Data Recording and Calculations to Determine Frequency
The field sheet to record frequency usually includes a species list and a tally is made for every quadrat where the species is recorded. Only a single tally is made for each quadrat, which represents the presence of the species regardless of its abundance. If nested quadrats are used, more than one column is needed to separately recorded each quadrat.
When data is collected in this manner, it is not possible to combine several species groups after the field work has been completed. For example, in a monitoring program to assess Lehmann lovegrass (Eragrostis lehmanniana) spread within a pasture, we may decide to distinguish seedlings from established plants by providing each a separate category in the species list on our field sheet, to find that seedlings occur at a 45% frequency and established plants at 30% frequency. However, this does not mean that frequency of the entire Lehmann lovegrass population is 75% (45% + 30%), because it is likely that seedlings and established plants both occurred in some quadrats, so that we may have double counted when only a single tally is permitted. This shortcoming can be overcome by recording species presence for each individual quadrat, but then the technique loses some of its speed and simplicity.
Once field work is completed, data is summarized to estimate frequency for each species in the sample. Procedures followed to summarize the data depend upon the definition of the sample unit, that was determined by the quadrat layout selected during the planning stages
- Randomly Located Quadrats to Determine Frequency
- Quadrats Arranged As a Group of Subsamples to Determine Frequency
References and Further Reading
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. 15-16.