Body size (M) was specifically referred to above-ground biomass of individual plant for both power law of self-thinning and energy equivalence law for plant populations in most studies. Little is known about the biomass-density relationships with respect to below-ground part and other plant components, such as stem, leaf and reproductive part during the course of self-thinning, although they were very important. The self-thinning law can also be defined as component biomass, below-ground biomass and total biomass when all biomasses of all yield components are measured, such as leaf, stem, reproductive biomass and below-ground biomass. Subsequently, the competition effects of total biomass or each plant component can be observed during the course of self-thinning as required. Since growth in plants is a modular process, it is rational ecologically to examine biomass-density relationships not only at population and individual levels, but also at plant part level.
The growth patterns of plant components, such as stems, leaves and reproductive parts, as well as roots, in relationship with plant density were studied with Fagopyrum esculentum populations in a pot experiment. There were 3 plant densities (8000, 24000 and 48000 plants/m²) with 4 replicates in a randomized complete design. There were 6 harvesting times with 4 pots being destructively harvested at each harvesting time. Results showed that the values of self-thinning exponents were -1.293 and -1.253 for mean above-ground biomass and total biomass, respectively, during the course of population development. These values, which were not significantly different from -4/3, can be regarded as evidence in favor of the -4/3 power law of self-thinning. The allometric exponents were -1.128, -1.263, -1.524 and -2.005 for mean biomass of roots, stems, leaves and reproductive parts, respectively. There were no significant difference between γ_root and -1(P > 0.05), γ_stem and -4/3(P > 0.05),γ_leaf and -3/2(P > 0.05),but γ_reproductive was significantly different from -3/2, -4/3 or -1. It is suggested that there is no universal biomass-density relationship associated with plant components. Competition within the population affects plant development and alters the biomass allocation patterns of plants and may lead to different biomass-density relationships for plant components in the population. Specific allometric relationships between plant component biomass and above-ground biomass can also resulted in slopes of the self-thinning lines which are different from -3/2 or -4/3. Populations of Fagopyrum esculentum obeys the law of energy equivalence during the course of self-thinning for above-ground biomass or total biomass, but for below-ground biomass.