The effect of different densities of tussocks (Festuca novae-zelandiae, Poa laevis) in short tussock grassland associations was studied using transplanted cocksfoot (Dactylis glomerate) seedlings as indicators of inter-tussock plant growth. The dense tussock associations considerably modified the microclimate, reduced frost damage and initially promoted cocksfoot growth. Sparse tussock associations did not generally promote cocksfoot growth unless plants were near the tussock base and sheltered from the prevailing wind
Seedlings of five native tussock species grown in the glasshouse on mountain subsoil showed outstanding responses to nitrogen and phosphorus fertiliser in combination. The most vigorous species was silver tussock but this produced less dry matter than an introduced grass, Yorkshire fog, grown on the same soil. Tussocks in general responded positively to applications of magnesium and potassium but growth was depressed by lime.
The low level of plant nutrients in exposed high-altitude subsoils, and the effects of soil frost and needle ice on plants attempting to colonise these subsoils combine to make natural revegetation very difficult. Artificial revegetation trials established in 1965 at three sites in the Canterbury mountains tested the effect of a fertiliser mixture which supplied a wide range of nutrients, and compared ten herbaceous species as providers of an initial protective cover, and of a cover that would persist.
Feral horses (Equus caballus L.) occupy 64 000 ha of montane- subalpine tussock grassland in the south-western Kaimanawa Mountains, an area zoned for military training. Since 1979, the population has increased at 16.7% per annum, reaching 1102 in 1990. The most extensive habitat, red tussock (Chionochloa rubra) grassland, was variably affected by horses; tussocks in restricted mesic sites were heavily grazed and mostly eliminated, but those in extensive xeric grasslands showed little impact.
Less than 4% of the non-bamboo grasses worldwide abscise old leaves, whereas some 18% of New Zealand native grasses do so. Retention of dead or senescing leaves within grass canopies reduces biomass production and encourages fire but also protects against mammalian herbivory. Recently it has been argued that elevated rates of leaf abscission in New Zealand’s native grasses are an evolutionary response to the absence of indigenous herbivorous mammals.