By examining pollen it is possible to determine when past changes in vegetation patterns occurred. In New Zealand, scientists such as Janet Wilmshurst and Matt McGlone drill down and collect cores of sediment from peat bogs or lake beds, and analyse the pollen grains preserved in these cores. Sudden changes in the types of pollen in different sediment layers indicate forest clearance by fires lit by the earliest permanent settlers. Although fire episodes are apparent in New Zealand’s pollen records before 1250 AD, they represent only minor forest disturbance. Because they were so small, these earlier fires cannot be confidently attributed to human activities.
Pollen from some North Island sites (collected from beneath the Kaharoa ash layer) suggests that vegetation changes occurred as early as 1280. As this matches the earliest radiocarbon date from an archaeological site (Wairau Bar 1288–1300), some scientists think that settlers began burning the forest soon after they arrived. But there is not enough evidence to say that this burning, just prior to 1300, is definitely attributable to humans. Still, it is interesting to think that if Polynesians had arrived by this time, those on the coast would have been showered with a few centimetres of ash falls from the Kaharoa eruption in about 1314. The ash layer deposited from this eruption is known as the Kaharoa ash, and erupted from the Mt Tarawera massif.
Nearly all New Zealand pollen records, especially those from drier regions, show that by 1300 there was a massive change from predominantly forest species to bracken fern and scrub, with a corresponding increase in charcoal. These widespread vegetation changes were due to settlers burning the forests for kūmara (sweet potato) cultivation, to create living space and to encourage bracken fern growth. The starch-rich underground stems of bracken fern formed an important part of the settlers’ diet.