Because each plant species has unique pollen grains or spores, by examining them in wetland peat deposits researchers can tell what types of plants grew there at a particular time. Fossil pollen or spores have been used to date Polynesian settlement of New Zealand. The coincidence of a sudden, very marked and sustained decline in pollen from trees with a sudden increase in bracken spores (a plant which colonises open ground) indicates that forests had been destroyed by deliberate burning, and that bracken occupied the bare ground.