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Annals of Botany 78: 353-364, 1996
© 1996 Annals of Botany Company
Effect of Localized Placement of Nutrients on Root Competition in Self-thinning Populations
School of Biological Science, University of New South Wales, Sydney, Australia
September 26, 1995 ; March 28, 1996
The hypothesis that increased root competition can lower the slope and/or intercept of the self-thinning line traversed by plant populations was tested using localized placement of nutrients to increase root competition. Localized placement of nutrients will result in increased root competition, if average inter-root distances are reduced, and if nutrients are in limiting supply. It was predicted that high-density, nutrient-limited populations of Ocimum basilicum L. grown with localized placement of nutrients would self-thin along a lower biomass–density line than non-localized controls. This was tested at two fertility levels on a soil-based potting medium in expt 1, and at one fertility level on washed sand in expt 2.
Localized placement of nutrients significantly reduced the elevation (intercept) of the self-thinning line for both shoot and root biomass in expt 2. In expt 1, at the higher-fertility level, localized placement of nutrients had no significant effect; at the lower fertility level, localization had no significant effect on thinning lines for shoot biomass, and resulted in a zero slope of the thinning line for root biomass.
Canopy-based models of self-thinning failed to account for the reduction in the thinning-line intercept observed in expt 2. In both experiments, localized placement of nutrients resulted in a higher proportion of total root length being located in the localization zone, which would result in a reduction in the average inter-root distance. This would intensify root competition under conditions of nutrient limitation. The hypothesis that intensified root competition would lower the self-thinning line is supported by the results of expt 2.
Localized placement of nutrients; root competition; shoot competition; root–shoot allocation; self-thinning; Ocimum basilicum ; sweet basil