New monitoring work has revealed startling proof of possum damage to native mountain tōtara and kaikawaka/mountain cedar trees in a Mid Canterbury forest.
The Department of Conservation used aerial photography to measure canopy die back in the Wilberforce and Mathias valleys last summer. The monitoring has revealed the Wilberforce forest is now in very poor condition, with roughly 50 per cent of the vulnerable tōtara and cedar trees impacted by possums.
Cedar and tōtara are highly palatable to possums, and their browsing causes canopy die off, limits forest regeneration and accelerates forest collapse. The upper Wilberforce and Mathias catchments contain a nationally significant cedar and tōtara forest type, which is rare in New Zealand.
DOC National Predator Control Programme Manager Peter Morton says an aerial pest control operation is being planned for this spring to reduce high possum numbers, as well as rats and stoats, and protect the forest from further damage.
"Seeing so many dead trees was a wakeup call. This really has become urgent action for us. If we don't control possums, these forests could collapse," he says.
Peter Morton says the Wilberforce is also a significant site for hunters, particularly for red deer trophy hunting, and DOC has engaged with hunting groups about the upcoming operation and agreed to the use of deer repellent.
"DOC is also supporting a community-led initiative to undertake ground-based predator control in part of the forest which will start soon.
"Stakeholders requested that operational timing avoid periods of grazing to minimise impacts on neighbouring farming activities, and avoid peak recreational use periods, including the deer hunting roar period, which we have accommodated."
Peter Morton says the last predator control operation in the Wilberforce area in 2018 was successful but possum and rodent numbers have slowly increased since.
"We want to ensure these rare forests don't disappear as these are nationally significant ecosystems found nowhere else in the world," he says.
DOC prioritises predator control to protect the most at-risk populations of native species, threatened forests and areas of high ecological value. The Wilberforce site is one of more than 60 forest sites throughout New Zealand protected through the National Predator Control Programme.
Most sites are returned to every 2-5 years depending on predator numbers and environmental conditions. Overall, DOC protects about 1.8 million ha or 20% of public conservation land on a sustained basis through predator control.
The Wilberforce/Mathias valley aerial operation using biodegradable 1080 cereal baits is planned for the first clear weather window between 14 September and 30 November.
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