Forest Mensuration. Brack and Wood

Bark Measurement©

Bark is the outer sheath of a tree. It protects the bole from drying out and other damage, especially damage from fire. Bark thickness varies with species, genetic constitution, location or site, tree age, health and size, rate of growth and position along the bole.

Bark is often seen as a by-product of wood production. However it is a potential source of energy and, in processed form, is a suitable growing medium for horticultural purposes being relatively rich in plant nutrients. Its value as a forest product is likely to increase in the future. Should this eventuate, a simple measurment technique or a reliable prediction function relating bark volume to tree dbhob and height will be needed.

Average bark volume ranges from 10 to 20% of overbark volume in most tree species. Thus, the percentage of bark in lorry loads of round timber may vary appreciably.

Bark Thickness Relationships

Bark thickness decreases more or less regularly from ground to tip, with the rate of taper usually, (but not necessarily), correlated with the rate of taper of the stem under bark. Despite this variation, a relationship between average bark thickness at breast height, bark volume and stem variables (dbhob, height) can usually be established. (See, for example Brack et al, 1985).

The ratio of diameter underbark (dub) to diameter over bark (dob) often follows a consistent and predictable pattern with increasing height on a tree trunk:

in many tree species, particularly conifers, the ratio of dub/dob increases up the bole, i.e. bark percentage by volume decreases with height.
in other species, e.g. many hardwoods, the ratio is almost constant with height and,
in a few species, dub/dob decreases with height.

Slower growing trees tend to have thicker bark than fast growing trees mainly because the faster growing trees shed more bark.

The most common relationship is that between bark thickness at breast height and dbhob. The relationship varies with the locality, e.g. bark is much thinner in South Australian radiata pine plantations than in the A.C.T. For practical purposes, twice bark thickness at breast height of radiata pine in the A.C.T. averages 14-16% of dbhob over all tree sizes, ages and sites.

The bark percent of a log may be computed using the formula:
Bark % = 100 x [(dob^2 - dub^2)/dob^2]
where dob and dub are diameter over and under bark respectively at the centre of the log.

Measuring Bark Thickness

Often the bark thickness is not directly of interest to the forest mensurationist. Rather, an underbark diameter is needed and this can only be determined by measuring the overbark diameter and then adjusting for the thickness of the bark. This bark measurement or adjustment is done in several ways:

Direct measurement - remove the bark and measure the underbark diameter directly.
Indirect derivation -
- measure the overbark dimension and bark thickness
- measurethe overbark dimension and refer to a previously established relationship between overbark and underbark dimensions.

Direct measurement

Direct measurement is confined to felled trees. If the tree has been cross cut into logs, the underbark dimension can be measured at the log ends. When the bole has not been cross cut, bark removal or measurement of bark thickness on a chip of bark is required. Measurement on a chip can give a biased result if the surface is compressed or some bark flakes off during removal.

Indirect measurement

Bark thickness can be assessed indirectly by gauge or by using a previously established bark function or relationship.

The increasing use of dendrometry in forest inventory and assessment will force the development of reliable bark functions permitting underbark diameters to be derived from overbark measurements.

Bark gauge

On standing trees, it is essential to use a gauge for estimating bark thickness (e.g. the Swedish Bark Gauge or the Bark Probe). A nail of known length and a short tape or ruler can also be used.

No satisfactory way of ensuring exact penetration of the gauge to the wood interface has yet been devised. Many of the gauges use different chisel tips to help the operator 'feel' the correct penetration point, but the main determinate of success is experience.

The gauges only sample the bark thickness. It is therefore necessary to take 3 - 4 sample points around the tree to determine the mean bark thcikness. Where calipers are used to measure diameter, the bark thickness should be measured at the axis points where the calipers touch. Bark thickness errors arising from the bark gauge are a common source of error in tree volume estimation. Extreme care is needed when using a gauge because even small bark thickness errors can result in significant errors in the parameters of interest, e.g. an error of 0.25 cm in radial bark thickness in a 25 cm log (DOB) gives rise to a 4% error in SAUB and, hence, volume. The corresponding error in SAUB and volume for 0.5 cm error in radial bark thickness is 8%.

Example bark functions / relationships

Exercises conducted at the Australian Forestry School on radiata pine in the A.C.T. demonstrated that bark volume varies between 10 and 20% of stem volume OB. For general purpose use, 15% is a reasonable figure to accept over all ages, sites, size classes and silvicultural practices (Carron 1968, p. 48). Bark percent of many tree species in the northern hemisphere, both broadleaf and conifer, roughly falls in the range 10-20%.

Simple functions are often adequate, e.g. a simple equation used in Tasmania to predict bark thickness in eucalypts:
B = d / (a + bd)
where B denotes twice bark thickness
d denotes DBHOB and
a and b denote regression parameters.

Twice bark thickness at breast height - P. radiata, Kowen, A.C.T. (ex Carron and Jacobs, 1964).

Dbhob (cm) 2BT (cm) %
12 1.5 12.5
14 2.0 14
16 2.4 15
18 2.8 16
20 3.1 15.5
22 3.5 16
24 3.8 16
26 4.1 16
28 4.4 16
30 4.7 16
32 4.9 15
34 5.l 15
36 5.4 15
38 5.6 15
40 5.8 14.5
42 6.0 14
44 6.2 14
46 6.4 14

Bark as a percent of overbark volume in Oak

DBHOB 5 m height class 15 m height class 25 m height class
10 21 19 -
20 18 18 -
30 18 18 16
40 17 16 15
50 15 14 13
60 14 13 12

Johnson and Wood (1987) developed a bark function for radiata pine in the A.C.T. The model predicts bark thickness at any point along the bole, relative to bark thickness at breast height, from the fourth power of relative diameter (defined as the ratio of diameter overbark at the point on the bole to the diameter overbark at breast height). The model is independent of site and age and appears to be unaffected by forest location, e.g. Kowen, Stromlo, Pierce's Creek, Uriarra. Using the model, the underbark volume of 109 trees was estimated to be 54.56 m3 compared with the actual volume of 54.88 m3 which represents an error of -0.6%. The model is:
RBT(i) = 0.12 + 0.84 x RDi^4
where RBT(i) denotes relative bark thickness at the i-th measure point
RDi^4 denotes relative diameter at the i-th measured point raised to the 4 power.

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

Dbhob (cm)	2BT (cm)	%
12	1.5	12.5
14	2.0	14
16	2.4	15
18	2.8	16
20	3.1	15.5
22	3.5	16
24	3.8	16
26	4.1	16
28	4.4	16
30	4.7	16
32	4.9	15
34	5.l	15
36	5.4	15
38	5.6	15
40	5.8	14.5
42	6.0	14
44	6.2	14
46	6.4	14

DBHOB	5 m height class	15 m height class	25 m height class
10	21	19	-
20	18	18	-
30	18	18	16
40	17	16	15
50	15	14	13
60	14	13	12