Weight-Density Relationships in Ramet Populations of Clonal Perennial Herbs, with Special Reference to the -3/2 Power Law

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Abstract
(1) The -3/2 power law relates the logarithms of mean plant weight and plant density through time as a population undergoes thinning. It describes the growth of populations of a wide variety of species with genets as their basic population units. In populations of clonal perennial herbs, however, ramet density is restricted by environmental controls and controls internal to the plant, and thinning of ramets does not occur as a direct result of their growth. The -3/2 power law, therefore, is not applicable to the ramet populations of clonal perennial herbs, and several sets of data are presented to illustrate this fact. (2) The sets of data fall, with one exception, into three families of curves. Most relationships follow the model curve presented by Hutchings & Barkham (1976), or exhibit a cyclic form returning to approximately the same position on the graph at 12-month intervals. In the former type, cohorts of shoots are completely separated in time, whereas in the latter type cohorts overlap, so that the site is continuously occupied by shoots. Species exhibit a range of types of behaviour between these two extremes. (3) Genet populations with efficient utilization of biological space self-thin along an `ultimate' thinning line with a gradient of -3/2. At maturity, most clonal perennial herbs closely approach this thinning line, but do not transgress it. This is an efficient strategy incorporating maximum utilization of environmental resources and high production of biomass without self-thinning. (4) It is suggested that the -3/2 power law may have operated to determine the optimum relationship between shoot number and biomass for each site when it has achieved a steady state with constant biomass from year to year.