Soil dilution as a surrogate for root competition: effects on growth of seedlings of Australian tropical rainforest trees

1. Previous studies on the combined effects of light and nutrient supply on the growth rates of plants have involved adding nutrients to soils to levels rarely (or never) met by the plants in nature. As a better test of the likely response of plants to root competition for nutrients. we grew for 1 year seedlings of six species of Australian rainforest tree on forest topsoil undiluted and on soil diluted 50/50, 25/75 and 5/95 with coarse sand. Seedlings were grown in tall tubes in 6 and 9% daylight, equivalent to gap-edge and gap-centre conditions. 2. The six species varied in their light requirements for establishment and onward growth. 3. One species grew significantly and markedly more quickly on soil diluted 50/50 with sand (Elaeocarpus grandis). Improved aeration is the most likely explanation. 4. The extent of inhibition by soil dilution correlated positively with the absolute amount of new dry mass accreted in one year, whether the larger accretion resulted from a combination of high relative growth rate (RGR) and small embryo size, or moderate RGR and larger embryo size. 5. Soil dilution generally reduced leaf mass fraction and leaf area ratio, but had no significant effect on specific leaf area. 6. In all species the overall concentration of nitrogen in the whole plant declined with soil dilution, in the four species tested, N concentration in leaves declined, and it is likely that the reduction in growth arose partly from a reduced unit leaf rate. Most species showed increases in the concentrations of P, K, Ca and Mg with soil dilution, or no change, but in Diploglottis bracteata the concentrations of Ca and Mg, especially in the older leaves, declined markedly with soil dilution, and one of these cations could have been limiting growth. 7. Extrapolation of the results to the field suggests that in single-treefall gaps the fastest growing species will the most inhibited by root competition. 8. The soil-dilution technique is likely to be more appropriate than nutrient addition as an indicator of sensitivity to root competition on a given soil.