you made on the first tree and close your other eye.

Combining Basal Area of Trees

When you hold your thumb up at each tree with only one eye open, you can either see the tree extending beyond the width of your thumb on either side or you won't see any of the tree's trunk beyond your thumb. If you do see some of the trunk, count the tree. If you can't see the trunk, don't count the tree. Turn a full 360 degrees and measure every tree in the area. Multiply the total number of trees counted by 10 to get the basal area of the spot of land.

The Purpose of Basal Measurement

Basal area doesn't have a lot of use for the common household gardener, but it does serve a couple of purposes for forestry experts. Taking several readings at several areas of a property can allow forestry experts to determine how many square feet of trees cover each acre of forestland, which can help keep track of deforestation. Basal area also allows experts to determine whether or not sufficient tree shade exists to keep wetlands cool and healthy.

References

• University of Minnesota; Making and Using Measurement Tools -- Basal Area; 2002
• University of Florida; Site Quality and Stand Density; August 2006
• Oregon State; How to Measure -- Special Techniques for Trees; Mark V. Wilson; 2007