Double Sampling Method

The double sampling method is designed to determine biomass by sampling in quadrats. Depending on the objectives of the study, the double sampling method can be used to describe any biomass property. It can be applied to a wide variety of vegetation types, particularly grasslands and shrublands, but becomes more complicated in vegetation with a diverse array of species or life-forms.

The double sampling method was developed as a modification of the weight-estimate method, to attempt to overcome the lack of precision among observers and the possibility of unchecked drift in an individual's estimate of biomass over time. In concordance with the weight-estimate method, data is collected by using defined weight-units for each species to visually estimate the biomass in each quadrat. However, a small second calibration data set is also collected, by clipping and weighing selected quadrats after estimation. Regression analysis is used to compare estimated and harvested values of the calibration samples, to determine if tended to underestimate or overestimate the visual estimation, and to provide the appropriate adjustments to be made to all field samples.

The number of samples selected for the calibration data set depends on the observer's ability to furnish accurate visual estimates, the sample variance of biomass estimates, the diversity of species on the site, and time restrictions. Ideally, calibration data should encompass the range of biomass values and the majority of species encountered during sampling. Fewer calibration samples are needed when the observer's visual estimates closely reflect harvested weights. However, the observer's proficiency cannot be confirmed until after the calibration quadrats are clipped and weighed! Clipping one out of every 5 - 10 quadrats for inclusion in the calibration data set provides a reliable calibration in most situations.

Data is usually collected form multiple quadrats located along a transect, so that the transect is the sample unit. Therefore, data must be collected from several transects to determine the precision of the sample, for statistical analysis of biomass data.

The double sampling method is regularly used to determine biomass in range inventory or monitoring programs. It is a little slower than the weight-estimate method and still requires extensive training in the preliminary stages, but these disadvantages are well compensated by improvements in accuracy and precision. By only clipping a selection of quadrats, it is more efficient than harvesting to determine biomass.

References and Further Reading

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Ahmed, J. and C.D. Bonham. 1982. Optimum allocation in multivariate double sampling for biomass estimation. Journal of Range Management 35:777-779. (pdf)

Ahmed, J., Bonham, C.D., and W.A. Laycock. 1983. Comparison of techniques used for adjusting biomass estimates by double sampling. Journal of Range Management 36:217-212. (pdf)

Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Sons, New York, NY. p 205.

Bureau of Land Management. 1996. Sampling vegetation attributes. Interagency Technical Reference, BLM/RS/ST-96/002+1730. pp 102-111.

Carande, V., and D.A. Jameson. 1986. Combination of weight estimates with clipped sample data. Journal of Range Management 39:88-89. (pdf)

Cook, C.W., and J. Stubbendieck. (eds). 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. pp 53-54.

Riech, R.B., Bonham. C.D., and K. Remington. 1993. Double sampling revisited. Journal of Range Management 46:88-90. (pdf)

Tadmor, N.H., Brieghet, A., Noy-Meir, I., Benjamin, R.W., and E. Eyal. 1975. An evaluation of the calibrated weight-estimate method for measuring production in annual vegetation. Journal of Range Management 28:65-69. (pdf)

Wilm, H.G., Costello, D.F., and G.E. Kipple. 1944. Estimating forage yield by the double sampling method. Journal of the American Society of Agronomy 36:194-203.