Diameter at breast height

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Diameter at breast height, or DBH, is a standard method of expressing the diameter of the trunk of a tree.

The trunk is measured at the height of an adult's breast; this is defined differently in different situations, with foresters measuring the diameter at 1.3 m above ground, while people measuring ornamental trees usually measure at 1.5 m above ground. In some 'post-inch' nations like New Zealand the breast height diameter is measured at the height of 1.4 m. In most cases this makes little difference to the measured diameter.

On sloping ground, the "above ground" reference point is usually taken as the highest point on the ground touching the trunk, but some use the average between the highest and lowest points of ground. If the DBH point falls on a swelling in the trunk it is customary to measure the girth below the swelling at the point where the diameter is smallest.

The two most common instruments used to measure DBH are a girthing (or diameter) tape and calipers.

A girthing tape actually measures the girth (circumference) of the tree; the girthing tape is calibrated in divisions of π centimetres (3.14159 cm), thus giving a directly converted reading of the diameter. This assumes the trunk has a circular cross-section, which is typically accurate for most plantation trees.

Calipers consist of two parallel arms one of which is fixed and the other able to slide along a scale. Calipers are held at right-angles to the trunk with the arms on either side of the trunk. Precision can be improved on non-circular stems by averaging two caliper measurements taken at right-angles. Electronic calipers are also available enabling highly accurate measurements to be taken and stored for further analysis.

DBH is used in estimating the amount of timber volume in a single tree or stand of trees utilising the correlation between stem diameter, tree height and timber volume, (Mackie, 2006). It can also be used in the estimation of the age of veteran trees, given that diameter increment is the only, "constant non-reversible feature of tree growth", (White, 1998).

[edit] References

• Hamilton, G. J. (1975). Forest Mensuration Handbook. Forestry Commission Booklet 39. HMSO, London.
• Mackie, E. D. and Matthews, R. W. (2006). Forest Mensuration, a handbook for practitioners. HMSO, Edinburgh. ISBN 0-85538-621-5
• White, J. (1998). Estimating the Age of Large and Veteran Trees in Britain. Forestry Commission Information Note 12. Surrey.