Farm Forestry New ZealandThe Pukaki wilding fire
Nick Ledgard, New Zealand Tree Grower November 2020.
The main spreading tree species on the western shores of Lake Pukaki is the worst one, contorta pine, although there is quite a bit of Douglas-fir and larch. Other spreading conifers, such as Scots pine, Corsican pine, ponderosa pine and radiata pine are also present in small numbers.
The original planted seed source was in shelterbelts and small woodlots alongside the Dusky, Mackenzie Peaks and Pukaki Downs homesteads, all submerged when the lake was raised in the early 1970s. A later seed source was in among amenity stands planted by the Ministry of Works alongside the lake after the lake was raised. At this time, the spread threat of contorta pine was well recognised and it was not supposed to be planted, but unfortunately some was present in a seeds recorded as ‘mixed conifers’.
In the 1980s, apart from some small stands close to the lakeshore, wilding trees were not very apparent to anyone other than those deliberately looking for them such as myself. But by the mid-1990s, major battles against wilding spread were starting to be waged. Since then, the main affected station, Pukaki Downs, has put in a significant amount of effort and hundreds of thousands of dollars trying to manage the spread.
Using fire for management
Over the years, I have written two reports about wilding spread and its management in the Pukaki area, particularly on Pukaki Downs station. As someone who started his working life as a Forest Service trainee, I am quite familiar with the use of fire as a means of removing woody species. Consequently, the reports mentioned fire as a potentially very effective control tool but also noted that ‘the resource consent procedure required to carry out a burn could well be onerous’. Indeed it was, and still is, so even though small burns have been used in research trials, no deliberate operational fire has been attempted.
Needless to say, my ears pricked up when I heard of the recent accidental fire on the western shores of Lake Pukaki. Even though the idea of ‘let it go’ crossed my mind, it was only a fleeting thought as there were
important farm buildings and infrastructure to protect.
All the same, I would have hoped that with all the funding, training and research which goes into fire fighting, a strategic plan could have been developed which aimed at saving what needs saving, while letting the unwanted such as wildings be devoured by the flames.
I feel that these days we have become almost paranoid about fires and tend to take a rather blinkered approach to its potential use and control. The Forest Service would use good all-round fire management skills – these days we only seem to know how to put fires them out.
Avoiding accidental fires
A few months ago, the NZ Wilding Management Group had a field-day at the property of one of our North Canterbury farm forestry members Hamish Roxburgh, who farms at the western end of the Amuri Range, near Hanmer. For generations, the Roxburghs have used fire to control unwanted vegetation, such as rank grass and woody species including wildings.
Hamish showed us how this can be done with minimal loss of important biodiversity, mainly by proper preparation and seasonal timing. If undertaken when soils are cool and damp but the above-ground vegetation is dry, the likes of geckos, skinks and ground invertebrates can escape the heat by retreating underground and in among stones. Hamish has seen geckos out and about within days of his fires crossing their habitat.
Therefore I support well managed burns to control unwanted vegetation, but I definitely do not support accidental fires, due to the variable range of effects they have on vegetation succession making the overall situation more difficult to manage. Having said that, there is one simple follow-up action you can take which significantly influences the vegetation succession following fire. That is sowing a herbaceous cover – usually grasses and legumes – which establish quickly and seal off the ground surface from slower establishing woody species such as wilding trees.
Such sowing was done following accidental wilding fires at Mt Cook and Flock Hill Stations in 2008 and 2015 respectively. At both locations, the incidence of wilding reversion was virtually zero at Mt Cook or minimal at Flock Hill. The photographs show the Scion trial site at Mt Cook. The first was taken in July 2008, six months after the accidental fire. Below is the same site 16 months later in November 2009. The vigour of grasses and legumes completely eliminated any chance of woody species establishing.
Starting in the grass
One final comment on forests and fires. We frequently hear, especially from the anti-forestry lobby, that forests increase fire risk. My fire research colleagues point out that this is not strictly correct, as fire risk has two main components – fire ignition and fire intensity.
Fire ignition mainly occurs in herbaceous cover, particularly grasses, but fire intensity is higher in forests due to the greater fuel loading. Most forest fires are started by grasses being ignited first – I am fairly sure
this was the case at Pukaki.
Nick Ledgard is a member of the North Canterbury branch.
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