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.
The former forest
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
Forest succession at Tūtira
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.