Allocation of Sample Units - Sampling Designs

Decisions relating to the arrangement of sample units within the site have a large influence on sample precision, sample efficiency, and statistical models used to analyze the data. These decisions are an important component of designing the inventory or monitoring program before the data is actually collected.

In vegetation where distinct boundaries of spatial patterns can be identified, sampling precision will be improved by stratified sampling. Under this approach, it is important to ensure that the sample unit is confined within the boundaries of the subunit. For example, when collecting data to determine range condition of a sandy loam upland range site, care must be taken that the transect does not extend into the adjoining loamy upland range site.

Vegetation often exhibits an identifiable gradient according to slope, soil depth, or distance from water, where the effect of orientation of sample units on sample precision becomes increasingly pertinent. In these situations where the gradient is too subtle to create distinct sampling areas, the sampling precision will be enhanced by orienting the quadrat or transect so it intersects across the gradient, with the aim of encompassing the range of variability within each sample unit.

Types of Sampling

The following are the standard sampling techniques used for rangelands:

References and Further Reading

(Note: pdf files require Adobe Acrobat (free) to view)

Bormann, F.H. 1953. The statistical efficiency of sample plot size and shape in forest ecology. Ecology 34:474-487.

Cook, C.W., and J. Stubbendieck. (eds). 1986. Range Research: Basic Problems and Techniques. Society for Range Management, Denver, CO. pp 221-250.

Daubenmire, R. 1968. Plant Communities: A Textbook on Plant Synecology. Harper Row, New York, NY. pp 82-86.

Greig-Smith, P. 1983. Quantitative Plant Ecology. Blackwell Scientific Publications, Oxford.3rd ed. pp 20-25.

Hurlbert, S.H. 1984. Pseudoreplication and the design of ecological field experiments. Ecological Monographs 54:187-211.

Kenkel, N.G., Juhasz-Nagy, P., and J. Podani. 1989. On sampling procedures in population and community ecology. Vegetatio 83:195-207.

Kent, M., and P. Choker. 1992. Vegetation Description and Analysis. Belhaven Press, London. pp 40-54.

Wester, D.B. 1992. Viewpoint: replication, randomization, and statistics in range research. Journal of Range Management 45:285-290. (pdf, 990K).