Above the treeline, the forest suddenly gives way to shrubland or grassland. Alpine shrubs grow as a dense band of scrub up to 200 metres above subalpine forest, or as a mosaic with tussock grasses and alpine herbs. With increasing altitude and exposure to wind, shrublands become stunted, and comprise dwarf and prostrate shrubs less than half a metre tall. On the highest mountains, conditions are too severe even for shrubs, and shrublands are replaced by grasslands or alpine herbfields.
The broadest and most impenetrable bands of alpine shrublands grow on the wet western side of the mountains, especially those that experience frequent fogs.
Common alpine shrubs include species of hebe, Dracophyllum, shrub daisies and coprosma, along with Ozothamnus vauvilliersii, snow tōtara (Podocarpus nivalis), bog pine (Halocarpus bidwillii), mountain toatoa (Phyllocladus alpinus) and mountain flax (Phormium cookianum).
In the South Island and on Mt Taranaki (Mt Egmont), mountain lacebark (Hoheria lyallii) forms tall shrublands and low forests on old slip faces and in gullies. It is one of New Zealand’s few deciduous plants.
Under or over
In the 1930s, before deer opened up tracks through leatherwood scrub, a botanist noted that the only way to get through it was ‘to worm one’s way along the ground or even walk upon the top of the shrubs’. 1
On the wet western sides of the mountains, leatherwood scrub often forms a near-impenetrable band just above the treeline. The broadest stretches grow above subalpine broadleaf forest, which is not found as high as beech forest.
Leatherwood scrub is dominated by one or more shrub daisies with broad, tough leaves. The main species is tūpari (Olearia colensoi), found from Mt Hikurangi in the North Island to Stewart Island in the south. It is a stoutly branched, chest-high shrub with semi-prostrate stems which seriously impede trampers’ progress. Tūpari’s serrated leaves are broad and leathery, glossy above and clad in long woolly hairs underneath.
Tūpari is absent from Mt Taranaki, but a lookalike shrub daisy, Brachyglottis elaeagnifolia, forms leatherwood scrub there. In the Ruahine Range, Brachyglottis elaeagnifolia often grows with tūpari. In the South Island Brachyglottis rotundifolia, another leathery-leafed shrub daisy, is usually present with tūpari and īnanga (Dracophyllum longifolium) in the leatherwood zone.
Muttonbird scrub, found around the coasts of the southern South Island and on Stewart Island and the Snares Islands, closely resembles leatherwood scrub. It is also dominated by one or two leathery-leaved shrub daisies, Olearia lyallii and Brachyglottis rotundifolia. It is named muttonbird scrub because the soils underneath are usually riddled with the burrows of a native petrel (Puffinus griseus), known as muttonbird, sooty shearwater or tītī.
Scotch heather (Calluna vulgaris) shrubland covers 52 square kilometres of the northern and western side of Tongariro National Park, in the central North Island. Heather was planted deliberately from 1912 to 1922, and spread rapidly through native red tussock grassland, especially after the tussock was burnt, up to 1,650 metres altitude.
In the absence of fire, mānuka and īnanga invade and overtop heather. This succession to native shrubland is fairly rapid on hill slopes (within 38 years), but slow on frosty basin flats (more than 75 years).