The girth (also called circumference) or the diameter of the trunk is the most measured parameter of monumental trees. For most trees in temporate climates (conifers and broadleaved trees) each year a growth ring is formed, so the gradually increasing circumference is an indicator of tree age. While a tree is alive it increases in girth annually, omitting the fact that some trees can collapse due to old age or can have different girths in dry or wet season (e.g. African baobab trees). For vigorous species this growth can be measured each year. Most old trees can best be measured each 5 to 10 years.
In most European countries girth is measured at breast height, which is 1.3 m above the ground (Circumference at Breast Height – or CBH in short). Some instructions (like those of the Tree Register of the British Isles (TROBI)) ask to measure above the highest ground point around the trunk when the ground is not entirely flat.
For this website (Monumental Trees (MT)) we propose to measure the CBH like most American tree measurers (for example the Native Tree Society (NTS)): at 1.30 m above the average ground level. The aim is to measure at the height where the tree germinated or sprouted or was planted as a small tree, so at the centre of the trunk. See instructions below. In the UK and Belgium girth is measured at 1.5 m (5 feet). A 1.3 m stick is useful to stand upright next to a tree for accuracy and handy to gain access trough bracken or nettles! Make sure the tape is level; if the tree leans ensure the tape is perpendicular to the trunk.
On level ground and a tree with a normal formed, slow tapering trunk measure at 1.3 m above ground level. If the ground level artificially or by root pressure has been raised or lowered by erosion, try to measure at 1.3 m above the original ground level and make a note.
When the tree has a distorted trunk, measure at “x” when “a” is smaller than “b”. When burrs or swellings are evident at 1.3 m, put the tape round the stem at various points below 1.3 m to obtain the smallest measurement.
With trees on a slope measure at 1.3 m above the centre of the trunk at the average level of the ground, so measure “where the tree germinated” or where it was planted.
Measure a low forking tree at the smallest point of the trunk below the forking and note the height (x) were was measured.
Measure a leaning tree perpendicular to the axis of the trunk at 1.3 m height along the ax from the center of the not leaning side.
When the trunk splits in two or more trunks below 1.3 m, measure the girth of the biggest trunk at 1.3 m height. Do this also when two trees have been planted close together.
When a tree is a fusion of stems apearing as a solid bole, but actually a bundle planting, the girth of the fused trunk can be measured but it should be classified as a multitrunk tree.
More on tree measuring: