Biases in the Step-Point Method on Bunchgrass Ranges

Abstract

During a study of grazing in the Great Basin, using the grazed plant method used for measuring utilization, we noticed a bias in the step point technique used to select plants. Subsequent mathematical calculations showed both the direction and order of magnitude of the bias: the step point method overestimates the number of large plants and underestimates the number of small plants; when used to estimate the basal area of bunchgrasses the method overestimates the area of small plants. The calculations were tested and verified on maps of real and artificial bunchgrass populations. When the plants are distributed at random, the biases can be removed. For estimation of numbers, one should select the plant whose center (rather than perimeter) is closest to the toe point to eliminate bias by size class; for estimations of basal area, the numbers in each size class should be scaled by the area of plants of that class.