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The non-random distribution of species within plant communities has been investigated by several workers and the problem reviewed by Greig-Smith (1957) and Kershaw (1958). Such investigations lead to an appreciation of factors influencing the distribution of species as well as providing information on the structure of plant communities. Among the factors which have been shown to control non-random distribution are (a) morphology of the rhizome system (Phillips 1954), (b) different density levels occurring in a dynamic system (Greig-Smith 1952), (c) soil moisture variations caused by topography (Harper & Sagar 1953), (d) soil depth (Kershaw 1958). Ashby (1948) emphasizes soil heterogeneity as a possible factor controlling nonrandom distribution of species, but little evidence has been published concerning this effect of soil factors on the micro-distribution of species, although Kershaw (1958) suggests that a mineral nutrient factor influences the micro-distribution of Agrostis. Data are also scarce on the variation of soil factors over short distances which might effect non-random distribution of species. Downes & Beckwith (1951) have studied the variation of soil pH, and Raupach (1951) has studied the variation in pH and several other soil chemical and physical factors over short distances in an edaphically heterogeneous area in Australia. Downes and Beckwith conclude that differences as great as 1 pH unit occur over a distance of 1 ft (0 3 m), and observed that the distribution of several annual species in the area was apparently determined by soil pH, though that of perennial species was not. Raupach found similar large variations in soil pH and also in content of the exchangeable cations Ca, Mg, K and Na and of clay and organic matter. Such results show the great variation in soil factors which can occur over short distances, at least under Australian conditions. The present study was designed primarily to ascertain the edaphic range of Trifolium repens L., white clover, in predominantly acid, base-poor, upland areas of North Wales, and to attempt to define the limiting edaphic factors. In the process of this study, soil samples were taken in areas where the species appeared to be near the limit of its edaphic tolerance. In the areas studied, T. repens showed low percentage cover and was apparently non-random in distribution. Under these conditions, at the limit of the distribution of the species, small variations in individual factors might be expected to have the maximum effect upon performance.
R. W. Snaydon (Thu,) studied this question.