POPULATION-DYNAMICS OF SIRATRO AND SHRUBBY STYLO IN SOUTH-EAST QUEENSLAND AS AFFECTED BY PHOSPHORUS, SOIL TYPE, STOCKING RATE AND RAINFALL

The effects of P application and stocking rate on the persistence of S iratro (Macroptilium atropurpureum cv. Siratro) and shrubby stylo (Sty losanthes scabra cv. Fitzroy) were monitored in 2 experiments over 9 y ears on 2 soil types in subcoastal, subtropical Queensland. Experiment 1 examined the effects of 2 stocking rates (0.8 and 1.2 steers/ha) an d annual rates of P application, ranging from 0-15 kg/ha P, on Siratro . Experiment 2 compared the response of Siratro. and Fitzroy to a rest ricted range of P and S treatments. Phosphorus application increased t he yield of both legumes, but had only a negligible effect on the surv ival of the original plants. However, P application to Siratro resulte d in the soil seed bank of the treatment receiving 15 kg/ha P (P15) be ing some 5 times higher than in the treatment receiving no P (P0). Thi s in turn led to more recruitment of Siratro in the P-fertilised plots and a 3-fold higher plant density in the P15 treatment compared with the PO treatment at the end of the experiment. However, the density of Siratro decreased with time as recruitment of new plants did not comp ensate for death of older plants. The lower than median rainfall may h ave adversely affected Siratro recruitment. There was very little effe ct of stocking rate or soil type (red earth or yellow podzolic) on Sir atro persistence. Fitzroy density increased with time whereas Siratro density declined. This is attributed to the higher seed set of Fitzroy which lead to reserves of soil seed being more than 100 times those o f Siratro after 9 years and to the successful recruitment of stylo pla nts from seedling strikes. Appreciable quantities of viable seed of Fi tzroy were excreted in cattle dung.