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.
Introduction | General Principles | Sampling Concepts | Vegetation Attributes
Rangeland Evaluation | Management Applications | Chapter Outline
Poorly functioning equipment can jeopardize sample accuracy and sample precision. All measuring equipment, such as scales used to weigh samples for herbage biomass, should be regularly checked to confirm their accuracy. Other sources of measurement error include plastic tapes stretching with use and variations in pace-length among observers.
The dry-weight rank method is specifically designed to determine species composition by providing a measure of the relative contribution of various species to the total biomass (based on dry matter content) for a site.
Sampling distributions describe general patterns in the spread and frequency of the values in the sample data. The type of sampling distribution generated during sampling depends on the vegetation attribute, spatial patterns of the vegetation, the sampling method, and characteristics of sample unit.
Distance methods to determine density that involve plotless sampling regimes are based on determining the mean area associated with each plant by measuring the spacing between them. This approach builds on the pattern that when plants are crowded together in dense vegetation there is a small distance between individuals.