Effects of added nutrients on species composition in native tussock grasslands of New Zealand

The effects of improvement with superphosphate and sown legumes on tussock grassland infested by Hieracium pilosella were investigated, and special attention given to the Hieracium spp. Vegetation in the Ashburton Catchment was surveyed to determine the major patterns and relate them to measured environmental variables. The dominant gradients were elevation and moisture. Soil fertility accounted for only a small amount of the species composition but soil phosphorus was positively correlated with the abundance of H. pilosella and H. praealtum. A large field experiment established in 1979 was used to assess the effect of improvement at low elevation range in the same area. The experiment consisted of nine different sown legumes and nine superphosphate rates (ranging from 0 to 800 kg P ha⁻¹). All species and their abundance were assessed in summer 1991/92, and soil cores for chemical analysis were taken in autumn 1992. Present vegetation was not strongly correlated with present soil conditions, except for inorganic soil phosphorus which reflected the initial superphosphate applications. Species richness decreased as fertility increased and native species were replaced by introduced grasses and sown legumes. Individual species responses along the gradient differed. Introduced grasses generally increased in abundance as inorganic soil P increased, while the opposite occurred for the native grasses. H. pilosella and H. praealtum decreased in abundance with increasing fertility and their response curves did not differ from Festuca novae-zelandiae. In contrast, H. caespitosum had highest abundance at the higher fertility levels; a response similar to Agrostis capillaris. It was concluded that H. pilosella was shaded by the legumes and introduced grasses in the higher fertility plots and H. caespitosum was likely to be shade tolerant. A glasshouse experiment assessed the responses of H. pilosella and Festuca novae-zelandiae to added phosphorus, nitrogen, sulphur and micronutrients, and the competition intensity of these species when they were grown together. Added phosphorus had the most effect, stimulating both species. The response of H. pilosella was enhanced when it grew in mixture, and added P further augmented the response. F. novae-zelandiae biomass was suppressed by H. pilosella. This study suggests that nutrient application to tussock grasslands changes their species composition in the long-term. Fertiliser application without maintenance may contribute to the degradation of these grasslands. Hieracium spp. differ in their responses to nutrients and shade tolerance, and need to be considered individually in terms of management.