Plant cover

Measurement

The most common way to measure plant cover in herbal plant communities, is to make a visual assessment of the relative area covered by the different species in a small plot (see quadrat). The visually assessed cover of a plant species is then recorded as a continuous variable between 0 and 1, or divided into interval classes as an ordinal variable. An alternative methodology, called the pin-point method (or point-intercept method), has also been widely employed.

In a pin-point analysis, a frame with a fixed grid pattern is placed randomly above the vegetation, and a thin pin is inserted vertically through one of the grid points into the vegetation. The different species touched by the pin are recorded at each insertion. The cover of plant species k in a plot, ${\displaystyle c_k}$, is now assumed to be proportional to the number of “hits” by the pin,

${\displaystyle c_k=\frac{y_k}{n}}$,

where ${\displaystyle y_k}$ is the number of pins that hit species k out of a total of n pins. Since a single pin in multi-species plant communities often will hit more than a single species, the sum of the plant cover of the different species may be larger than unity when estimated by the pin-point method. The sum of the estimated plant cover is expected to increase with the number of plant species in a plot and with increasing 3-dimensional structuring of the plants in the community. Plant cover data obtained by the pin-point method may be modelled by a generalised binomial distribution (or Pólya–Eggenberger distribution).