Forest Mensuration. Brack and Wood

In many circumstances, the point 1.3 m above the ground may not be clearly recognised or tree measurements at this height may be unrepresentative. Standard instructions help ensure all measurements are taken at the same height on the trunk by all measurement crews throughout the country.

Where is 1.3 m above ground level?

1. Sloping ground: measure 1.3 m up from the uphill side of the stem.
2. Leaning trees (on level ground): locate the point on the under-side of the tree parallel to the axis of the stem.
3. Leaning trees on sloping ground: Imagine rotating the earth so that the tree is vertical and then locate the point as for a sloping tree. Apply common sense.
4. Coppice crops should be measured at 1.3 m from ground level, not from stool level.
5. Displace loose mounds of soil and litter.
6. Remove loose material at breast height - vines, moss, loose bark.
7. Trees forked below breast height - treat as a double stem (DS)
8. Trees forked above breast height (DL - double leader; ML - multiple leader) - treat as a single stem.
9. Trees forking at breast height or slightly above are measured at the point of minimum diameter below the fork.
10. Locate breast height using a 1.3 m stick. In some applications, the dbh point may be located by eye, e.g. measurement of pulp size material (2 or 3 cm size classes).
11. Measure at right angles to the stem axis. Keep tapes taut.
12. In tropical high forest, buttressing and fluting are common. Special instructions must be issued to field crews in such situations to ensure standardisation and comparability of records. Normally, measurement is made above the buttress/fluting. Where this extends well up the bole, an arbitrary height is specified, e.g. 3 m above ground.

Unrepresentative points

• Presence of branches.
• Nodal swellings.
• Malformities.
• Wounds.

1. Take two measurements equidistant above and below the nominal point of measurement. Mean the two measurements if there is little difference between them. If the difference is raltively large or the parameter is to be used in volume calculations, take the quadratic mean (an equivalent of the mean of the sectional areas). A quadratic mean is the mean of the square of the 2 diameters:
   

        Dq = sqrt((D1^2 + D2^2)/2)
        where Dq denotes the quadratic mean diameter
              D1 and D2 denote the upper and lower diameters
              sqrt denotes taking the square root.
   

2. Alternatively, select one position of representative size - a single point of measurement is faster than the above 'above' and 'below' method. The bias of a subjectively selected representative point is not likely to be serious particularly if one is more concerned with increment. However, if basal area is the main concern, bias from a subjective point may be important. In periodic measurement (sample plots), a single representative point may be located not by eye, but by trial and error based on the mean of 'above' and 'below' measurements.

http://online.anu.edu.au/Forestry/mensuration/BH.HTM
Cris.Brack@anu.edu.au
Fri, 20 Dec. 1996