Pests in their sights

Culling deer, possums, goats, feral pigs and other invasive mammals could let established native forests recover to the point where they sucked in 15 per cent of New Zealand’s yearly greenhouse gas emissions, says a report from Forest & Bird.

The conservation advocacy group has used a report by Crown science agency Scion, as well as independent estimates of the numbers of feral mammals and other information, to estimate how much eradicating pests could benefit the climate.

More than a decade of monitoring at native forest plots scattered the length of the country shows New Zealand’s established forests are in equilibrium – sucking in roughly as much carbon dioxide as they release.

But while reports to government agencies often stress that forests are overall in balance, Forest & Bird argued looking at the national average was missing the point.

The natural forests inventory tracks hundreds of representative plots of forest. It’s the nearest to an official count of how much carbon the nation’s oldgrowth forests are gaining and losing.

It shows that ka¯ mahi-podocarp forest, which covers vast swathes of Westland, is losing significant amounts of carbon. That roughly balances out the climate benefit from other types of forest that are gaining carbon, such as regenerating ka¯ nuka forest.

Forest & Bird believes the carbon losses from ka¯mahipodocarp forest are probably caused by invasive pests. Its report, released today, said that, as well as being an excellent storer of carbon, ka¯ mahi (a common forest tree) is attractive and easy to eat for possums, deer and goats.

While each deer, possum, or other mammal eats only a tiny proportion of a forest’s foliage, the total becomes significant once spread over vast forests, the report said.

Warm-blooded browsers also eat seedlings and kill young trees, which doesn’t remove much carbon immediately, but can disrupt the next generation of trees from growing in a mature forest.

The forest inventory is out of date – covering the years from 2002-14 – however, the results were only recently published. Forest & Bird’s Kevin Hackwell said there was no reason to think carbon losses from ka¯ mahi-podocarp forest had stopped since 2014, and, if extrapolated over two decades, would add up to almost as much as the country’s annual emissions.

Per year, counting foliage eaten directly and other impacts, the group estimated 8.4 million tonnes of carbon dioxide could be saved by culling pests to the lowest possible level.

The group also tallied the methane belched by deer, goats and other feral mammals, not including pigs, which don’t burp much of the greenhouse gas – concluding methane from pests was also a significant contribution to warming. However, the group’s report acknowledged its estimates of feral mammal numbers were uncertain, since many of the studies are decades old.

It called for a doubling of the number of forest plots surveyed

by the official monitoring programme, to better capture gains and losses.

Other researchers are increasingly trying to get a handle on how carbon sequestration varies between forests, and why – including iwi-led initiatives.

For example, Niwa research recently found that forest in Fiordland, which is predominantly covered in beech forest, was sucking in more carbon dioxide than expected.

The East Coast’s Rauku¯ mara Pae Maunga project is a conservation partnership programme between the Crown, Nga¯ ti Porou and Te Wha¯ nau-a-Apanui to restore the 250,000ha Rauku¯ mara forest with intensive pest control and monitoring.

Environmental and indigenous rights leader Tina Ngata has previously said that partnering with CarbonWatch NZ (a Niwaled collaborative research programme) will allow iwi to track the impacts of pest control on carbon uptake in the Rauku¯ mara, a forest ravaged by possums and other treemunching introduced species.

About one-third of the country is forested, much of it on the South Island’s West Coast, and New Zealand’s indigenous forests store an astonishing estimated 1.7 billion tonnes of carbon (or about 6.5 billion tonnes of carbon dioxide) – the equivalent of 80 years of New Zealand’s greenhouse gas emissions, or 13 per cent of the world’s annual greenhouse gas footprint.

However, changes in carbon storage by pre-1990 forests aren’t counted as part of New Zealand’s targets under the international Paris Agreement, meaning any carbon losses don’t count against the country’s progress as reported to the United Nations. Methane emissions from feral animals are not tallied in the country’s official estimates of its climate impact, either.

Globally, it has been estimated that saving old forests from further damage and degradation is even more crucial to halting climate change than planting new trees. That is because new trees will take centuries to reach the same level of carbon storage.

Typically, New Zealand’s mature indigenous forests hold twice as much carbon per hectare as tree plantations, and much more too, than younger regenerating native forests.

The Forest & Bird report is not the first time the climate benefits of culling pests have been studied. In 2015, the Department of Conservation got scientists at Landcare Research to estimate how much carbon could be gained by better pest control in the vast conservation estate.

The researchers found conservation land could store almost 700 million tonnes of additional carbon dioxide – equivalent to almost nine years’ worth of New Zealand’s emissions – through a mix of reforesting cleared land, helping shrublands regenerate, and letting existing mature forests recover from animal over-browsing.