Beech forest seeding events - or masts - are known to drive explosions in pest numbers, adding extra pressure on New Zealand's native species. Photo / Brad Windust
Beech forest seeding events - or masts - are known to drive explosions in pest numbers, adding extra pressure on New Zealand's native species. Photo / Brad Windust
Another plague-making beech forest seeding event could be on the cards in parts of New Zealand next summer - with a warmer-than-usual March raising the odds.
The warning comes just two years after the largest beech mast in half a century prompted the Department of Conservation (DoC) to launch its biggest-ever pest control operation.
At the same time, scientists have suggested that climate change may be playing an increasing part in mast events, which produce extremely heavy flowering and seeding.
In beech trees, which make up much of New Zealand's wilderness forest, mast seeding kicked off when the average summer temperature was more than a degree higher than the average temperature of the preceding summer.
Long in the past, before pest predator species like rats and stoats arrived on our shores, mast seasons offered seed-eating birds like kea and kaka a veritable buffet to make up for those lean years in between.
Other insectivorous birds like robins and tomtits would have gorged on all of the insects also showing up for the feast.
But today, with enough predators lurking in our bush to lay waste to some 25 million birds each year, mast years spell even greater danger to those native species still hanging on.
The extra food only boosted rodent numbers – and in turn stoat numbers – with the plague turning on our native birds, bats, lizards and insects after the seed was gone.
DoC's latest climate modelling showed a beech mast was very likely in southern Fiordland next year - and could also occur in parts of North Otago, South Westland, North Canterbury and in the central North Island.
"After a couple of years with little beech seeding anywhere, it's looking like there will be a beech mast in a handful of places around the country in 2022," DoC principal scientist Graeme Elliott said.
"Although this past January and February weren't especially warm, March was unusually hot in some places, which increased the overall summer temperature."
Scientists have suggested that climate change may be playing an increasing part in beech mast events, which produce extremely heavy flowering and seeding. Photo / Supplied
As the temperature difference wasn't as great outside of southern Fiordland, there was less certainty full seeding would result in those areas, he said.
"We will be monitoring flowering next spring using satellite imagery, and then in summer sampling seed in some forests to confirm where a mast is occurring."
Rodents and stoats are also monitored, and predator control operations planned for priority conservation areas to protect our most vulnerable wildlife.
In 2019, DoC spent tens of millions of dollars beating back pest numbers with 1080 poison drops and ground trapping, in a sprawling operation covering some 908,000ha of wilderness.
That followed smaller, but still significant events, in 2017, 2016 and 2014.
This year's programme covered about 500,000ha of priority conservation areas, and DoC would be watching mast projections closely when planning next year's work.
There was emerging evidence that climate change was increasing the frequency of forest seeding.
One recent study, co-authored by Professor Dave Kelly of the University of Canterbury, suggested that reproduction in beech trees in Britain, specifically, was being negatively affected by climate-driven changes to seed production systems.
Kelly thought it interesting the prediction for next summer was based on a warm March, given earlier research had suggested temperatures over January and February had the most influence.
"DoC have been doing a lot of work refining these temperature prediction models, and based on that think a very warm March can set things off, which is certainly plausible," he said.
"Also the effect is quantitative, so a very hot March might do things that a warm March would not."
Over time, Kelly said, we were seeing more extreme temperatures.
"And as temperatures vary from year to year, we will gradually see all the possible combinations of cold months followed by hot - and find out for sure what the plants do."
In New Zealand, his own research had pointed to potential changes in plant responses from big temperature swings.
"For example, in my work at Mt Hutt on snow tussocks, in 2018 we had a huge temperature difference where January 2018 was more than 4C warmer than January 2017, which is far outside the normal range of a degree or two either way," he said.
"This gave us the big flowering year in 2019. All this means I expect that as the climate warms up, we'll see greater swings from year to year and this may drive more extreme seed events - but we don't know the details well yet."
More broadly, climate change was predicted to pose a wide range of problems to our already struggling biodiversity - and 2018's spring and summer brought just the kind of hotter, wetter weather we could expect from climate change.
Under these conditions, rats in the North Island rats were starting to behave more like they did on tropical islands – with longer breeding seasons, and bigger populations.
Meanwhile, in the South Island, invasive mice were climbing to higher altitudes than ever before, pushing above the treeline into alpine areas.
