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Correction factors for coarse woody debris sampling

by

John Parminter
Ministry of Forests, Research Branch
Victoria, British Columbia
August 1998

Introduction

Coarse woody debris (CWD) refers to the larger dead and down woody material on the forest floor in natural stands as well as that found in harvested areas and managed forests. Standing dead trees (snags) and stumps are usually excluded. Coarse woody debris is most often sampled along a line transect of varying length arranged singly, in pairs (emanating from a common point or crossing at their midpoints) or as a triangle and the data collected for each piece are:

- tree species,

- decay class,

- diameter at transect crossing, and

- tilt angle of each piece away from the horizontal

an optional attribute is

- piece length

which permits calculation of the number of pieces per hectare by assigned length classes.

Volume calculation

The standard formula for calculating coarse woody debris volume (V) is based on the length of the transect sampling line (L) and the diameter of each piece (d) at the point of measurement:

V = (p ² / 8L) x S d²

When V is in m³, L is in m and d is in cm, the formula becomes:

V = (1.234 / L) x S d²

If, for example L = 25 m the formula becomes:

V = (0.04936) x S d²

Correction factors

Converting a sloping line transect to its equivalent horizontal distance and accounting for the tilt angle of each piece of coarse woody debris sampled introduce two correction factors. These calculations and correction factors were described in detail by Van Wagner (1982). The purpose of this discussion is to describe the relative importance of each correction factor.

Using a true horizontal line length changes the last equation above, based on the new value for L as determined by the slope. Slope distances (SD) are converted to horizontal distances (HD) based on the formula:

HD = SD / Square root of [1 + (% slope / 100)²]

Van Wagner (1982) described the primary rationale for slope correction as the need to estimate actual ground area for fire behaviour prediction or horizontal map area for residue surveys. Correction factors were given by McRae et al. (1979) and showed that only above a slope of 50% does the slope correction amount to more than 10%.

Correcting for piece tilt angle changes the volume estimation formula to:

V = (1.234 / L) x S (d² x a)

Where a is the secant (reciprocal of cosine) of the tilt angle (away from the horizontal) of each piece sampled. When an angle of 25° is reached the correction factor is 1.10, at 37° the correction factor is 1.25, at 48.5° it is 1.50 and at just over 55° it is 1.75. At just over 60° the correction factor is 2.0, at 70° the correction factor is nearly 3.0. There is a steep increase in secant above angles of about 35° .

Brown and Roussopoulos (1974) stated that tilt errors are significant in fresh logging slash up to 7.6 cm in diameter, averaging about 13%. Brown (1974) provided correction factors for logging slash and non-slash fuels of 1.13 for pieces up to 7.6 cm in diameter and 1.00 for pieces greater than 7.6 cm in diameter. In a later version of the procedures Brown et al. (1982) did not address the issues of piece tilt and error correction.

The Canadian Forest Service does not correct for piece tilt (McRae et al. 1979) nor the procedures for measuring logging slash in B.C. (Trowbridge et al. 1989). The Alberta Forest Service uses Brown's correction factors of 1.13 and 1.00 for small and large fuels respectively (Alberta Forest Service n.d.) It may be that most agencies do not consider these factors to be important enough or that too much extra time and effort would be required to deal with them.

Contribution of correction factors

The relative contributions of the corrections for ground slope and piece tilt were assessed for three data sets from coastal western hemlock - western redcedar forests. It became apparent that the volume correction for piece tilt may be less than, more or less equal to or greater than the correction for slope. It is not possible to determine the specifics based on just three plots picked at random for comparison purposes but the range of variation is evident.

Plot 141 had a moderate number of tilted pieces (averaging 7.4° ) and a slope of 27%. The correction for tilt is less than half the correction for slope. Plot 124 had most pieces tilted (averaging 31.1° ) and a slope of 50%. In that case the corrections were more or less equal. Plot 99 had few tilted pieces (averaging 6.1° ) and a slope of only 11%. The piece tilt correction was greater than the slope correction. Table 1 shows the relative contributions of the correction factors to the increase in calculated volume per hectare.

Table 1. Correction factors and coarse woody debris volume estimates

If corrections are to be made for piece tilt and ground slope then both should be done in all cases. It would be too difficult to produce guidelines as to when one should or shouldn't correct for tilt and/or slope. Additionally, to correct only some of the samples would introduce bias. With a potentially wide variation in the orientation of each piece of coarse woody debris there will be samples with a mixture of very tilted and perfectly horizontal pieces. Similarly, if the sample lines are established in pairs (emanating from a common point and separated by a 90° angle is a common approach) there will be cases where one line has no slope (along the contour) and its companion will be steeply sloping (up or down slope). Treating these lines as equivalent in true length by not correcting for slope would be inconsistent.

The issue of sampling intensity required is another concern but needs to be examined from a practical standpoint. The purpose of the CWD sampling is to determine relative levels of coarse woody debris in stands of different ages as well as different disturbance histories and growth and decay conditions rather than carrying out a detailed residue survey to estimate actual coarse woody debris volumes for valuation or research purposes.

Conclusions

Corrections should always be made for piece tilt and ground slope since it would be too difficult to produce guidelines as to when one should or shouldn't correct for tilt and/or slope. Additionally, to correct only some of the sample lines would introduce bias into the dataset. In the sample datasets analyzed here, the total increase in calculated volume due to the corrections ranged from 2.36 to 26.4%. While the first may be insignificant, that latter is certainly not.

Literature Cited

Alberta Forest Service. n.d. Measurement and description of fuels in natural stands in Alberta. Alberta Forest Service Handbook. 29 p.

Brown, James K. 1974. Handbook for inventorying downed woody material. USDA Forest Service General Technical Report INT-16. Ogden, Utah. 24 p.

Brown, James K. and P.J. Roussopoulos. 1974. Eliminating biases in the planar intersect method for estimating volumes of small fuels. Forest Science 20:350-356.

Brown, James K., R.D. Oberhu and C.M. Johnston. 1982. Handbook for inventorying surface fuels and biomass in the interior west. USDA Forest Service General Technical Report INT-129. Ogden, Utah. 28 p.

McRae, Douglas J., M.E. Alexander and B.J. Stocks. 1979. Measurement and description of fuels and fire behavior on prescribed burns: a handbook. Great Lakes Forest Research Centre Report O-X-287. Canadian Forestry Service, Sault Ste. Marie, Ontario. 44 p. Appendices.

Trowbridge, R., B. Hawkes, A. Macadam and J. Parminter. 1989. Field handbook for prescribed fire assessments in British Columbia: logging slash fuels. FRDA Handbook No. 001. 63 p.

Van Wagner, C.E. 1982. Practical aspects of the line intersect method. Petawawa National Forestry Institute Information Report PI-X-12. Canadian Forestry Service, Chalk River, Ontario. 11 p.

Author: John Parminter
Submitted by: John Parminter
Peer review: No