The New Zealand bracken fern rhizome, Pteridium esculentum (G.Forst): a toxic food plant of pre-European Māori

Abstract

The two species of bracken fern, Pteridium esculentum and Pteridium aquilinum, are well known to produce neoplastic lesions and thiamine deficiency when consumed by mammals, with severe consequences to health. New Zealand Pre-European Māori are known to have consumed rhizomes of P. esculentum as food with little or no recorded consequences to health. Processing methods by Māori prior to consumption may have helped to detoxify this food. We carried out LDH toxicity tests on rhizomes that had been pre-processed before simulated digestion to test this possibility. We tested rhizomes harvested each month of the year, different components of the rhizome, both raw and roasted rhizomes, rhizomes stored for up to 12 months, and rhizomes leached for up to 24 hours. All specimens remained equally toxic within experimental error. We carried out a detailed analysis of nutrients in bracken rhizome and compared this with kūmara, Ipomoea batatas, another important food plant for pre-European Māori, and found that bracken rhizome has c. 70% of the caloric value of kūmara. A cost/benefit analysis of the two plants suggested that the reward for effort is greatest for kūmara by a modest amount. Analysis of historic ethnographic observations relating to bracken rhizome from AD 1769 to the 1840s provides complex and contradictory evidence of the role of bracken rhizome in the Māori economic system. Although there is clear evidence that Māori greatly favoured chewing rhizomes, this fondness may result from the presence of one or more plant secondary metabolites (PSM), such as ecdysone, which are known to be addictive. Our analysis of the evidence favours the plant being essentially a famine food, filling in the period between planting and harvest of kūmara, known as the ‘hungry gap’ between October and April in the southern hemisphere. However, it would also have provided an important source of food for travellers, as fern-lands are widespread. Our analysis of archaeological information did not produce unequivocal direct evidence of bracken rhizome consumption. However, the presence of extreme tooth wear and a unique pattern of first molar dislocation, attributed to the use of teeth to strip starch from rhizomes, has been shown to be present at all periods of New Zealand prehistory. This is contrary to the finding of some other researchers.