The girth (also called circumference) or the diameter of the trunk is the most often measured parameter of monumental trees. For most trees (conifers and broadleaved trees) in temperate climates, a growth ring is formed each year, so the gradually increasing circumference is an indicator of tree age. While a tree is alive it increases in girth annually, although some trees can collapse due to old age or can have different girths in dry and wet seasons (e.g. African baobab trees). For vigorous species this growth can be measured each year. Most old trees are best measured every 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). In the UK and Belgium girth is measured at 1.5 m (5 feet). 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 ‘ground level’ is the top of the soil and does not include leaf litter or moss.
The aim is to measure girth at 1.30 m above the ground from where the tree germinated or sprouted, or was planted as a small tree. This is assumed to be at the centre of the trunk - see instructions below. A 1.3 m stick is useful to stand upright next to a tree as a height marker and handy for gaining access through bracken or nettles! Make sure that the tape is level; if the tree leans then ensure that the tape is perpendicular to the trunk.
A tree with a normal formed, slow tapering trunk on level ground should be measured 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 of this.
If the tree has a distorted trunk, and “a” is smaller than “b”, then measure at height “x”. If burrs or swellings are evident at 1.3 m then put the tape round the stem at various points below 1.3 m to determine the height with the smallest measurement.
With trees on a slope, 1.3 m above the centre of the trunk can be estimated by taking the average of the highest and lowest points of the ground level around the trunk.
Measure a low forking tree at the smallest point of the trunk below the forking and note the height (x) above the average ground level where this was measured.
Measure a leaning tree perpendicular to the axis of the trunk, with the average height of the tape at 1.3 m above the average ground level.
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.
If a tree is a fusion of stems, appearing as a solid bole, but is actually a bundle planting of more than one tree, then the girth of the fused trunk can be measured but it should be classified as a single multi-trunk tree.
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