Diameter at breast height

Diameter at breast height, or DBH, is a standard method of expressing the diameter of the trunk or bole of a standing tree. DBH is one of the most common dendrometric measurements.

Tree trunks are measured at the height of an adult's breast. However, this is defined differently in different countries and situations. In continental Europe, the UK, and Canada the diameter is measured at 1.3 metres above ground. In the US, Australia, New Zealand, Burma, India, Malaysia, and South Africa, breast height diameter is measured at a height of 1.4 metres. Previously 4.5 ft (1.37 m) was used. In many cases the height makes little difference to the measured diameter. Ornamental trees are usually measured at 1.5 metres above ground.

However, some authors maintain that the term DBH should be abolished precisely because the heights at which the diameter is measured are so variable and because it may strongly influence forestry calculations such as biomass. Instead D_x was proposed whereby the x denotes the exact height above the floor (and along the stem) at which the diameter is measured. For instance D₁₃₀ denotes a diameter measured at 130 cm above the floor and along the stem.

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. Other ambiguous settings for determining the exact place where to measure the diameter is given in Dahdouh-Guebas & Koedam (2006).

Treetec, a company based in Melbourne, has created a free online calculator to determine the DBH for multi-stemmed trees as per AS 4970-2009 Protection of Trees on Development Sites. [1]

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). The measure assumes the trunk has a circular cross-section and gives a directly converted reading of the diameter. It is accurate for most plantation trees.

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