Continuous forest cover
Before people arrived, more than 80% of New Zealand was covered in dense forest. Pollen and charcoal records from over 150 lake, swamp or peat bog sediment core samples give a clear picture of the country’s vegetation and fire history since the end of the last ice age, about 14,000 years ago.
Only small, occasional fires occurred in the forests. These are shown by minor, temporary declines in pollen from tall forest and shrub species, along with increases in charcoal, and in spores and pollen from bracken, grasses and other pioneer species (the first plants to grow in a cleared area).
Some scientists believe that these minor bracken and charcoal peaks from 100 CE are evidence of human-lit fires, and that there were people in New Zealand before the 13th century. However there are equally convincing natural explanations, including lightning strike after droughts, and volcanic eruptions. There is no other supporting archaeological evidence for human occupation.
Deforestation by fire
Sediment records show a huge increase in charcoal and bracken spores around the 13th and 14th centuries. At the same time, there was a massive decline in pollen from forest trees, marking a striking and devastating change: up to 40% of the forest was burnt within 200 years of Māori settling in New Zealand. Radiocarbon dating shows that deforestation began at the same time throughout the country. There are also many archaeological sites dating from this time.
The forests in the drier eastern regions burnt rapidly, and were cleared quickly and completely. In wetter or mountainous areas, clearance occurred later and was more piecemeal.
In most sites, charcoal continues to appear after the first forest clearance, suggesting that Māori used fire to stop tall forest and scrub from regenerating.
Purpose of burning
Reasons for clearing the forest included opening up the landscape to make it more habitable. Crops could be grown, and bracken fern (Pteridium esculentum) was encouraged for its edible starchy rhizomes. Burning also kept tracks clear and made travel easier. The first forested areas to be cleared must have been easy to burn, particularly during or after a drought.
Swamp and lake sediments
Pollen records from swamp and lake sediments show major changes in the plants that grew around wetland edges. Raupō (bulrush, Typha orientalis) became more abundant around swamps, because of increased nutrients and water flow. Māori used the pollen from raupō flower spikes to make cakes, so they may have encouraged its growth by burning lake-edge vegetation.
Erosion did not always follow deforestation, even in areas that are now very erosion-prone. The dense bracken with its network of rhizomes, and the tree stumps that remained after burning, would have protected against the landslides and soil erosion caused by heavy rain.