Sampling in Quadrats to Determine Cover
Although other approaches are generally preferred, a variety of methods are available to determine cover quadrats. Depending on the technique, measurements can be made for ground cover, basal cover or canopy cover. Species cover and species composition can also be estimated with most methods.
The general sampling principles involving sample unit shape, sample unit size, and sample size apply to these techniques, and need not be discussed with further. Because each quadrat represents a only a very small area of the site, sample variance is generally high, many quadrats must be taken to obtain a sample size that adequately represents the site.
Early attempts, mainly for plant demographic studies, involved the labor intensive procedure of mapping quadrats, followed in the office by a second step to estimate cover from the paper charts using an approach of point sampling to determine cover or line sampling to determine cover. Similar procedures followed today have the advantages of modern technology; where quadrats can be initially depicted by vertical photography, and cover digitally estimated using computer-based analysis packages. Although time consuming, these techniques have the advantage of providing additional detailed information indicating the source of changes in cover over time.
Other techniques have been designed to estimate the area represented by vegetation within the quadrat, by assuming that plants are circular in shape and measuring their diameter. Clear ground rules are needed, particularly guiding boundary decisions, because including material extending outside the quadrat would overestimate cover. In other cases, cover has been quantified by placing a grid over the quadrat, and counting the number of squares dominated by vegetation. In a 10 x 10 grid, each square filled by vegetation represents 1% cover. Conversely, counting those intersections of the grid which 'hit' plants is a type of point sampling to determine cover.
A third approach to determine cover in quadrats is based on ocular estimation. Some techniques, such as the square-foot-density method and the area-list method, attempt to directly estimate a cover value (e.g., according to 1% or 5% intervals), but the more successful approaches to assess cover adopt broad cover classes. Both approaches rely on extensive training and are vulnerable to the personal bias associated with subjective estimation. However, broad cover classes confer the advantages of speed, somewhat less training, and lower the demand that each quadrat be finitely assessed.
The Daubenmire Cover Class Method is discussed in greater detail as a method for determining cover using quadrats that has been most widely adopted in the USA for rangeland monitoring.
References and Further Reading
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Daubenmire, R.F. 1959. Canopy coverage method of vegetation analysis. Northwest Science 33:43-64.
Elkson, L. 1942. A comparison of methods quadratting short grass vegetation. Journal of Agricultural Research 64:595-614.
Goebel, C.J., DeBano, L., and D. Lloyd. 1958. A new method of determining forage cover and production on desert shrub vegetation. Journal of Range Management 11:244-246. (pdf)
Hanson, H.C., and D. Love. 1930. Comparison of methods of quadratting. Journal of Ecology 11:743-748.
Hatton, T.J., West, N.E., and P.S. Johnson. 1986. Relationships of the error associated with ocular estimation and actual total cover. Journal of Range Management 39:91-92. (pdf)
Pease, K. 1945. An area-list method for measuring range plants. Ecology 16:573-579.
Poulton, C.E., and T.W. Tisdale. 1961. A quantitative method for the description and classification of range vegetation. Journal of Range Management 14:13-21. (pdf)
Smith, A.D. 1944. A study of the reliability of range vegetation estimates. Ecology 25:441-448.
Stewart, G., and S.S. Hutchings. 1936. The point-observation-plot (square-foot-density) method of vegetation assessment. Journal of the American Society of Agronomy 28:714-727.
Wells, K.F. 1971. Measuring vegetation changes on fixed quadrats by vertical ground stereography. Journal of Range Management 24:233-236.
Wright, R.G. 1972. Computer processing of chart quadrat maps and their use in plant demographic studies. Journal of Range Management 25:476-478. (pdf)