Two measurements that support nearly everything
If you only recorded two things about a tree, the most useful pair would be trunk diameter and height. From them, foresters estimate volume, growth and, through equations, biomass and carbon. They are the building blocks of nearly every forest inventory, and the quality of the larger estimates depends on getting them right.
Measuring diameter
Where to measure. Because trunks are wider at the base, diameter is measured at a standard height so that trees can be compared. This is called diameter at breast height, usually abbreviated DBH. In many countries the standard is 1.3 meters above ground; in the United States it is 4.5 feet, which is about 1.37 meters. The key is to follow the standard of the program you are working with and use it consistently.
Tools.
- A diameter tape has a scale that converts the circumference into diameter directly, so you wrap it around the trunk and read the number. It relies on the relationship diameter = circumference ÷ π.
- A regular measuring tape can be used to measure circumference, followed by a calculation.
- Calipers are a sliding jaw device that clamps across the trunk. They are quick for small trees but awkward for large ones.
- A Biltmore stick is a simple ruler held at a fixed distance from the eye that gives a rough diameter. It is less accurate than a tape.
A hypothetical example: if a tape wrapped around a trunk reads a circumference of 94.2 centimeters, the diameter is 94.2 ÷ 3.1416 ≈ 30 centimeters.
Tricky cases. The measurement point has rules for awkward trees. On a slope, you measure from the uphill side. For a leaning tree, the height is measured along the underside of the trunk. If there is a swelling, branch or wound at breast height, the measurement is taken slightly above or below, and the position is noted. For trees that fork below breast height, programs have rules about whether to treat the stems separately. Buttressed trees common in tropical forests have flared bases, so the measurement is taken above the buttress, which makes it a different point from the standard one.
The tape should be snug, level around the trunk and clear of loose bark, moss or vines. Even small errors in diameter have a notable effect on later calculations because volume and biomass equations are steep.
Once diameter is known, a useful derived number is basal area, the cross-sectional area of the trunk at breast height. For a 30 centimeter trunk, area = π × (0.15 m)² ≈ 0.0707 square meters. Adding the basal area of all trees in a plot and scaling to a hectare gives a measure of stand density, as covered in the article on forest inventory plots and sampling.
Measuring height
Height is harder because the top is far away and often hidden by other crowns. Several methods are common.
A clinometer or hypsometer measures angles. You stand at a known horizontal distance from the tree, sight the top of the crown, and read the angle. Basic trigonometry then gives the height above eye level: height above eye = distance × tan(angle to top). To get the full height, you add the height of your eye above the ground, and if the base of the tree is above or below eye level, you adjust using a second reading.
A hypothetical worked example: you stand 20 meters from the tree on level ground. The angle to the top is 40 degrees. The value of tan 40° is about 0.839. So the height above your eyes is 20 × 0.839 ≈ 16.8 meters. If your eye is 1.6 meters above the ground, the total height is about 18.4 meters.
Many clinometers show a percent scale. A reading of 84 percent corresponds to the same 40 degree angle, since percent is 100 × tan(angle). At a distance of 20 meters, 84 percent gives 16.8 meters again.
Laser rangefinders and electronic hypsometers measure distance and angle automatically and calculate height. They are faster, but leaves, light and dense foliage can confuse them, so the operator must be sure they hit the highest point and not a nearer branch.
A height pole can be used for short trees and saplings.
The stick method uses a straight stick held at arm’s length, aligned with the tree, in the classic way people estimate height. It is rough and best suited to educational activities.
Shadow methods compare the shadow of the tree with the shadow of a known object. This works only in clear sun on level ground.
Remote tools. Airborne scans and drone surveys can determine canopy height across large areas, as described under forest lidar. These tools are good at the tops of the canopy but still need ground checks.
Common mistakes
- Wrong distance. Standing too close makes the angle steep and magnifies errors. A distance roughly equal to the tree’s height is often recommended where the view allows.
- Sighting the wrong top. In a broadleaf tree with a rounded crown, the highest point is often not directly above the trunk. It can be hard to be sure.
- Slopes. Distances should be horizontal, not along the slope.
- Leaning trees. A tilted trunk is measured from a direction perpendicular to the lean to avoid underestimating or overestimating.
- Broken or dead tops. These should be recorded as such, since the tree might have been taller earlier.
Other tree measurements
Foresters also record crown width, height to the base of the live crown, species and condition. Crown and condition notes feed into the checks described under forest health indicators. Growth is derived from remeasurement; increment borers, which extract a thin core of wood, let people count tree rings to learn about age and past growth, a topic taken up in the article on tree rings.
Why consistency matters
A tape and a clinometer are modest tools, but careful and consistent use gives data that can be compared over decades. Training crews, writing down the protocol, marking the measurement point on the trunk, and double-checking some trees all reduce error. These measurements are the foundation that larger methods build on, from sample plots to calibrating remote sensing.