Variable Plot Method
The variable plot method to determine cover was originally developed by European foresters, before being introduced to the USA by Cooper in the late 1950's as a technique for rangeland sampling. It is also referred to as the Bitterlich method (after the original German inventor), or the angle-gauge method (describing the instrument used to collect data).
To conduct the variable plot technique, the observer views along a gauge consisting of a bar held horizontal to the ground and perpendicular to the eye, with a shorter crossbar attached to the far end. The observer slowly rotates 360° at a sampling location, to count the number of plants that have a canopy extending beyond the width of the crossbar. In this manner, plants with smaller canopies will be included close to the sampling location, but only larger plants will be included as they lie further from the sampling location. Therefore, the method is based on a plotless approach because the dimensions of the sample unit are not clearly defined. Instead the actual size of the sample unit depends upon the dimensions of the plants viewed, according to established geometric theory (which can be followed in Cooper 1957). The viewing angle, formed by the length of the bar relative to the length of the crossbar, is critical in subsequent calculations, to derive actual cover values from the raw data.
The variable plot method is most appropriate for sampling shrub canopy cover, but a modified gauge has been successfully used to measure bunchgrasses. Because counting is involved, it is not a suitable method to sample vegetation where the identification of individuals is difficult. The value derived at each location represents the sample unit, so that data must be collected from several locations to conduct statistical analysis of cover data. Sampling locations should be spaced so that the same plants are not included on two different occasions.
Most studies have found that the variable plot method provides accurate and precise data. Some inaccuracies could arise from failing to count distant plants obscured by large closer ones. Therefore, the method is not recommended in situations where canopy cover is greater than about 35%. The geometric theories supporting this method assume that the plant canopy is round, but adjustments can be made for elliptical-shaped plants. Sampling efficiency is relatively high because there is a low sample variance among sampling locations, and fewer records are required to adequately represent the population.
References and Further Reading
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Cooper, C.F. 1957. The variable plot method to estimate shrub density. Journal of Range Management 10:111-115. (pdf)
Cooper, C.F. 1963. An evaluation of variable plot sampling in shrub and herbaceous vegetation. Ecology 44:565-569.
Fisser, H.G. 1961. Variable plot, square foot plot, and visual estimate for shrub crown cover measurements. Journal of Range Management 14:202-207. (pdf)
Grosenburg, L.R. 1952. Plotless timber estimates - new, fast, easy. Journal of Forestry 50:32-37.
Hyder, D.N., and F.A. Sneva. 1960. Bitterlich's plotless method for sampling basal ground cover of bunch grasses. Journal of Range Management 13:6-9. (pdf)
Kinsinger, F.E., Eckert, R.E., and P.O. Currie. 1960. A comparison of the linear interception, variable plot, and loop methods as used to measure shrub crown cover. Journal of Range Management 13:17-21. (pdf)