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

The hypothesis that changing the fertility level of the substrate would cha nge the self-thinning line (different slope or intercept) followed by high- density populations was tested by solving 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, mor e mortality occurred per unit increase in biomass as fertility level declin ed (the slope of the thinning line became flatter). The slope of the log sh oot biomass vs. log density relationship was -0.5 at the F2-, zero at the F 1-, 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 we ll with observed exponents of self-thinning lines at the F2- and F1-fertili ty level. Plants at the F2-fertility level required more canopy space to su pport a given shoot biomass than plants at F1, indicating that shoot compet ition 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 gi ven shoot biomass. Density-dependent mortality in populations grown at the F0-fertility level was highly unusual in having a positive slope for the sh oot biomass vs. density relationship. Shoot growth per plant was static vs density declined in the F0-populations; however, root growth per plant incr eased. All measurements of shoot growth (mass, height, canopy extension, le af area) remained static in the F0-populations: root mass and length increa sed in comparison. It is argued that root competition became sufficiently i ntense to cause the density-dependent mortality seen at the F0-fertility le vel, with little contribution of shoot competition to mortality. (C) 1999 A nnals of Botany Company.