Line Intercept Method
The line intercept method was developed by Canfield in the 1940's to estimate cover in the grasslands of southwestern USA and has been widely adopted in rangeland inventory and monitoring applications. To follow this method, a tape is extended to create a transect across the site. The observer proceeds along the line-transect, identifies plants intercepted by the tape, and records intercept distance. Cover is calculated by adding all intercept distances and expressing this total as a proportion of tape length. Each transect is regarded as one sample unit, so multiple transects must be measured to estimate sample variance and conduct statistical analyses of cover data.
Measurements can be made for either basal cover or canopy cover, according to the objectives of the study. Cover of individual species is easily obtained by collecting intercept data on a species basis. Species composition can be estimated from the proportional representation of each species. Transect length depends on the vegetation and type of plants which are to be measured. In many instances, 15 m transects have been found suitable in dense vegetation, while 30-50 m is needed to obtain a representative sample in sparse vegetation.
The line intercept method is easy to learn, simple to use, and provides an accurate estimate of cover. In fact, line intercept sampling is often used as the standard comparison when testing other methods to determine cover. Its primary drawback is that sampling can be time consuming, particularly in dense vegetation or when intercept distances are difficult to define because of many gaps or irregular edges within the canopy. Therefore, the line intercept technique is best suited for vegetation characterized by discrete plants, such as bunchgrasses or compact shrubs.
References and Further Reading
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Brun, J.M., and T.W. Box. 1963. A comparison of line intercepts nd random point frames for sampling desert shrub vegetation. Journal of Range Management 16:21-25. (pdf)
Bureau of Land Management. 1996. Sampling vegetation attributes. Interagency Technical Reference, B.M./RS/ST-96/002+1730. pp 65-69.
Canfield, R.H. 1941. Application of the line interception method in sampling range vegetation. Journal of Forestry 39:399-394.
Hanley, T.A. 1978. A comparison of the line-estimation and quadrat estimation methods of determining shrub canopy coverage. Journal of Range Management 31:60-62. (pdf)
Hormay, A.L. 1949. Getting better records of vegetation changes with line interception method. Journal of Range Management 2:67-69. (pdf)
Kinsinger, F.E., Eckert, R.E., and P.O. Currie. 1960. A comparison of the line interception, variable plot, and loop methods as used to estimate shrub ground cover. Journal of Range Management 13:17-21. (pdf)
Parker, K.W., and D.A. Savage. 1944. Reliability of the line interception method in measuring vegetation on the Southern Great Plains. Journal of American Society of Agronomy. 36:97-110.
Ruthven, D.C., Fulbright, T.E., Beason, S.L., and E.C. Hellgren. 1993. Long-term effects of root plowing on vegetation in the eastern south Texas plains. Journal of Range Management 46:351-354. (pdf)