DBH — Diameter at Breast Height

When you start looking into tree data, you will run into DBH, or Diameter at Breast Height. It’s actually very self-explanatory for a scientific measurement: DBH measures how wide the trunk is at a particular height. Wikipedia helpfully calls it “one of the most common dendrometric measurements”.

The fun comes in with what the height is. There are national differences: 1.37 m (4.5 feet) in the US, typically 1.3 m in Europe, and sometimes 1.4 m, as in New Zealand forestry guidance and Australian tree-protection practice. Even within a country, the protocol matters: Australian forest measurement guidance also uses 1.3 m.

Use of DBH dates back at least to the 18th century, but it gained much greater significance with the German sustainable-yield forestry revolution, as scientific methods were applied to forest management. Different forestry traditions and systems of units helped produce different standards. In the US, the 4.5-foot convention can be traced to Gifford Pinchot, the first chief of the United States Forest Service and a founder of the Yale Forest School.

There’s an entertaining analysis of this history in Yasha A. S. Magarik’s “Roughly Speaking” (2021). It suggests that 4.5 feet may simply have been a convenient height for Pinchot’s own chest. That’s a hypothesis, rather than a settled explanation - the paper considers other options - but it’s our favorite.

Then the measurement itself has nuance

If a tree is on a slope, the height might be taken from the highest point on the ground touching the trunk, or from halfway between the highest and lowest points, depending on the protocol. The New York State DEC guide uses the uphill side. For a leaning tree, it measures along the stem.

On a slope

With a lean

Rust marks the measuring point. Slopes and leaning stems change where the height starts and how it is followed. Drawn from NYS DEC’s guide, p. 2.

And if there happens to be some form of unusual swelling, the measuring point moves. DEC uses the narrowest point below 4.5 feet for abnormal taper, and a point below an obstructing branch or bump. Its guide notes that the US Forest Service instead measures above the branch or bump. Record the actual measurement height.

Unusual taper

Branches & bumps

Rust marks the measuring point. Avoid distorted sections; record the height used. DEC’s branch-and-bump case is on page 2. Drawn from NYS DEC’s guide, p. 1.

Multi-trunk trees get even more exciting. A fork near breast height calls for measuring the main trunk below the fork. But for several stems splitting higher than six inches above ground, DEC offers three methods; the inventory’s scope of work should say which one is used.

A fork near DBH

Several stems

Rust marks the measuring point. A fork near breast height and a tree with several distinct stems call for different records. Drawn from NYS DEC’s guide, p. 3.

1. Combine the stem areas. Measure the DBH of each trunk, take the square root of the sum of all squared stem DBHs, and round to the nearest whole number. Note that the tree is multi-stem.
2. Average them. Measure the DBH of each trunk and find the average; again, note that it is multi-stem.
3. Add them. Measure each trunk separately, add them together for the DBH column, and list the individual DBHs in the comments. This is the guide’s least-preferred option: it has led to confusion because the tree is not as “large” as the raw number would suggest.

For three stems measuring 5, 6, and 8 inches, those approaches give you 11 inches (rounded from √125), about 6.3 inches (the mean, 19 ÷ 3), or 19 inches. Same tree, very different numbers.

If the split is at or within six inches of ground level, the guide treats the stems as separate trees and records their shared cluster.

A split at ground level

Rust marks the measuring point. A split at or within 6 inches (15.24 cm) of ground level is recorded as a cluster of separate trees. Drawn from NYS DEC’s guide, p. 4.

Or, if Jupiter is ascending, one should… you get the drift.

So why keep using it?

This imprecision has led some to argue that the term DBH should be abolished, because the height it describes is variable and imprecise, affecting downstream calculations. Brokaw and Thompson (2000) proposed writing the height explicitly: D₁₃₀, for example, means diameter measured at 130 cm above the ground along the stem. That makes the convention clearer, though the awkward trees still need rules of their own.

For what it’s worth, there doesn’t seem to be a magic measurement that makes all these field decisions disappear. We may be stuck with DBH for the moment.

In our datasets, we are particularly concerned with a few wrinkles:

• DBH is not universally populated. Missing values limit what we can infer.
• DBH conventions differ across inventories and continents. We generally choose not to adjust for differences in measurement height.
• DBH is one of our best available predictors of age and crown size when those fields are missing. Those are estimates: species, growing conditions, and management such as pollarding all matter.
• DBH varies because of incorrect entry and measurement choices, including the multi-stem methods above. An impressive number can hide a rather ordinary cluster of trunks.

We encourage anyone finding misleading or ridiculous DBH values to submit a correction on the map!

References

• Magarik, Y. A. S. (2021). “Roughly Speaking”: Why Do U.S. Foresters Measure DBH at 4.5 Feet? Society & Natural Resources, 34(6), 725–744.
• New York State Department of Environmental Conservation. Tree Diameter Measurement: Urban and Community Forestry Grant Program Round 15 — Guidelines for Awardees and Consultants, pp. 1–4. The sketches above are original drawings illustrating the guide’s seven scenarios.
• Brokaw, N., & Thompson, J. (2000). The H for DBH. Forest Ecology and Management, 129(1–3), 89–91.