A large proportion of Australia's rangelands is arid or semi-arid and is dominated by soils low in total phosphorus. Productivity is controlled chiefly by the season and distribution of rainfall, although fire plays a major role in determining species composition, especially of the scrublands in these regions. Two legumes, common to the Pilbara region of Western Australia, Acacia ancistrocarpa and Senna notabilis, regenerated quickly after fires in summer 1995-1996. We examined their growth, mineral nutrition and carbon fractions in relation to their role as possible feed for herbivores, both native and introduced, with and without added phosphorus both in the field and the glasshouse. In the field, both species showed no significant changes in concentrations of total phenolics, condensed tannins, non-structural carbohydrates or lignin in response to fertiliser addition. Both species accumulated nitrogen and phosphorus after summer rain but the allocation of those resources varied between species. In the glasshouse, the concentration of nitrogen in both species was not affected by phosphorus fertilisation but the concentration of phosphorus in foliage increased linearly as the amount of phosphorus increased. Phosphorus application resulted in an increase in the dry mass of both species. Both species are low in in vitro dry organic matter digestibility and maintain high concentrations of total phenolics, condensed tannins and lignin. All nutritional, including major minerals, and anti-nutritional components varied significantly with season.
