A low average temperature is the defining characteristic of alpine zones. Other environmental factors vary widely. Differences in rainfall, cloud cover, aspect, rock type, soil fertility and drainage mean that alpine vegetation across New Zealand is diverse. For simplicity, however, the country’s alpine plants can be grouped into four main vegetation and habitat types:
- stable rock
- rock debris.
Unlike the short, summer-green alpine grassland in many other parts of the world, those in New Zealand are dominated by large, long-lived, evergreen tussocks of Chionochloa, collectively known as snow tussock. Some alpine grasslands are formed by shorter, evergreen grass species, notably in the genera Chionochloa, Poa and Rytidosperma.
Which grass dominates is determined by a complex set of interacting variables. These include seasonal rainfall levels (mostly an east–west gradient), north–south species distribution patterns, soil characteristics, how long snow lies on the ground, and disturbances such as avalanches.
Short-statured shrubs are associated with the grasses, especially in the lower half of the alpine zone. They include snow tōtara (Podocarpus nivalis), a number of Hebe and Coprosma species, and some heath and heath-like shrubs (Gaultheria, Archeria, Leucopogon and Dracophyllum).
Many herbaceous flowering plants grow between the snow tussocks. Among them are numerous species of mountain daisy (Celmisia), spaniard (Aciphylla), Anisotome, buttercup (Ranunculus), Ourisia, eyebright (Euphrasia), Parahebe and gentian (Gentianella). Compared with continental alpine areas the flowering season in New Zealand is prolonged. Some buttercup and marsh marigold (Psychrophila) species are in full flower from November to December, Celmisia daisies bloom from December to March with a peak in January, and gentians flower from late January into April.
In waterlogged and aquatic sites (lakes, tarns, bogs, marshes and swamps) the shape of the land prevents water from draining away, or water is supplied continuously from higher ground (flushes, streams and waterfalls). Widespread in alpine wetlands are mosses (including peat-forming Sphagnum), liverworts, sedges, rushes, golden-flowered lilies (Bulbinella species), white and pink-flowering willowherb (Epilobium macropus) and algae.
In the South Island a distinctive orange-brown, glossy alpine rush (Marsippospermum gracile) is often the dominant species where there is more or less continuous seepage in high-altitude grasslands. It is visible from afar.
Carnivorous bog plants
Sundews (Drosera species) obtain essential nutrients, especially nitrates, from infertile, acidic, waterlogged bog sites by trapping and digesting small insects. They use specialised, flypaper-like leaves that are covered with sticky glandular hairs. Of six native sundews, three are alpine.
Wetlands where cushion-forming plants such as comb sedge (Oreobolus pectinatus) dominate are known as cushion bogs. They are extensive in areas where alpine terrain was planed and hollowed by glacier ice in the last glaciation but which has been free of ice for thousands of years. On the eastern side of the central Southern Alps, glaciation was so extensive and the ice retreat so relatively recent that a number of characteristic alpine bog species including alpine bog cushion (Donatia novae-zelandiae) and pygmy pine (Lepidothamnus laxifolius) are absent.