Phosphorus availability and the growth, mineral composition and nutritive value of ephemeral forbs and associated perennials from the Pilbara, Western Australia

Abstract

The effect of phosphorus fertiliser on plant growth and the quality of leaf tissues for herbivores were investigated in field and glasshouse experiments. In the field, the relative abundance of ephemeral forb species was strongly affected by the seasonal variation in rainfall. In winter, C3 ephemeral forbs were abundant, whilst in summer, C4 ephemeral grasses dominated. During the dry months, growth of all species was poor. After rain, grasses to which phosphorus had been added increased growth significantly. Nutrient concentrations in ephemeral forbs were significantly greater than those in perennial or ephemeral grasses. Phosphorus concentrations were low in perennial and ephemeral grasses and declined during the dry months. The nitrogen : phosphorus ratio of ephemeral forbs and perennial shrubs suggested a deficiency of phosphorus, whilst that of ephemeral grasses suggested a deficiency of nitrogen. A glasshouse experiment investigated the response to phosphorus of 2 common and abundant ephemeral forbs – Ptilotus macrocephalus which responded to all treatments up to a maximum rate of 200 kg P/ha, and Ptilotus exaltatus which increased in growth up to a maximum rate of 100 kg P/ha. In both species, the concentration of phosphorus increased significantly with phosphorus supply, while that of nitrogen did not vary significantly among phosphorus treatments. Generalisations about growth and nitrogen and phosphorus nutrition of native species based on more mesic plant communities are not readily applied in the arid and chronically phosphorus-poor Pilbara environment. Instead, plant life-cycle and life form play major roles in determining nitrogen or phosphorus limitations and plant responses to added nutrients.