Self-thinning and competition intensity over a gradient of nutrient availability

(1) High-density populations of Ocimum basilicum L. were grown in sand culture at three nutrient levels to investigate patterns of self-thinning in stands grown over a range of nutrient supply. (2) Populations of Ocimum at each of three nutrient levels showed self-thinning with different biomass-density relationships (thinning lines) for each level. The lines for the stands at the two lower nutrient levels lay below that followed by the stands grown at the high level. Thus competition was more intense in the stands grown at low nutrient levels, when compared at a common plant mass. However, because thinning proceeded fastest in the stands grown at high nutrient supply, competition at a common time was most intense in these stands. (3) It is proposed that the mechanism underlying intensified competition (irrespective of rate of development) is the extent of overlap of resource depletion zones to achieve given plant mass. If this overlap increases at lower resource levels to achieve a given plant mass, competition will intensify at the lower resource levels. However, the rate at which overlap develops at each resource level will determine the intensity of competition when stands are compared in time. (4) Plants of Ocimum from stands grown at lower nutrient levels tended to have less radial extension of the canopy for a given height, and to be shorter and have less radial extension for a given shoot weight. Thus they packed their shoot weight into a smaller canopy volume per plant than plants at high nutrient levels. If canopy packing had determined the relative positions of thinning lines, populations grown at low nutrient levels would have followed higher thinning lines than those at high levels. (5) Biomass packing, when measured at the population level, showed the reverse trend, i.e. it was lower in the stands at low nutrient levels. This occurred because of the wider spacing between plants in these stands than in the high-resource stands at a given biomass level. Canopy interactions between plants grown at low nutrient levels were less than in plants grown at high nutrient levels. Thus the major determinant of plant spacing in the stands at low nutrient levels was below rather than above ground. (6) Plants competing for nutrients can increase uptake by increasing the absorbing power of the root, and/or by growing more root. Root and root hair measurements failed to detect any morphological evidence of increased absorbing power in the plants grown at low