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Forest loss in NZ reveals fire prevention ploys.

By Dave Armstrong - 08 Nov 2014 14:46:0 GMT
Forest loss in NZ reveals fire prevention ploys.

In Whanganui National Park, thousands of endangered brown kiwi (here) and blue duck survive in the ancient podocarp forests, this time in North Island ; Kiwi image; Credit: © Shutterstock

Our NZ cousins, the Maoris, landed 750 years ago and did what humans always do. They trashed the forest. Considering what the British did later, with sheep, stoats and cats, the damage was less, but still destroyed many species of flora and fauna, leaving half of the 13th century native forest for the later colonisers to destroy in the 18th century. This would have been evergreen closed-canopy broadleaf forest dominated by podocarps.

The Initial Burning Period of the Maoris can be checked in pollen and charcoal remains and found to have taken place in mere decades. One vulnerable site was found that was dry while a wetter site was thought to be less liable to be affected. David B McWethy with others from Montana State University joined with several colleagues from New Zealand’s Landcare Research, and the University of Auckland to investigate this unique example of anthropogenic colonisation effect. They publish their paper as- A High-Resolution Chronology of Rapid Forest Transitions following Polynesian Arrival in New Zealand.

The particular interest is in how small groups of transient humans can be so effective in this environment that is made so vulnerable to fire. Part of the answer seems to have been the fire-prone open shrubland vegetation appeared in place of the ancient podocarp forest, just as it appears in some present-day deforestation situations.

Land use has shifted recently to accommodate frequent large fires. To fight this regular and very wide occurrence, study of forest loss could lead to solutions involving a negative feedback to fire disturbance. We know that scrub, grassland and shrubby vegetation encourages fire. The encouragement of resistance to fire would save lives, livings, habitat and ancient woods.

The 3 hectare Lake Kirkpatrick (dry, lowland) and 1.5 hectare Dukes Tarn (wetter, high elevation) in South Island represent two vulnerabilities to fire. AMS (accelerated mass spectrometry) results from the sites were able to give accurate radiocarbon dates from the lake sediment cores. 22 dates were obtained from the 2 sites, using invaluable charcoal and terrestrial plant macrofossils. Lake Kirkpatrick still has some stands of native silver beech while the Tarn has patches of black beech. The first human-set fires rapidly created forest transitions. It took only 17 years at Lake Kirkpatrick and 48 years at the Tarn for a significant decline in native plants to take place. Every 50-100 years after that, more fires devastated the flora until 1600, when the activity decreased.

Southern beech, Nothofagus menziessi, Prumnopitys and podocarps declined from 99% to 47% of the pollen at the first fires, with ferns and grasses increasing, then decreasing around 1642. There was some variability before 1792, after which native trees’ pollen reduced to <30% in the 20th century with the advent of exotic pines, docks and dandelions!

All of this may seem inconsequential, but the podocarp forest had a wealth of New Zealand endemics within it. We know what happened to the giant moas, but the interest lies in smaller flora and fauna, some of which may have never been viewed by human eyes.

Without making the topic too exciting, simply consider how many flightless parrots would be left if human and other animal influences had never taken such a great toll? The Maori gardens created insignificant clearings compared to the later sheep farming, especially in the sparsely-inhabited South Island. Population levels have not played as conclusive a part as many models often predict in fire vulnerability studies.

What is clear is that high mature canopy cover makes fires die because the understory lacks sufficient fuel and the moisture levels are very high. Flammability can therefore decrease with the age of the forest. Opposing fire seems linked to maintaining forest without small ignition events, allowing moisture levels to increase and kindling to decrease.

The natural tipping pint of old forests in many countries is therefore likely to be approached in the near future, given the land-use changes now taking place. If fire had been removed from the New Zealand equation, perhaps the podocarp forest would have renewed, as many tropical forests have, in the past. Because the disturbed areas continued for a century or so, more fires then maintained their integrity, preventing any resurgence of native flora for 750 years. The warning is there for rapid transitions unless some native forest is retained or rehabilitated.

Simply as an illustration of how ecology can affect flammability of forest, here is an older Canadian example, affected by pine beetle infestation.