Population dynamics of Stylosanthes hamata and S. scabra in north-eastern Queensland: effects of superphosphate application, timber treatment, and stocking rate

Abstract

The effects of pasture management (superphosphate application, timber treatment, stocking rate) on the herbage yields and population dynamics (seed production, seedling numbers, plant longevity) of Stylosanthes hamata cv. Verano and S. scabra cv. Seca grown in grass-legume mixtures were measured from 1982 to 1992 at 2 sites, Hillgrove (extractable phosphorus, 50 mg/kg) and Cardigan (extractable phosphorus, 6 mg/kg), near Charters Towers, north-eastern Queensland.

Stylosanthes hamata was higher yielding during the first 3 years at both sites but subsequently S. scabra produced the larger yields. This resulted in S. hamata dominating the legume component in the early years but the proportion of S. hamata then declined, so that by the final year, S. hamata contributed <20% of the legume mixture at both sites.

Both seed yields and seedling numbers varied widely between years for both species at both sites. In the early years there were more S. hamata seedlings but in the later years S. scabra had greater seedling numbers. Seed yield and seedling numbers were not significantly affected by superphosphate but both were higher for S. hamata on plots with live trees than on plots with killed trees; numbers of seeds and seedlings of S. scabra were not affected by tree killing. Seed and seedling numbers of both species tended to be greatest at the intermediate stocking rates. Survival of both S. hamata and S. scabra at both sites conformed to Deevey"s Type II curve (i.e. death rate was constant with age). The S. scabra plants lived considerably longer than the S. hamata plants. Pasture treatments had less influence on longevity than on seed production and seedling numbers.

Seed production and seedling establishment of both species were marginal for their persistence in the region, and management should aim for seed set and subsequent seedling establishment to occur sufficiently frequently to allow the legumes to persist.