Tree girth measurement
Tree girth is a measurement of the circumference of tree trunk. It is one of the most ancient, quickest, and simplest of foresters' measures of size and records of growth of living and standing trees. The methods and equipment have been standardized differently in different countries. A popular use of this measurement is to compare outstanding individual trees from different locations or of different species.
Tree girth measurements
Girth is a measurement of the distance around the trunk of a tree measured perpendicular to the axis of the trunk. In the United States it is measured at breast height, or at above ground level. Elsewhere in the world it is measured at a height of,, or. The base of the tree is measured for both height and girth as being the elevation at which the pith of the tree intersects the ground surface beneath, or where the acorn sprouted. On a slope this is considered as halfway between the ground level at the upper and lower sides of the tree.This "breast height" value is a measurement grandfathered from decades of forestry applications. It was developed because of the simplicity and ease of measurement. There is no one ideal height at which to measure girth. Tree trunks flare outward at their base. In some trees this flare or buttressing extends only a short distance up the trunk, while in others it may extend or more up the tree, but the measurement is still taken at this default height for consistency. If the flare at the base of the tree extends above this default girth height, then ideally a second girth measurement should be collected where possible above the basal flare and this height noted.
Tree girth is one of the parameters commonly measured as part of various champion tree programs and documentation efforts. Other commonly used parameters, outlined in Tree measurement include height, crown spread, and volume. Additional details on the methodology of Tree height measurement, Tree crown measurement, and Tree volume measurement are presented in the links herein. American Forests, for example, uses a formula to calculate Big Tree Points as part of their Big Tree Program that awards a tree 1 point for each foot of height, 1 point for each inch of girth, and ¼ point for each foot of average crown spread. The tree whose point total is the highest for that species is crowned as the champion in their registry. The other parameter commonly measured, in addition to the species and location information, is wood volume. A general outline of tree measurements is provided in the article Tree Measurement Overview with more detailed instructions in taking these basic measurements is provided in "The Tree Measuring Guidelines of the Eastern Native Tree Society" by Will Blozan.
Maximum girth records
The tree with the largest girth was the Glencoe Baobab in South Africa with a diameter near ground level of, equivalent to a girth of. In 2009 the hollow tree split into two parts. The Árbol del Tule in Santa Maria del Tule, Oaxaca, Mexico has a girth of and a height of, with a crown as measured by Dr. Robert Van Pelt in 2005. The Tule tree therefore has a diameter of as extrapolated from the tape wrap values. However, as the tree is heavily buttressed, and irregular in shape, a calculation of nominal diameter, defined as the cross-sectional wood area expressed as a circle, gives this tree a diameter at breast height of —a much smaller number, but a more accurate representation of the tree's size. Some have argued that the Tule tree is a multi-trunk tree consisting of three separate trunks emerging from the same root mass that have grown together to form the massive base of the tree and therefore its girth cannot be fairly compared to those trees with just a single trunk. Many of the large girth baobabs may be multi-trunk clusters as well. The General Grant Tree in Kings Canyon National Park in California is clearly a single trunk tree. It was measured to have a girth of measured at a height of. There are historical accounts of trees with extremely large girths. These should not be just accepted at face value. In these older accounts the girths were often taken at ground level and incorporated considerable basal flare at the base of the tree. In other cases, the trees measured were multiple trunk masses or coppices treated as single trees in the girth measurements.Single-trunk versus multi-trunk trees
In many, but not all champion tree lists, and for data collected for scientific purposes, there is a need to distinguish between a single trunk tree and a multi-trunk tree. Two smaller trunks that grow together will achieve a large combined girth much faster than will a single trunk tree growing in the same conditions, so if the data will be biased if combined into a single dataset.A single trunk tree is defined as one that would only have a single pith if cut at ground level. A multi-trunk tree would have two or more piths at ground level. In this definition it does not matter if the trunks have grown together, nor if they are genetically the same and growing from a single root mass. If the tree has more than one pith at ground level, it is a multi-trunk tree. Separating data from single trunk and multi-trunk trees is critical to maintain a valid database of measurements. Data from both forms are worth collecting, but they should be considered different forms and the number of trunks included in the girth measurement should be listed for those trees with more than one trunk.
Direct girth measurement procedures
The girth measurement is commonly taken by wrapping a Measuring tape around, and in the plane perpendicular to the axis of, the trunk, at the correct height. In spite of the apparent simplicity of wrapping a tape around a tree trunk at breast height, errors in this measurement are common. The most common error is mixing measurements of single trunk trees with those of multi-trunk trees and not distinguishing between the two. Even with single trunk trees irregular bumps and hollows are common. Some trees have low branches that split below breast height. Other trees have epicormic sprouts, suckers, or dead branches. Some tree trunks stand slanted at an angle rather than vertical. Girths of trees with these features may be measured by competing methods by different surveyors and result in differences.The basic guidelines for dealing with the above difficulties were developed by American Forests, and most of the guideline used by other tree measuring groups around the world are based upon American Forests guidelines. The Native Tree Society measurement guidelines also generally follow the American Forests prescription, with some additional elaborations.
Many trees have burls bumps, and knots along their trunk. If these occur at the 4.5 feet girth measurement height, including them in the measurement would falsely inflate the girth measurement. The girth measurement should then be taken at the narrowest point below the odd growth and the height of the girth measurement noted. In some cases a girth taken just above the odd growth will be more representative of the actual girth of the tree. In these cases the measurement should be taken there and the height above the base of the tree noted.
Some trees have branches at or lower than a height of. Since the purpose of a girth measurement is to get a full measure of the tree's trunk, measurements should be taken at the narrowest point below any significant branching. When taking a girth measurement at a non-standard height the height of that measurement above the base of the tree should be noted. Epicormic sprouts, suckers, and dead branches can be ignored. Some guidelines have suggested that if a tree branches below breast height, that the girth of the largest branch should be measured at breast height ignoring the other branches. However, if a good portion of the trunk volume or cross-sectional area has been split from the total by measuring above a significant branching, then this is not giving a full and fair measure of the trunk's girth. If the pith of the branch does not intersect the pith of the main trunk above ground level, it is not a branch but a separate trunk and this tree should be considered a multi-trunk tree.
If the tree is leaning, measure the circumference at along the axis of the trunk. The distance should be measured along the side of the trunk from the base point of the center of the tree. The measurement is taken at a right angle to the trunk. Some groups recommend measuring the girth at breast height on the upper side when the tree is on a slope rather than from midpoint on the slope. One example is the Tree Register in the UK. There are advantages to either option. Measuring on the upslope side if often easier, it is also higher on the tree and likely will include less of the flare at the base of the trunk, and when measuring extremely large trunk on a slope the upslope side of the girth loop will always be above ground level. Measuring the girth from a reference point at midslope also has advantages. Consider, the tree started as a single sprout and grew upward and outward from that point. This is the point where the pith of the tree would intersect the ground surface supporting the tree. This is the logical base point from which to measure the height of the tree and by extension the girth should be measured with respect to the same base point. This point is fixed at the same location over time as the tree grows. In addition this is a reference point that is present and consistent in all trees no matter the height of the girth measurement. Even if the girth is measured at a non-standard height because of low branching, a large burl, or even on the upslope side of a large girth tree on sloping ground, all heights can still be consistently referenced to this single point present on all trees. Measuring midpoint on the slope is the recommended option.
Trees with very large girths, such as some of the sequoias growing the western United States, can also pose girth measurement problems. If they are growing on even a gentle slope, if girth is measured at 4.5 feet about where the pith of the tree emerges from the ground, the upside of the tape could easily be below ground level. In this case a better option would be to measure the standard girth measurement at 4.5 feet above ground level on the high side of the tree and note this in the measurement description The height of this measurement point above the standard base point at midslope should also be noted.
If measuring a mountaintop forest of stunted trees only six feet tall, a girth measurement made at 4.5 feet would be meaningless. In the case of these stunted trees a girth taken at 1-foot above the base might be more appropriate. The point is that the girth measurements should be taken at the standard heights whenever possible. Where this measurement is not meaningful, an additional girth measurement should be taken at a more appropriate position and that height noted.
Converting the girth measurement to a diameter will always overstate the cross-sectional area of the trunk, therefore it is best to record the raw girth numbers directly rather than convert them to diameters. The conversion of girth values to approximate diameters can always be done later if needed for other types of analysis.
Measuring tree girth directly is a common educational technique allowing students to learn about their local environment in a practical manner. It is often used in the primary setting to introduce topics such as measuring, using numbers and simple calculation. Simple techniques can be used to estimate tree age.