Mānuka (Leptospermum scoparium) is New Zealand’s main pioneer shrub, and thrives after fire. Like bracken, it grows throughout the country, from the coast to the mountains, and tolerates wet or dry soils. It is extremely adaptable and hardy, and can flower and seed when just a few centimetres tall. Its woody capsules split open when dry or burnt, releasing thousands of fine light seeds that are spread by wind.
Germinating mānuka seedlings need full sunlight to grow, as they have few food reserves in their tiny seed. They germinate prolifically – thousands to a square metre. These compete intensely for nutrients, water and light. Over time most mānuka seedlings and saplings die, until just one tree dominates every few square metres.
Mānuka is senile at 30–50 years, when it dies from insect and wind damage. As its canopy opens, other species germinate and grow, as it is too shady under the canopy for a second crop of mānuka.
Talking of mānuka
European settlers pronounced mānuka in various ways. Some stressed the first syllable, some the second. In 1948, L. R. C. Macfarlane tried to sum it up: ‘North Islanders call this plant “marnaka”, South Islanders “manooka”, while the New Zealand farmers refer to it as a “bloody nuisance”.’ 1 The correct pronunciation is actually mahnooka.
Kānuka (Kunzea ericoides) often grows with mānuka. Both are known as tea trees – kānuka as white tea tree and mānuka as red tea tree, because of their wood colour. Like mānuka, kānuka plays a pioneering role on cleared land. If the two grow together, kānuka eventually replaces mānuka because it grows taller and lives longer (up to 150 years). Not as adaptable as mānuka, kānuka does not tolerate wet or very infertile soils, or grow in subalpine areas. Where fires are frequent, mānuka usually gains the upper hand as fire destroys kānuka seed capsules.
Gorse (Ulex europaeus) was introduced sometime before 1835 for farm hedges. A fast-growing, short-lived shrub, it quickly became the first plant on cleared land, replacing bracken, mānuka and kānuka. Its seeds spread and germinated on farmed land, forming impenetrable, spiny thickets.
Gorse was declared a noxious weed by Act of Parliament in 1900. By then it was widespread over the lowlands and hill country. Farmers consider gorse a curse, but environmentalists like it because forest regenerates rapidly beneath it.
Gorse succession at Hinewai
At Hinewai Reserve on Banks Peninsula, native forest has regenerated through gorse. Gorse grew in grazed pastures, and dominated within two years if there was no further grazing. After about 10 years, the stems began to collapse, the canopy opened and native seedlings grew in the gaps. Gorse cover changed to a young broadleaved forest after another 5 to 10 years – a much shorter period than succession through mānuka or kānuka.
Elsewhere, succession from gorse to broadleaved forest can take much longer, especially if native seed sources are distant or if browsing animals (sheep, deer, cattle, goats and possums) eat the regenerating plants.
Gorse versus kānuka
One study compared stands of gorse and kānuka, and found they were quite distinct habitats. Some native plants and animals prefer to live among kānuka, and an undergrowth of native orchids, divaricating shrubs and conifers was more common. Few native birds are seen in gorse stands, but native fungi, gnats and beetles may be common.