Self-Thinning in Ocimum basilicum Grown at Three Soil Fertility Levels With and Without Mycorrhizal Inoculum E. Charles Morris (Communicated by D. Keith) Morris, E.C. Self-thinning in Ocimum basilicum grown at three soil fertility levels with and without mycorrhizal inoculum. Proc. Linn. Soc. N.S.W. 115: 89-107 (1995) . To investigate whether mycorrhizal status affected the course of self-thinning of plant populations grown over a range of soil fertility, stands of Ocimum basilicumwere estab-lished at three levels of soil fertility and two sowing densities on a soil-based potting mix that was either pre-heated to kill fungi, or not heated. Sampling of roots from the pre-heated soil mix failed to find mycorrhizal infection at any subsequent harvest, while roots from the non-heated soil showed mycorrhizal infection by second harvest. Self-thinning lines for shoot biomass at each fertility level did not differ in slope; however the line for the highest-fertility level was significantly lower in elevation (intercept) than a pooled line for the two lower-fertility levels. Self-thinning lines for shoot biomass showed no effect of mycorrhizal status: the relative position of thinning lines due to fertility level was the same for popula-tions in both the mycorrhizal and non-mycorrhizal treatments. Self-thinning lines for root biomass differed significantly in slope. The root thinning line for the highest-fertility level generally lay under the line for the lowest-fertility level, while the line for the intermediate-fertility level crossed both lines. This is the first reported case of populations from the highest-fertility level thinning along the lowest line on a biomass — density plot; in previous experiments, either popula-tions from all fertility levels thinned along a line of common slope and intercept, or popu-lations from the lowest-fertility level thinned along the lowest line. While root and shoot competition were not measured directly in this experiment, examination of root and shoot growth suggested that root competition was not the major determinant of the position of self-thinning lines. However shoot competition increased most quickly as shoot biomass accumulated in the stands grown at the highest-fertility level. The canopy volume required to support given shoot biomass was greatest in populations grown at the highest-fertility level, and this difference accounted for the separation of self-thinning lines on the shoot biomass — density plot. E. C. Morris, School of Biological Science, University of New South Wales, Sydney, 2052, Australia. Current address: School of Science, University of Western Sydney Hawkesbury, Locked Bag 1, Richmond NSW 27 5 3, Australia: manuscript received 6 September 1994, accepted for publication 14 December 1994. KEYWORDS: self-thinning, mycorrhizae, mycorrhizae and competition, shoot competi-tion, soil fertility and competition, soil fertility and self-thinning Introduction The presence of neighbours in even-aged plant monocultures results firstly in a restriction of plant growth at high densities, and may result ultimately in death of suppressed plants (Shinozaki and Kira 1956, Yoda et al. 1963). These effects of competi-tion are attributed to resource depletion caused by neighbours. When stands show this density-dependent mortality, or self-thinning (Yoda et al. 1963), increases in mean biomass (B, g/m 2 ) are related to decreases in mean density ( iVper m 2 ) by the relationship log B= K-$ log N (1) where Kis the intercept and (3 the slope (Yoda et al. 1963; Weller 1987). Initially, it was argued that P had an ideal value (=-0.5) (Yoda et al. 1963, White 1985); more recent work has shown that (3 takes a wider range of values (Weller 1987; Lonsdale 1990) . Since competition for resources is presumed to be the cause of self-thinning, vary-Proc. Linn. Soc. N.s.w., 115, 1995
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