Tree GirthIt is important to note that numerous authorities around the world measure circumference or diameter at different heights (referred to as Breast Height - BH ). In the USA, Breast Height is measured at 4.5 ft (137 cm) above ground level. The UK Tree Register and British tree standards use 1.5 m (150 cm). The IUFRO (International Union of Forest Research Organisations) Standard is 1.3 m. Authorities in Australia and New Zealand have adopted 1.4 m. as their standard height to record the girth or diameter of a tree above ground level.
New Zealand Breast height (BH) = 1.4 m
Tree girth is usually measured at 1.4 m from ground level adjacent ‘to where the acorn fell’. This measurement is usually referred to as girth at breast height (GBH) or circumference at breast height (CBH). The real objective here is to get a true measurement of a tree's stem, outside the influence of buttress or branch swellings. Some trees can have very large buttresses that can unduly influence girth measurements.
Trees growing on sloping ground present difficulties establishing where GBH should be taken from. Some authorities recommend GBH on these trees should be measured at the median of the top and bottom of the slope adjacent to the trunk. Problems can occur on very steep sites or if down slope roots make it difficult to establish where the base of the tree is. Another approach is to take the measurement from the base of where the centre-line of the trunk meets the ground i.e. mid-slope. As most people will be able to sight the centre-line of a tree without difficulty, the later approach may result in more reliable measurements being recorded.
The following rules apply to measuring tree girth.
Always record girth measurements in your field notes. Girth will be converted to diameter on the website.
Always make sure the tape is level - slide the tape around the tree to obtain the smallest measurement.
Always measure girth from mid-slope on trees growing on slopes.
If a tree forks or abnormally swells at or below 1.4m, then the smallest measurement below 1.4m should be recorded and the height from ground level recorded in your notes.
If your girth measurement includes multiple/fused stems, record this in your notes. Record how many stems or piths are contained in the girth measurement.
If a tree has multiple stems, the largest stems can be recorded and additional stems indicated by use of a + sign. e.g. 34cm + 29cm girth @ 1.4m + + +.
In all cases the girth is taken perpendicular to the axis of the trunk.
It is important to note that Champion Tree Points are NOT calculated on the NZTR for multi-stemmed trees. Scores are only shown for trees with a single stem measurement.
For additional information on measuring tree girth go to www.nativetreesociety.org/
This site includes many diagrams and excellent discussion on the science of tree measuring.