Point Frame Method
The point frame method, based on point sampling to determine cover, was first suggested as an instrument to measure cover by Levy and Madden in the early 1930's. It consists of a standing frame that holds a set of vertically aligned pins, that are lowered as sampling points to record hits. The optimum number and arrangement of points within the frame depends on vegetation spatial patterns, distance between plants, and size of individual plants. A common configuration consists of 10 pins each 5 cm apart. Actual dimensions of the point frame should be selected so that the same plant is not hit by all pins.
Each point frame is usually considered the sample unit, so commonly cover data can be assessed in 10% intervals. Data from several frames are required for statistical analysis of cover data to compare differences between years or among sites. Sometimes separate data is kept for each pin within the frame, but a possible lack of independence between points can lead to problems in statistical analysis.
According to the objectives of the study, either ground cover, basal cover, canopy cover, or leaf area index can be determined by this method. A periscope can be attached to the point frame to determine overstory cover. Species composition is also obtained from the point frame technique by recording the species associated with each hit.
The point frame provides reasonably accurate cover data, as long as enough points are observed. It is a much slower technique than sampling using the step point method, but eliminates much of the bias arising from subjective pacing. The point frame is best suited for grasslands and other low-growing vegetation. The point frame becomes impractical in taller shrublands because of difficulties in placing the point frame above tall plants. It is also best suited to vegetation with dense cover, where there is less likelihood that all points within the frame will record bare ground.
References and Further Reading
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Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Sons, New York, NY. pp 21-24, 110.
Bureau of Land Management. 1996. Sampling vegetation attributes. Interagency Technical Reference, B.M./RS/ST-96/002+1730. pp 78-85.
Fisser, H.G., and G.M. Van Dyne. 1966. Influence of number and spacing of points on accuracy and precision of basal cover estimates. Journal of Range Management 19:205-211. (pdf)
Glatzle, A., Mechel, A., and M.E. Vaz Lourenco. 1993. Botanical components of annual Mediterranean grassland as determined by point-intercept and clipping methods. Journal of Range Management 46:271-274. (pdf)
Goodall, D.W. 1952. Some considerations on the use of point quadrats for the analysis of vegetation. Australian Journal of Scientific Research, Series B. 5:1-41.
Levy, E.B., and E.A. Madden. 1933. The point method for pasture analysis. New Zealand Journal of Agriculture 46:267-279.
Morrison, R.G., and G.A. Yarranton. 1970. An instrument for rapid and precise point sampling of vegetation. Canadian Journal of Botany 48:293-297.
Sharrow, S.H., and D.A. Tober. 1979. A simple, light weight point frame. Journal of Range Management 32:75-76. (pdf)
Stanton, F.W. 1960. Ocular point frame. Journal of Range Management 13:153. (pdf)