A COMPARISON OF THREE METHODS FOR ESTIMATING PLANT COVER

SUMMARY (1) Cover is among the most widely used measures of abundance of plant species because it is not biased by the size or distribution of individuals. This study compared cover estimates obtained by line interception, point interception, and cover-class estimation from 136 sample lines located systematically in sagebrush steppe in western U.S.A. (2) Line interception estimates of shrub cover were significantly higher than those obtained by point interception; estimates of 'bare ground and litter' by point interception were higher than those of line interception by the same amount. These offsetting differences resulted from the assumption, used in line interception sampling, that shrubs completely cover the areas within the outlines of their canopies. Otherwise, point interception and line interception estimates were very similar. (3) Cover-class estimation provided reliable estimates only for the dominant shrub species. Typically, estimates for grasses and other species with small or rare individuals were high in comparison with the other techniques. This is a consequence of the assumption that cover values are uniformly distributed about the mid-points of the cover classes. (4) Point interception achieved about the same degree of precision as line interception in one-third less sampling time. Point interception is the most efficient of the three methods where estimates for most of the species in a community are needed. (5) Optimal combinations of numbers of lines and numbers of sample units per line in relation to the time involved for a particular level of precision were investigated. In general, increased precision was achieved by sampling more lines rather than more sample units per line.

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