Theoretical Relationships Between Mean Plant Size, Size Distribution and Self Thinning under One-sided Competition

As yet there is no comprehensive theory in plant population ecology to explain relationships between mean plant size, size distribution and self-thinning. In this paper, a new synthesis of plant monocultures is proposed. If the reciprocal relationship between plant biomass and plant population density among various stands of even-aged plant populations holds, the same reciprocal relationship must exist between cumulative mass and cumulative number of plants from the largest individual within a population, assuming strict one-sided competition (which is an extreme condition for competition for light among plants). The two parameters of the relationship between cumulative mass and cumulative number within a stand both correlate with maximum plant height in the stand. One parameter equals the reciprocal of the potential maximum plant mass per area, which is expressed by the product of maximum plant height and dry-matter density. The other parameter correlates with the potential maximum individual plant mass, which is allometrically related to maximum plant height. As a stand develops, the growth rate of the smallest individuals will become zero due to suppression from larger individuals, and they will die; i.e. self-thinning will occur. The slope of the self-thinning line is expressed through the coefficients of allometry between height and mass and between dry matter density and height. When the former coefficient is 3 and the latter is 0, the gradient exactly corresponds to the value expected from the 3/2 power rule, but it can take various values depending on the values of the two coefficients. Competition among individuals determines size-density relationships among stands, which in turn determine the size structure of the stand. The size structure constrains the growth of individuals and results in self-thinning within the stand.