Washington: A new research has indicated that the speed of early forest clearance following human colonisation of the South Island of New Zealand was a lot faster and intense than previously thought.
Charcoal recovered from lakebed sediment cores has shown that just a few large fires within 200 years of initial colonization destroyed much of the South Island`s lowland forest.
Dave McWethy and Cathy Whitlock from Montana State University led the international team that carried out the study.
Previous research by co-authors Matt McGlone and Janet Wilmshurst at Landcare Research in New Zealand showed that closed forests covered 85-90 percent of New Zealand prior to the arrival of Polynesians (Maori) 700-800 years ago, but by the time Europeans settled in the mid 19th century, grass and shrubs had replaced over 40 percent of the South Island`s forests.
The team reconstructed the environmental history of 16 small lakes in the South Island, New Zealand. They used pollen records to reconstruct past vegetation, charcoal fragments to document fires, and algae and midge remains to quantify changes in lake chemistry and soil erosion.
"The impacts of burning were more pronounced in drier eastern forests where fires were severe enough to clear vast tracts of forest and cause significant erosion of soils and nutrients. Because the initial Maori populations were small, we can only conclude that forests were highly vulnerable to burning," said McWethy.
Wilmshurst said archaeological evidence has suggested that successful cultivation of introduced food crops, such as kumara and taro, was only possible in warmer northern coastal areas and the starch-rich rhizomes of bracken fern, which replaced the burnt forests, provided an essential part of Maori diets in colder regions.
Newly derived records of past climate enabled the team to disprove the hypothesis that unusual climate conditions encouraged fire at around the time of Maori settlement.
"Our evidence suggests that human activity was the main cause of the fires, and that these fires were not related to any unusually dry or warm conditions at the time," said McGlone.
Before human arrival in New Zealand, fire was naturally rare in most forests, with lightning-started fires occurring perhaps only once every 1-2 thousand years.
This study has shown the extent to which a small number of settlers can transform a vast and topographically complex landscape through land-use change alone, and highlights how exceptionally vulnerable New Zealand forests were to fire in the past.
The findings were published in the journal Proceedings of the National Academy of Sciences.