Over-Dispersion in Grassland Communities and the Use of Statistical Methods in Plant Ecology

Abstract

ASSUMPTIONS are frequently made as to the mode of distribution of the of plant species in natural but there is actually very little exact information on the point. In a paper dealing with The structure of prairie vegetation, T. L. Steiger (1930) published tables which give the number of of each species observed within 80 m. quadrats. Forty of these quadrats were placed in vegetation described as high prairie, dominated by Andropogon furcatus, and the other forty in prairie , with several grasses co-dominant. These data are valuable as a basis for obtaining exact information as to the mode of distribution of the species recorded, and they have been consequently analysed statistically for that purpose. The units counted by Steiger are described as individuals , but, as Romell (1930) has pointed out, this term must be taken to mean rooted shoots, since it is impossible to use any other satisfactory criterion of individuality. The quadrats were all 1 sq. m. in area. The forty quadrats on low prairie were selected at regular intervals along four parallel lines. A similar number were selected on the upland with the express purpose of including various portions of tract upon which the vegetation was much more diversified. The two sets of quadrats can be supposed; therefore, to represent more or less random samples of two communities recognised by Steiger.