Weight-Estimate Method
The weight-estimate method is designed to determine biomass from sampling in quadrats. Depending on the objectives of the study, the weight-estimate method can be used to describe any biomass property. It can be applied to a wide variety of vegetation, particularly grasslands and shrublands, but becomes more complicated in vegetation with a diverse array of species or life-forms.
Observers must initially learn to visually estimate plant biomass for important species in the vegetation by reiteratively comparing estimated weights against clipped weights. Rather than attempting to directly determine total biomass, observers learn to identify a consistent weight-unit for each species, and then estimate the number of weight-units found in the quadrat. Weight-units should be small enough to easily visualize, yet not so small that estimation becomes arduous or haphazard. Estimations are made simpler by selecting a weight-unit that corresponds to a discrete portion of each species, such as an average sized bunchgrasses or a branch for shrubs.
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.
Although the training process is tedious, subsequent sampling concentrates on the rapid visual estimation of biomass with few interruptions for clipping. Biomass is usually assessed on a green-weight basis in the field, but corrected to dry-matter content after drying a subsample of each species.
Experienced observers are able to obtain fairly accurate results using the weight-estimation technique, particularly in relation to the speed with which data can be collected. For these reasons, it was used extensively to conduct range surveys throughout western rangelands of USA in the 1930's. However, shortcomings of the technique, such as the potential lack of precision among observers, the possibility of unchecked drift in an individual's estimate over time, and a tendency to overestimate quadrats with low biomass while underestimating quadrats with high biomass (leading to artificially reduced variances), make it a less suitable technique when an accurate assessment of biomass is required.
References and Further Reading
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Cook, C.W., and J. Stubbendieck. (eds). 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. pp 53-54.
Francis, R.C., Van Dyne, G.M., and B.K. Williams. 1979. An evaluation of weight estimation double sampling as a method of botanical analysis. Journal of Environmental Management 8:55-72.
Goebel, C.J. 1955. The weight-estimate at work in southeastern Oregon. Journal of Range Management 8:212-213. (pdf)
Hutchings, S.S., and J.E. Schmautz. 1969. A field test of the relative weight-estimate method for determining herbage production. Journal of Range Management 22:408-411. (pdf)
Pechanec, J.F., and G.D. Pickford. 1937. A weight-estimate method for the determination of range or pasture production. Journal of the American Society for Agronomy 29:894-904.
Shoop, M.C., and E.H. McIlvain. 1963. The micro-unit forage inventory unit. Journal of Range Management 16:172-179. (pdf)
Smith, A.D. 1944. A study of the reliability of range vegetation estimates. Ecology 25: 441-448.