Point-Centered Quarter Method
The point-centered quarter method was developed by Cottam and Curtis in the 1950's as a plotless technique to estimate density. It assumes that the key species follow a random spatial pattern, and is sensitive to bias when plants exhibit a more contagious arrangement. It also requires extra field effort and computational input than many of the other distance methods. Despite these disadvantages, the point-centered quarter method has been used in a wide variety of vegetation types, including some attempts to sample herbaceous vegetation.
In the point-centered quarter method, a set of points (usually positioned along a transect to traverse the area) is initially selected. The area around each point is divided into four 90° quadrants, and the plant closest to the point in each quadrant is identified. The distance between the central point and selected plant in each quadrant is measured, and then averaged across the four to represent the distance (d) at each sample point. At the conclusion of data collection, the average distance (
) for all sample points is calculated.
Mean area (MA) can then be estimated as:
Density (plants/area) is then derived by calculating the inverse of MA.
When following this procedure, each set of points is regarded as a separate sample unit, contributing to the values used in statistical analysis of density data.
References and Further Reading
Beesom, S.L., and H. Haucke. 1975. A comparison of four distance sampling techniques in South Texas live oak mottes. Journal of Range Management 28:142-144.
Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Sons, New York. pp. 159-164.
Cottam, G., and J.T. Curtis. 1956. The use of distance measure in phytosociological sampling. Ecology 37:451-460.
Dix, R.L. 1961. An application of the point-centered quarter method to the sampling of grassland vegetation. Journal of Range Management 14:63-69.
Heyting, A. 1968. Discussion and development of the point-centered quarter method of sampling grassland vegetation. Journal of Range Management 21:370-380.
Penfold, W.T. 1963. A modification of the point-centered quarter method for grassland analysis. Ecology 44:175-176.
Risser, P.G., and P.H. Zedler. 1968. An evaluation of the grassland quarter method. Ecology 49:1006-1009.