PASTURE MANAGEMENT IN SEMIARID TROPICAL WOODLANDS - EFFECTS ON SPECIES-DIVERSITY

The effects of pasture management options (sowing introduced legumes a nd grasses, timber treatment, applying fertilizer, cultivation before sowing, and stocking rate) on species diversity were measured at two e xperimental sites (Hillgrove and Cardigan) near Charters Towers, north east Queensland. Species were divided into three groups (sown, native and exotic) and diversity was measured as species density (number of s pecies recorded in each plot and number of species/quadrat) annually f rom 1982 to 1992. The responses of individual native and naturalized s pecies to treatment were also determined. All management options affec ted diversity but the responses varied with site and season, and with the different measurement scales. The density of sown species either i ncreased or was unaffected by all the management options; there were n o significant decreases. The density of native species showed both pos itive and negative responses; it increased at high stocking rates and with tree killing at Hillgrove, and decreased with pasture sowing and cultivation. The density of exotic species increased as stocking rate was increased and decreased when pastures were sown (although not at t he quadrat scale at Hillgrove). Overall the most diverse vegetation wa s on plots grazed at high stocking rates; at the plot scale these were native pastures but at the quadrat scale the sown pastures had more s pecies. Among the native and naturalized species, only Portulaca spp. were more frequent on the oversewn plots than the native pasture plots ; 48% (Hillgrove) and 68% (Cardigan) of the species were less frequent on the oversewn plots. Fertilizer application had little effect on sp ecies frequencies, while timber treatment resulted in both increases a nd decreases in frequency of a small number of species. The species we re divided into four groups on the basis of their responses to stockin g rate: a grazing-sensitive group (e.g. Themeda triandra), two grazing -tolerant groups which either slightly decreased (e.g. Chrysopogon fal lax) or slightly increased (e.g. Sida spinosa) in frequency as stockin g rate increased, and a fourth group of species which were frequent on ly at high stocking rates (e.g. Bothriochloa pertusa). There were no c lose relationships between herbage yield and species density.