As any gardener or farmer knows, cleared soil in New Zealand soon sprouts fast-growing weeds. In time the weeds are overtaken by woody shrubs. These in turn are gradually replaced by long-lived trees.
When one group of plants gradually replaces another, it is called plant succession.
When Mt Tarawera erupted in 1886, it buried vegetation under a thick layer of ash and scoria. For about 10 years the area was bare. Then toetoe (Cortaderia fulvida), bracken fern (Pteridium esculentum) and tree tutu (Coriaria arborea) appeared on the lower slopes. Above 600 metres, the first plants were Racomitrium moss, lichens and mat-forming daisies (Raoulia species).
Thirty years after the eruption, there was a young forest of pōhutukawa (Metrosideros excelsa), rewarewa (Knightia excelsa) and kāmahi (Weinmannia racemosa) on the lower slopes. Tree tutu was spreading up the volcano’s sides.
By the 1990s, kāmahi forest clothed the lower slopes. Some kāmahi was growing in tree tutu shrublands on the upper slopes. Tree tutu had spread upwards onto the flat area of the summit and was becoming a closed canopy over the low-growing mats of mosses, lichens and daisies.
At the end of the last Ice Age, glaciers retreated up their valleys and left moraines – glacial dumps of rubble – behind. On the West Coast, at Franz Josef, various plant communities can be seen growing on moraines of different ages. These have developed over more than 10,000 years. They show the different stages of a succession on moraine.
The first plants to grow on the fine gravels of moraine are similar to those on volcanic surfaces – mat-forming daisies, willow herbs (Epilobium species), lichens, and Racomitrium moss. Within 10–20 years a shrubland develops, dominated by tree tutu and tree broom (Carmichaelia arborea).
Seedlings of kāmahi and southern rātā (Metrosideros umbellata) establish themselves early, but in their first years, faster-growing shrubs overtop them. In time, the kāmahi and southern rātā – longer-lived and taller than shrubs – overtake the shrubs. They then dominate the forest for 300–400 years.
These first-generation kāmahi and rātā eventually die. After this, rātā are unable to germinate in the shade of the forest that has emerged. They are replaced by conifers: rimu (Dacrydium cupressinum), miro (Prumnopitys ferruginea) and Hall’s tōtara (Podocarpus hallii). Kāmahi is shade-tolerant, so it establishes a second generation. Rimu–kāmahi forest has lasted for at least 11,500 years on the oldest moraines.
Many plants are adapted to just one phase of a forest succession. They suit either the first (pioneer) phase, or the mature forest phase, or somewhere in between. As a succession proceeds, some plant and soil trends are fairly universal.
During a succession, plant height increases, soil builds up and soil nutrients increase. Plants change:
On volcanic surfaces and exposed glacial moraines there is a common sequence. Lichens, mosses and small herbaceous plants appear first, soon followed by nitrogen-fixing shrubs.
All living things need nitrogen to grow. When a primary succession starts, little or no nitrogen is available from the surface, which is often ash, or gravel. Some plants are able to ‘fix’ nitrogen – bacteria in their roots change nitrogen from the air into a useable form. This allows them to live on nutrient-poor surfaces. Over time, the dead leaves and roots of these plants mix into the soil. The nitrogen content increases, so other plants are able to grow there too.
Native nitrogen-fixers include tree tutu, native brooms, blue-green algae, some lichens, kōwhai (Sophora species) and matagouri (Discaria toumatou). Introduced nitrogen-fixers include clovers (Trifolium species), tree lupin (Lupinus arboreus), Scotch broom (Cytisus scoparius), and gorse (Ulex europaeus). These plants all appear early in primary successions.
Occasionally, natural fires from volcanic eruptions or lightning destroy areas of New Zealand forest. Typically they happen only every 1,000–2,000 years, so mature forest has time to re-establish. This is secondary succession – where plants establish themselves on pre-existing soil.
Polynesians, the ancestors of Māori, arrived in New Zealand around 1250–1300 AD. In some areas, they used fire to clear forest. There was massive deforestation of lowland New Zealand, especially on the country’s dry eastern side. Bracken fern (Pteridium esculentum) spread from its original coastal dune habitat to newly cleared areas. A sudden rise of bracken spores in the pollen fossil record coincides with Polynesian settlement of the country.
Bracken fern is one of the first plants to appear at repeatedly burnt sites. It spreads by spores or underground rhizomes, from which it re-sprouts after fires. Bracken can form dense 2–3-metre-tall stands in sheltered fertile areas, or stunted 20-centimetre-high cover on exposed sites with poor soils. Throughout New Zealand, bracken grows from sea level to subalpine regions, but fails to thrive in shaded or swampy habitats.
When Charles Darwin visited Waimate in the Bay of Islands in December 1835, he saw a bracken-covered land that had once been kauri forest. ‘Some of the residents think that all this extensive open country originally was covered with forests, and that it has been cleared by fire. It is said, that by digging in the barest spots, lumps of the kind of resin which flows from the kauri pine are frequently found.’ 1
After they were cleared by Māori, the hills around Lake Tūtira in Hawke’s Bay were covered in bracken. In the 1870s, European settlers established pasture, but some places were difficult to keep clear of bracken.
The farmer and naturalist Herbert Guthrie-Smith settled at Tūtira. From 1893 to 1940, he observed plant succession on a slope burnt by settlers. After a burn-off, mānuka seedlings and bracken sprouted. For seven years both grew up equally, then mānuka overtopped bracken and shaded it out, growing into a tall, spindly thicket. After some years, this opened up. Some trees were killed by insects and wind, and broadleaved shrubs and tree ferns grew up in the gaps.
The site was resurveyed in the 1980s. A few old kānuka (Kunzea ericoides) trees remained – so kānuka must have been growing along with the mānuka, although Guthrie-Smith did not mention it. Saplings and young forest trees were growing, including kōwhai, rewarewa, tōtara, miro, kahikatea, karaka, tawa and tītoki. In less than a century, a young forest (similar to the nearby mature forest) had grown up.
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.
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.
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.
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.
Southern beech (Nothofagus species) forests grow in harsh environments with steep slopes and shallow soils. Every decade or so vast swathes of beech forest are destroyed by a catastrophic event such as a storm or landslide. Beech species are adapted to cope with disturbance. The seeds and seedlings need light, and grow rapidly on open sites. Beech acts like a pioneer species. It starts the return back to mature beech forest, perpetuating itself.
Beech flowers and produces seed intermittently. Some years no beech seed, or very little, is produced. If a catastrophe occurs in a seedless year, the beech forest may fail to regenerate – instead, another type of forest will develop.
On 2 February 1936 the North Island was hit by New Zealand’s most destructive storm of the 20th century. It laid waste to hectares of beech forest in the western Tararua Range, uprooting trees and snapping trunks. Sixty years later, broken tree trunks were still visible among the regenerating forest.
If mānuka and bracken fern are growing near a place where beech forest has been destroyed, they may colonise the area instead of beech. Once dense mānuka or bracken has become established, beech will not grow, as its seedlings do not thrive in shade. Instead, kāmahi is usually the first tree. One of New Zealand’s most widespread trees, kāmahi grows from lowland to subalpine regions in the forest’s understorey or its canopy. As it regenerates freely in shade, or in the small gaps left by fallen canopy trees, kāmahi can persist in an area for centuries.
In many mature stands of conifer–broadleaf forest, the largest trees are conifers, but there are few young conifer seedlings and saplings. For some years this puzzled and worried New Zealand’s forest scientists who talked of the ‘conifer regeneration gap’. What had caused the poor regeneration and what would happen to the forests if there were no juveniles to replace the adults?
Two early theories addressed the regeneration gap.
Modern scientists have discarded both theories. The answer is simpler.
It seems that like beech, conifer regeneration is stimulated by disturbance. When a fire or storm leaves an opening in a forest, seedling and sapling growth of many species is prolific, and one or more species grows up filling the opening. This is known as gap-phase regeneration and occurs in New Zealand’s main forest types: kauri, conifer–broadleaf and beech forests.
For long-lived species like kauri (1,000–2,000 years) or other conifers (600–1,200 years), some disturbance is likely to affect the forest within their life spans. As the forest gradually grows back, broadleaved trees may dominate for 200–500 years – but this is just part of the life span of kauri or rimu. As long as some conifer seedlings establish themselves in openings created when broadleaved trees die, kauri or conifer–broadleaf forest endures.
Clarkson, Bruce D., and others. Botany of Rotorua. Rotorua: Forest Research Institute, 1991.
McGlone, Matt S., and others. ‘An ecological and historical review of bracken (Pteridium esculentum) in New Zealand, and its cultural significance.’ New Zealand Journal of Ecology 29, no. 2 (2005): 165–184.
Wardle, Peter. Vegetation of New Zealand. Cambridge: Cambridge University Press, 1991.
Wilson, Hugh D. Hinewai: the journal of a New Zealand naturalist. Christchurch: Shoal Bay, 2002.