Density-dependent mortality induced by low nutrient status of the substrate

The hypothesis that changing the fertility level of the substrate would change the self-thinning line (different slope or intercept) followed by high-density populations was tested by sowing populations of Ocimum basilicum L. at two densities on a soil-based potting mix adjusted to three fertility levels (F0, F1 and F2). Fertility level significantly affected the slope of the thinning lines for both shoot and root biomass. For shoot biomass, more mortality occurred per unit increase in biomass as fertility level declined (the slope of the thinning line became flatter). The slope of the log shoot biomass vs. log density relationship was -0.5 at the F2-, zero at the F1-, and 0.94 at the F0-fertility. For the log root biomass vs. log density lines, slopes were zero at the F2- and F0-fertility levels, and -0.32 at F1. Packing of shoot biomass into canopies of individual plants correlated well with observed exponents of self-thinning lines at the F2- and F1-fertility level. Plants at the F2-fertility level required more canopy space to support a given shoot biomass than plants at F1, indicating that shoot competition was more intense at the F2-fertility level for a given biomass. Leaf area index and size inequality also increased with fertility level for a given shoot biomass. Density dependent mortality in populations grown at the F2-fertility level was highly unusual in having a positive slope for the shoot biomass vs. density relationship. Shoot growth per plant was static as density declined in the F0-populations, however, root growth per plant increased. All measurements of shoot growth (mass, height, canopy extension, leaf area) remained static in the F0-populations: root mass and length increased in comparison. It is argued that root competition became sufficiently intense to cause the density-dependent mortality seen at the F0-fertility level, with little contribution of shoot competition to mortality.