A magnified bracken spore

A magnified bracken spore

Because each plant species has unique pollen grains or spores, by examining New Zealand’s wetland deposits researchers such as Janet Wilmshurst can tell what types of plants grew in a particular area at a particular time. 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 have been destroyed through deliberate burning, and that bracken has occupied the bare ground.