Frequency
Frequency is the vegetation attribute that describes the probability of finding a species within a particular area. The probability is based on the occurrence of that species in a series of sample units. For example, if a species has a frequency of 75%, we expect it to occur in three out of every four quadrats examined.
Frequency is expressed as a value between 0% and 100%, representing the proportion of quadrats where the particular species was found during sampling. For example, if we observed 200 quadrats and found the target species in 156 of those quadrats, frequency would be
Frequency is a simple vegetation attribute to measure because it only requires identification of the species in each quadrat, and does not require that individuals are distinguished, measured, or counted. Therefore, data collection is usually a more rapid procedure than for other vegetation attributes such as biomass, cover, or density, which involve counting or subjective quantification. This advantage is most apparent in rangeland vegetation characterized by a relatively low species richness, but diminishes in vegetation with complex species composition.
Frequency values are determined for individual species because an overall frequency for the entire vegetation cannot be obtained, in contrast to other attributes such as biomass, cover, or density. Likewise, it is not possible to obtain a meaningful expression of species composition from frequency data because the absolute abundance is not measured, only the presence of a species is measured.
Frequency can be a sensitive method to detect vegetation changes at a site. Its ease and speed of data collection means that frequency is suited to large areas, so it is sometimes adopted by State and Federal agencies for descriptive rangeland inventory or monitoring programs.
However, causes of the differences in frequency estimated between repeated sampling can be difficult to interpret because frequency is an index that integrates the density and spatial patterns of key species. For example, if frequency declines over time, it may be because individual plants are more sparsely scattered over the entire area, or because the range of the species distribution has constricted. In some cases, just knowing that there has been a change in the vegetation may meet the objectives of the monitoring program. In other situations, frequency can be used as a general monitoring procedure, that is followed by more detailed investigations to understand the cause of a change in frequency.
Special Considerations for Frequency Sampling
The following aspects of sample units are also important to consider when designing sampling protocols to determine frequency.
- Nested Quadrats
- Rooted or Canopy Dimensions
- Species Groups
- Specific Guidelines on Sample Unit Size for Frequency
- Sources of Sampling Error
Measurement Techniques and Statistical Analysis
References and Further Reading
Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Son, New York, NY. pp. 90-96.
Cook, C.W., and J. Stubbendieck. (eds). 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. p. 63.
Daubenmire, R. 1968. Plant communities: A textbook on plant synecology. Harper Row, New York, NY. pp. 53-57.
Despain, D.W., Ogden, P.R., and E.L. Smith. 1991. Plant frequency sampling for monitoring rangelands. In: G.B. Ruyle. (ed). Some methods for monitoring rangelands and other natural area vegetation. University of Arizona, College of Agriculture, Extension Report 9043. pp. 7-25.
Greig-Smith, P. 1983. Quantitative plant ecology. Blackwell Scientific Publications, Oxford. 3rd ed. p. 9.
Hironaka, M. 1985. Frequency approaches to monitor rangeland vegetation. In: W.C. Krueger. (chairman). Proceeding 38th Annual Meeting, Society for Range Management, Salt Lake City, UT, February 1985. pp. 84-86.
Mueller-Dombois, D., and H. Ellenberg. 1974. Aims and methods of vegetation ecology. John Wiley and Sons, New York, NY. pp. 70-76.