Sample Unit Shape

Sample units used to sample vegetation attributes are generally circular, square or rectangular in shape. In a monitoring program, it is essential to use sample units of the same shape for all subsequent or compared measurements. Sometimes the shape that is selected for a study is determined by convention or past practices, although it is a factor that can significantly affect sample accuracy and sample precision.

The shape of sample units influences sample accuracy by controlling the likelihood of bias associated with boundary decisions. Sampling precision is influenced by manipulating sample unit shape in a manner that considers vegetation patterns, so that more variability is encompassed within rather than among sample units.

A general principle is that circular sample units enhance sample accuracy, while elongated types of sample units promote sample precision; leading to the rule of thumb that long, narrow sample units are most suitable in sparse or clumped vegetation, but circular sample units are more convenient in dense vegetation with a more uniform spatial distribution.

This rule of thumb is guided by the following considerations:

  1. Circular sample units have a lower boundary length: area ratio. Fewer boundary decisions are encountered in circular sample units compared to square or rectangular sample units of the same size, improving the accuracy and precision of the sample (Table 1).
Table 1. Effect of sample unit shape on boundary length
Sample Unit Shape Area
(m2)
Dimensions
(m)
Perimeter
(m)
Area:Perimeter Ratio
Circle 10 3.6 (diameter) 11.2 1 : 1.1
Square 10 3.2 X 3.2 12.6 1 : 1.3
Rectangle 10 5.0 X 2.0 14.0 1 : 1.4
Rectangle 10 50.0 X 0.2 100.4 1 : 10.0
  1. Long narrow sample units reduce sample variance. In many vegetation communities, plants feature a patchy spatial pattern. Sampling using a circular sample unit promotes an 'all-or-nothing' situation, where some encompass most of a vegetation patch while others fall in the interspaces between vegetation patches, leading to data with a large sample variance. Under these conditions, it is more desirable to have long narrow sample units which include both the patch and the interspace within their boundaries.
  2. Long narrow sample units are subject to greater measurement error. Long narrow sample units are difficult to examine in their entirety. As well as the errors arising from boundary decisions, small plants are more likely to be overlooked in long, narrow sample units. Additionally, techniques requiring a subjective estimation or ranking, such as the comparative yield method to estimate biomass, the dry-weight-rank method to determine species composition, or the Daubenmire cover class method to estimate cover, are more difficult to execute in elongated quadrats.

References and Further Reading

Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Son, New York, NY. pp 33-35.

Clapham, A.R. 1932. The form of the observational unit in quantitative ecology. Journal of Ecology 20:192-197.

Cook, C.W., and J. Stubbendieck. (eds). 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. pp 220-221.

Daubenmire, R. 1968. Plant communities: A textbook on plant synecology. Harper Row, New York, NY. pp 87-88.