Tree basal area is the cross-sectional area of a tree's trunk at 1.3 meters off the ground, which is approximately chest height. It is used to determine the volume of the tree, the productivity of the forest and competition between trees for resources.

Measure 1.3 meters up the tree trunk from the ground.

Measure the circumference of the tree at 1.3 meters by wrapping the measuring tape around the trunk of the tree.

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Calculate the radius from the circumference by dividing the circumference by 2?. For example, if the circumference is 10 meters, dividing by 2? gives you a radius of 1.59 meters.

Calculate the basal area by squaring the radius and multiplying by ? (Basal Area = ? * R^2). If the radius were 1.59 meters, the basal area would be 7.94 meters squared.

If you want to calculate the basal area directly from the circumference, you can use the following formula where C is the circumference: Basal Area = ? * (C / 2?) ^ 2.