On the lowland plains and in the hill country, most shrublands are secondary growth – they occur on sites that were formerly forest. In the absence of disturbance (fire and heavy browsing by mammals), the forests regenerate.
Mānuka (Leptospermum scoparium) is the most abundant and versatile native shrub in New Zealand. It is a variable species, usually growing to 3–5 metres in height, but capable of persisting in a dwarfed form (10–30 centimetres tall) in exposed or infertile situations.
Mānuka shrublands occur in two distinct ecological settings:
- permanent shrublands, on infertile, dry, exposed, or poorly drained soils
- temporary shrublands on disturbed sites that formerly carried forest, as one stage in forest regeneration.
In moist, low-altitude areas that were once forest, mānuka is an important pioneer plant, starting forest regeneration. It forms dense shrublands that persist for decades (30–70 years, depending on the location), but are later replaced by tall forest.
In the lowlands and fertile hill country, mānuka is often accompanied by kānuka (Kunzea ericoides). This long-lived tree overtops mānuka, forming firstly a tall scrub cover, and then a forest. Areas of regenerating mānuka and kānuka scrub cover some 3 million hectares – especially in moist hill country where farming is marginal.
Although fire usually kills mānuka shrubs, it promotes seeding. On its branches, mānuka often has unopened fruit capsules, which split open in a fire and release thousands of tiny light seeds.
Broadleaved shrublands and scrub develop on moderately fertile and fertile soils. They usually comprise fast-growing, short-lived trees such as wineberry (Aristotelia serrata), rangiora (Brachyglottis repanda), māhoe (Melicytus ramiflorus), five-finger (Pseudopanax arboreus) and tree ferns. Near the coast, ngaio (Myoporum laetum) and taupata (Coprosma repens) are often present. At higher altitudes, the dominant tree species are pigeonwood (Hedycarya arborea), kāmahi (Weinmannia racemosa) and broadleaf (Griselinia littoralis). Broadleaved scrub is usually a stage in the regeneration of mixed conifer–broadleaf forest.
Tauhinu (Ozothamnus leptophyllus) is a small, bluish-grey shrub daisy. It forms temporary shrublands that last for 15–20 years on coastal farmlands. Close to forest, tauhinu shrublands quickly revert to broadleaved scrub. Away from forest, they often develop into a mixed shrubland of small-leaved twiggy plants, known as grey scrub.
Gorse and Scotch broom
The introduced shrubs gorse (Ulex europaeus) and Scotch broom (Cytisus scoparius) form young regenerating shrublands covering 800,000 hectares of New Zealand. They are especially prominent on low-producing hill pastures and gravel flood plains. Gorse and broom are able to ‘fix’ atmospheric nitrogen and change it into a useable form, which gives them an advantage over most native shrubs. Both gorse and broom have seeds that stay viable for years when buried in soil or deep litter.
Unlike most native shrubs, gorse resprouts after fire, so it soon dominates native plants on sites that have been frequently burnt. Broom tolerates drier and frostier sites than gorse, and is especially common on the east coast of the South Island.
If left undisturbed, gorse and broom shrublands are invaded by native tree seedlings, which eventually overtop the shrubs and develop into mixed broadleaved forests.