Reference Unit Method

The reference unit method was developed by Andrew, Noble and Lange in Australia in the 1970s as a non-destructive method to determine biomass. Sampling is usually restricted to a key species, and can describe any biomass property depending on the objectives of the program. The reference unit method is most commonly used to estimate browse biomass of shrubs, but is equally applicable to herbaceous species with discrete growth forms such as bunchgrasses. For meaningful interpretation, the data must be converted from a biomass per plant to a biomass per area basis, requiring an additional estimate of density.

The reference unit method is a natural evolution of the weight-estimate method and the double-sampling method, in that a standard unit is selected as a biomass comparison for other plants during sampling. The reference unit should reflect 10-20% of the foliage on an average sized plant in the area to be sampled, usually consisting of a shrub branch. However, unlike those methods, the weight of the reference unit is not quantified, so that biomass is estimated according to the number of reference units rather than by plant weight. After sampling, the estimate of biomass is obtained by multiplying the number of reference unit tallies by the weight of the reference unit. Possible bias toward overestimating or underestimating biomass can be corrected by applying regression analysis on a calibration subsample, to compare reference unit counts against harvested biomass.

References and Further Reading

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Andrew, M.H., Noble, I.R., and R.T. Lange. 1979. A non-destructive method for estimating the weight of forage on shrubs. Australian Rangeland Journal 1:225-231.

Andrew, M.H., Noble, I.R., Lange, R.T., and A.W. Johnson. 1981. The measurement of shrub forage weight: three methods compared. Australian Rangeland Journal 3:74-82.

Bonham, C.D. 1989. Measurements for Terrestrial Vegetation. John Wiley Sons, New York, NY. pp 228-229.

Cabral, D.R., and N.E. West. 1986. Reference unit-based estimates of winterfat browse weights. Journal of Range Management 39:187-189. (pdf)

Kirmse, R.D., and B.E. Norton. 1985. Comparison of the reference unit method and dimension analysis methods for two large shrubby species in the Caatinga Woodlands. Journal of Range Management 38:425-428. (pdf)