All recorded measurements should be rounded down to the nearest whole number.
Foresters round down in tree measurements instead of rounding up, because the tree has not yet reached the higher measurement. They keep to whole numbers because of the relative accuracy of repeatability – roughed up bark and even relative humidity can make small differences, as can having the tape measure less than perfectly perpendicular to the centerline of the trunk.
Is It One Tree or Two (or More?)
Determine whether a tree has a single trunk or whether it represents two or more stems growing very close to one another. Trunks that have clear separation or include bark at or near the ground line should be considered separate trees; trunks of different species should also be considered separate trees, no matter how close together. When following the circumference rules below, if the point below the lowest fork places the measurement at the ground line, the stems should be considered separate.
First, find the Diameter at Breast Height (DBH) point which is, as a general rule, 4.5 feet up from the ground. (see Example A below)
Then, find the smallest trunk circumference between the DBH point and the ground.
Measure and record, in inches, that smallest trunk circumference. If the tree forks, measure below the lowest fork. (see Example B below)
Lastly, convert the circumference to diameter with the formula D = C / 3.1415
Enter the diameter measurement in the online data sheet for the tree.
Considerations for Determining DBH Point
Tree on Slope: Measure up 4.5 feet along the axis of the trunk on high and low sides; DBH point is midway between these two planes. (see Example C below)
Leaning Tree: Measure 4.5 feet along both the top and undersides of the trunk; DBH point is midway between these two planes. (see Example D below)
Low Branches: When determining where on the trunk to measure circumference, ignore portions that do not form part of the tree's crown, such as dead branches or forks, and epicormic sprouts, which are ones that grow from the trunk or branches.
Obstruction at DBH: If there is a bump, burl, branch, or other obstruction at the DBH point, measure the circumference above and below the obstruction and record the smaller value. A buttress that forms between the trunk and root system as a natural feature of the species (e.g. baldcypress, water tupelo) should not be considered an obstruction.
|A. Diameter at Breast Height||B. Forked Tree||C. Tree on Slope||D. Leaning Tree|
Illustrations copyright International Society of Arboriculture
Download a Tree Measurement Guidelines handout (PDF, 2MB)