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Sustainability of New Zealand high‐country pastures under contrasting development inputs. 7. Environmental gradients, plant species selection, and diversity

Abstract The change in plant species relative abundance under different fertiliser and management inputs over 19 years is reported for two grazed multiple‐species trials on a Pukaki/Tekapo high‐country soil. One trial was 30 combinations of 5 superphosphate rates (0–500 kg ha‐1 yr‐1) × 3 stocking rates × 2 stocking methods, and the second was 31 combinations of S, P (0–100 kg ha‐1 yr‐1), and micronutrient fertilisers. Both were overdrilled with a 25‐species pasture mixture. There was rapid initial separation in relative abundance of species, principally according to fertiliser level. Main features were Hieracium pilosella remaining dominant in the absence of fertiliser; the initial success of Trifolium hybridum; the dominance and long‐term persistence of Lupinus polyphyllus at low fertiliser inputs; the transition to Dactylis glomerata dominance at high fertiliser inputs following a legume phase, in the middle years; the slow vegetative increase of Trifolium ambiguum to become dominant in most of the moderate and high fertiliser treatments in the second decade; and the increase of Bromus tectorum in later years. Species distributions were predominately determined by P fertiliser rates, or P by S fertiliser interactions. The effects of different grazing managements on late spring pasture composition were small during the first decade but increased over time, with the principal changes under moderate to high rate set‐stocking. Diversity considerations showed that the number of vascular plants in a plot (n), or their proportional distribution in biomass (k diversity), gave only a weak and inconsistent correlation with secondary production (mean sheep carrying capacity), or its stability (CV of annual grazing capacity), after fertiliser and grazing management treatment effects were considered. The only significant trend was in one trial where production tended to be inversely related to the number of plant species (2.2% decrease per species).