Two general approaches can be adopted to determine density. With the first approach, density is directly determined by counting plants within a defined sample unit, whereas the second approach is a plotless method based on measuring the distance or spacing between plants.
Both these methods are described in further detail in the following sections.
Density describes the number of individual plants in a given area. In situations where identification of individuals is ambiguous, density measurements may be based on some other counting unit, such as culms or shoots for sod-forming grasses or the basal stems for shrubs.
Critical areas are sampling locations deliberately chosen in rangeland inventory or monitoring programs because of unique values or special concerns such as riparian zones, restricted habitats for threatened or endangered species, or range sites highly susceptible to erosion.
Cover is the vertical projection of plant material onto the ground when viewed from above. It is usually expressed as a percentage value. For example, 18% cover indicates a birds-eye-view would reveal 18% of the surface area as vegetative material with the remaining 82% as bare ground. In some forestry situations cover is expressed on an area basis, such as square meters/hectare or square feet/acre.
Species composition refers to the contribution of each plant species to the vegetation. Botanical composition is another term used to describe species composition. Species composition is generally expressed as a percent, so that all species components add up to 100%.
Species composition can be expressed on either an individual species basis, or by species groups that are defined according to the objectives of the inventory or monitoring program (eg., Aristida spp., perennial forage grasses, etc.).