New Zealand has an abundance of twiggy shrubs. They grow in various combinations from the coast to alpine slopes, forming shrubland and scrub communities.
Shrublands occur alone, or mixed with grassland and forest, over some 7.5 million hectares – 28% of New Zealand. Some are temporary, forming one stage in the development of forests. Others are permanent, growing in harsh environments where trees fail to prosper – exposed coasts, wetlands, infertile soils, alpine areas, and very dry hill country.
Shrublands in New Zealand include:
- mānuka shrublands, which have an important role in forest regeneration
- alpine shrublands, found above the treeline in mountain areas
- matagouri scrub, which occurs among the South Island’s tussock grasslands
- heathlands, found on very infertile soils.
Shrub or scrub?
Shrubland is open vegetation where shrubs are the tallest plants; scrub is denser, with a continuous or near-continuous canopy of shrubs and small trees. Both are usually differentiated from bush – a New Zealand term for tall native forest.
What is a shrub?
A shrub is a small woody plant, usually with multiple stems that start close to the ground. There is no clear distinction between a small tree and a large shrub. Some species may take a shrub form when growing in exposed conditions, but develop into a small tree in moist sheltered situations.
New Zealand shrubs
New Zealand has around 445 native shrub species, more than twice the number of native trees. Just over a third (155) are uncommon or threatened. One, Logania depressa, is assumed to be extinct.
Around 230 shrubs have small leaves (less than 2 centimetres long). Of these, about 60 have an unusual, densely branched growth form that seems to be peculiar to New Zealand and is called divaricate, filiramulate or zigzag growth.
The prostrate shrub Logania depressa was collected just once – by the missionary and botanist William Colenso in 1847, east of Waiōuru in the central North Island. Despite further searches, it has never been found since. The area where it grew has been damaged by fire and browsing animals, and much of it is now covered by Lake Moawhango, formed by a hydroelectric dam.
Reasons for divaricate form
Divaricating shrubs have small leaves and wiry, interlacing branches, set at wide angles. Why this habit occurs in at least 17 plant families in New Zealand is uncertain. Some biologists think that divaricates evolved from large-leaved relatives in response to cold, frosty and windy environments during the Pleistocene ice ages. Others suggest that the form was an adaptation against excessive browsing by moa – large flightless herbivorous birds, now extinct, that lived in open country and forests.
With a few exceptions in very dry periods, New Zealand’s shrubs are evergreen – they hold their leaves year-round. Many have features that help them survive in dry, windy environments by preventing excessive transpiration (evaporation of water from the leaf):
- They have a dense network of interlacing branches, with small leaves held in the interior of the shrub.
- Leaves are leathery, waxy on top, and often covered in hairs (tomentum) underneath.
- Vertical, needle-shaped leaves are held at the tips of stiff, upright branches.
- Leaves are reduced to scale-like structures, and are closely pressed to the branch stems.
- The edges of the leaf curl inwards on the underside.
Plants in New Zealand’s shrublands
Although shrublands are extremely varied in their species composition throughout New Zealand, some generalisations can be made. Dominant or common plants include:
- divaricating shrubs
- the heath family (Ericaceae)
- small conifers
- shrub daisies
- hebe and coprosma species
- introduced shrubs, especially in the lowlands.