Axial ranges: a chain of mountains
As the Australian Plate to the west collides with the Pacific Plate to the east, they thrust up a chain of tall mountains running the length of New Zealand, from Fiordland northwards to the Volcanic Plateau. This comprises the Southern Alps in the South Island, and the Tararua, Ruahine, Kaweka, Kaimanawa and Raukūmara ranges in the North Island.
The mountains are not old by global standards – they began forming with movement along the Alpine Fault around 10 million years ago. The central Southern Alps may have reached their current height (the peaks are over 3,000 metres high) only two million years ago.
During the coldest period of the last glaciation (about 18,000 years ago), a near continuous field of glaciers extended down the Southern Alps from north-west Nelson to Fiordland. Elsewhere, in the lowlands from Taupō south, treeless vegetation and bare ground dominated.
Glacial erosion has strongly shaped the mountains in the South Island, creating jagged peaks and U-shaped valleys. Rivers draining the glaciers have formed broad gravel and silt terraces.
Greywacke sandstones and schists make up most of the axial ranges although in north-west Nelson and the far south, granites, gneisses and marbles are common.
Soils are usually shallow and of low fertility, leached of nutrients by high rainfall. Soils have formed from the low-nutrient greywacke sandstones that make up most of the mountain ranges.
The mountain environments have been created by a combination of factors: high rainfall, cloudy skies, cool summers, cold winters, shallow soils and plentiful disturbance through earthquake, heavy rain and wind.
A profusion of species
The mountain ecoregion covers almost all land in New Zealand above 800 metres (including the tops of the North Island volcanoes). Most uplands are clad in mountain beech or silver beech forest. Elsewhere there is diverse conifer–broadleaf forest of cedar, tōtara, toatoa, five finger, olearia and dracophyllum.
The upland slopes and valley basins were generally covered by beech forest, predominantly black beech, red beech and mountain beech, although the central Southern Alps were almost entirely free of beech: tōtara, toatoa and cedar were the main forest trees.
Above the often abrupt beech treeline is a zone of low shrubs. Higher up, tall tussocks give way to herb fields, then bare ground, and then snow and ice. This alpine zone has a spectacular diversity of plants and animals.
Denis Glover wrote of the Southern Alps in his epic poem ‘Arawata Bill’. The ‘birch’ are beech trees.
‘The mountains send below
Their cold tribute of snow
And the birch makes brown
The rivulets running down.’ 1
Shrubs (hebe, coprosma, dracophyllum and olearia), grasses (chionocloa, poa), rosette herbs (Aciphylla species, celmisia), buttercups, gentians and willow herbs have evolved into a host of different species in different habitats.
Plants with unusual growth forms include the whipcord hebes with their conifer-like scale foliage, and the spiny tussocks of the spaniards (Aciphylla species). There is an array of cushion plants, reaching an extreme form in vast pillows of the vegetable sheep (Haastia pulvinaris).
Peculiar, dark deep-rooting plants grow in the huge shingle slides, known as scree, that are an ancient feature of the mountains.
Insects such as cicadas, cockroaches, grasshoppers and stoneflies have adapted to mountain environments, evolving into many species with different forms. Some animals that are rarely found in the alpine zone elsewhere in the world do occur in New Zealand’s mountains. Examples are the parrot known as the kea (Nestor notabilis) and cicadas. New Zealand’s lizards are more cold-tolerant than those elsewhere, and some geckos and skink species occur at the treeline.