Diameter Breast Height – Where Does it Start From, Ground Level Right?

A discussion recently arose on “what should be considered as the starting point for measurement of diameter breast height, that is at what point does the stump height / tree stem start”. Seems simple doesn’t it; “high side of the tree at ground level” Well this forest type had a range of buttressing, exposed roots, was mostly on flat ground with spot and line cultivation evident. We were intrigued in the lack of clarity here that sparked this debate, so to offer some clarification and to help guide standards, let’s look over the guidelines available to us in this area.

Figure 1 – DBH Height measurement examples.

Firstly a look at the trees in question (Figure 1). Red mark-up shows correct / accepted technique: locating highest ground soil level immediately around tree base (after removing needles / leaves), see Figure 20 diagram D below. Yellow lines show the incorrect method of selecting the general ground surrounding ground level thus ignoring the spot mounding effect of solid soil being present higher on the tree than the surrounding ground level. A simple practical application of this guideline is “can you put a chainsaw through the stump at this point without clogging the chain with soil”.

Guidance on the Assessment of Diameter Breast Height

Interpine developed a comprehensive guide for the placement of breast height for the Ministry for the Environment, LUCAS Carbon Measurement Planted Forest Measurement Manual (Herries et al, 2010), which has also been used in the Emissions Trading Scheme Forest Measurement Approach (ETS FMA) manuals published by the Ministry of Agriculture and Forestry in New Zealand (MAF, 2011), Figure 20 below. These are published documents and serve a basis for some clarification, specifically diagrams C,D.

Figure 20 – Standard Points of Diameter Breast Height Assessment (extracted from Herries et al, 2010)

Guide to Figure 20:

(A) 1.4m above ground level, 90 degrees to the tree axis.
(B) leaning tree, 1.4m on the inside of the lean.
(C) sloping ground, taken on the high side.
(D) uneven ground, taken on the higher side.
(E) large swelling, two diameters are taken at equal distances from breast height, record averaged diameter. Record the bottom height in DBH height column and the other actual measurement point as a comment in data file.
(F) small swelling, single diameter taken if moving <+/-15cm from breast height (ensure the height of the diameter is recorded).
(G) forked below breast height, two diameter are taken each being considered a separate tree.
(H) fork at breast height, single diameter taken where practical below fork (ensure the height of the diameter is recorded).
(I) butt flaring or buttressing at breast height, single diameter taken where practical above;
(J), (K), (L) small bent and crooked stems showing breast height measured 1.4m in a straight line from the base of the tree.
(M), (N) are down live trees with tree-form branches growing vertical from main bole: when a down live tree, touching the ground, has vertical (less than 45 degrees from vertical) tree-like branches coming off the main bole, first determine whether or not the pith of the main bole (averaged along the first log of the tree) is above (M) or below the duff layer (N). If the general pith line of the main bole is above the duff layer, use the same forking rules specified for a forked tree as shown in (M). If the general pith line of main tree bole is below the duff layer, ignore the main bole, and treat each tree-like branch as a separate tree; take DBH and length measurements from the ground, not necessarily from the top of the down bole. However, if the top of the main tree bole curves out of the ground towards a vertical angle, treat that portion of that top as an individual tree originating where the pith leaves the duff layer, as shown by (N).

Literature Review of Breast Height Measurement

A quick review of literature shows the ruling above to be consistent. Of interest is the use of root collar (RC) / point of germination (POG) or high side soil / ground level whichever is greater (Figure 2 below), which deals with trees growing on substrates other than soil (rocks, mounds, swamp species etc.), is useful and could be added to our general definition in the future, although is not often seem in New Zealand or Australia forest plantations.

“High side is defined as the highest point of the ground around the base of the tree. Kick aside any loose litter and debris. If obstacles obstruct the base of the tree at the high side, measure the DBH from the high side of the ground and not from the top of the obstacle. If high side is lower than the point of germination (POG), then measure DBH from the POG. If the lower portion of the stem has sweep, pistol grip or the tree is on the ground, then measure DBH along the curve and parallel to the centre line of the tree” (BCFS, 2010)
“Breast Height (BH) is defined as (X distance) from the ground level on the high side of the tree. Ground level is determined by clearing away any obviously loose sticks and leaf litter (i.e. a quick brush sideways with foot), then pressing firmly (with foot or height pole)” (Herries et al 2007)
“The vertical distance between ground level and the top of a stump (Source: The Canadian Forest Inventory Committee, its Subcommittees and delegates.). On slopes, ground level is generally taken on the upper side of the stump. Stump height may be the actual height of a cut stump, or some arbitrarily selected standard. In rain forests and in mountainous terrain, the point of germination (POG) is used in place of ground level” (WOD,2012)
“When trees are growing on objects, such as rocks or logs, measure at (X distance) above the root crown rather than above the forest floor. (Figure 37.1N). [Source: FSH2409.12-2000] Trees that reside in water much of the year can also produce prop-like roots, measure diameter in a similar method at (X distance) above the root crown” (USFS 2010).
“Ground level excludes loose leaves and litter that is not incorporated into to soil. Clear it away before taking measurements of height. Consistent definition of ground level on sloping ground or for leaning trees is essential to maintain precision of measurements. Conventionally, ground level on sloping ground is taken to be the uphill of a vertical tree. If a tree is leaning, imagine which would be the uphill side of the tree if the ground was rotated to make the tree vertical. Thus, on level ground, ground level would be defined from the underside of the tree” (Brack, 1999)
“Where trees are planted on turfed or ploughed ground, the diameter should be measured (X Distance) from the root collar or highest ground level around the tree, whichever is higher” (Hamilton, 1996)