Pohutukawa spread by humans

The magnificent red blaze of pohutukawa flowers seen on many offshore islands may be far more a result of human intervention than widely thought.

A study on the Poor Knights Islands has found that before humans arrived the island vegetation was more like that in northern North Island lowland forests.

Dominant species before human settlement included nikau palms and conifers, particularly rimu.

Researchers said that returning the islands to that state, if considered desirable, could be difficult, partly because of the many seabirds now living there.

The researchers – Janet Wilmshurst, of Landcare Research, and colleagues from Landcare, Otago University, and Ngatiwai – based their findings on an 85-centimetre soil core. It was taken from the head of an ephemeral stream – one that exists only intermittently – on the 163-hectare Tawhiti Rahi, the largest island in the Poor Knights group.

The core was divided into three zones: prehuman from about AD230 to 1280, a Maori gardening period from about 1280 to 1823, and the period from 1823 to the present.

Volcanic ash found in the soil core early in the Maori gardening period was from the eruption of Mt Tarawera about 1314, the largest eruption in this country in the past 1000 years.

Up to 40 per cent of pollen in core samples from prehuman times came from the palm Rhopalostylis, probably the nikau palm R. sapida. Podocarps accounted for up to 30 per cent of the pollen, including rimu which produced up to 15 per cent. After human settlement both nikau palms and rimu disappeared from the island.

In the Maori gardening period, repeated human-lit fires caused rapid deforestation. Pollen from tall forest trees decreased by 50 per cent, charcoal particles were relatively more common and abundant, and pollen from bracken fern and grasses appeared.

After an intertribal massacre of Maori on the Poor Knights in 1823, a tapu was declared, prohibiting habitation and gardening on the islands, the study said.

Rather than the prehuman podocarp-dominated forest returning, a forest dominated by angiosperms (flowering plants) took over, with pohutukawa providing most of the pollen in the shallowest segments of the core sample.

"The current angiosperm-dominated forests are a stark contrast to prehuman forests," the study said.

"The abundance of palms and the presence of podocarps in the prehuman forests on Tawhiti Rahi were unexpected given the long-held belief that coastal forests dominated by angiosperm trees are climax communities on northern New Zealand islands."

The large list of species represented in the pollen record from the prehuman era of Tawhiti Rahi – at least 60 naturally related taxonomic groups – could provide information for the restoration of deforested islands, the study said.

Restoring populations of nikau palms could be a realistic goal. The palm was common on fertile soils in warmer coastal regions and might succeed on islands where seabird colonies increased soil fertility, although it might be limited by moisture availability.

"However, the currently large populations of seabirds on Tawhiti Rahi, which burrow the soils and elevate soil nitrogen and phosphorus concentrations may have favoured angiosperm dominance over conifers since reforestation after AD1823," the study said.

"It is possible that current high densities of Buller's shearwater, which have increased substantially during the 20th century, are a relatively recent feature of Tawhiti Rahi following the transformation of the vegetation.

"Since deforestation, the development of a shorter and simply statured canopy and accumulation of a deeper organic soil may have provided more habitat for the burrowing seabirds than was present in the tall forests of the prehuman period. If this is the case, restoring conifers on islands with substantial colonies of burrowing seabirds may be difficult."