Effects of Density and Extinction Coefficient on Size Variability in Plant Populations

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Abstract
The effects of density and extinction coefficient on size variability, as measured by the coefficient of variation of plant weight in even-aged monocultures, were investigated theoretically using a diffusion model of growth and size distribution and a canopy photosynthesis model over the range of densities at which self-thinning (size-dependent mortality) does not occur. Size inequality (the coefficient of variation of plant weight) increases with increasing density or leaf area index at each growth stage. Plants with erect leaves are prone to lower size inequality than plants with horizontal leaves. These results agree well with existing observations on even-aged plant monocultures and suggest that competition between plants is mainly one-sided (competition for light). One sided competition affects size variability through a G(t, x) function (mean growth of plants of size x at time t per unit time). Two-sided competition (including competition for nutrients) affects size variability through a D(t, x) function (variance of growth of plants of size x at time t per unit time). In this case, size inequality decreases with increasing density. The importance of studying size variability is emphasized.