Climate change is throwing beech forests out of synch – a new finding that could have big implications for pest-fuelling "mast seeds" in New Zealand.
A new study co-authored by a Kiwi scientist has shed more light on how the tree species is responding to a warming world, suggesting that its reproduction was being negatively affected by climate-driven changes to seed production systems.
It took researchers closer to answering the key question of how climate change will ultimately affect the frequency of big mast seed years – events that typically occur at irregular intervals, and which can bring about pest explosions.
Last year's so-called "mega-mast" proved the largest in half a century, and prompted the Department of Conservation to launch against a rat plague its biggest pest control operation in history.
The new study, drawing on nearly 40 years of data, focused on a European genus of beech found throughout Britain.
"At the start of the study, the beech trees gained major benefits from irregular big seed crops," said Professor Dave Kelly of the University of Canterbury.
"Pollination worked better, and losses to seed predators – a native moth – were kept low because the moth had little to eat in the famine years between big seed crops. Benefits like this are the evolutionary reasons that trees show mast seeding in the first place."
But over the decades of the study, seed production by the beech trees steadily increased due to the warming climate.
That should have been good news for the plant, but instead turned out to only be good for the seed-eating moth.
The data eventually illustrated how the characteristic "boom and bust" seed production patterns in UK beech trees disappeared as the climate warmed, driving an increase in moth populations.
"As a result, we have seen a dramatic increase in the percentage of the seeds that are eaten by the moth larvae – up from around one per cent in the 1980s to 40 per cent in recent years," said lead author Dr Michał Bogdziewicz, of Poland's Adam Mickiewicz University.
The net effect was that complex interactions between species nullified any benefit to forests – and any benefit the trees might have gained from this increased reproductive effort has been almost entirely offset by higher seed consumption by the moth larvae.
What did that mean for New Zealand and its biodiversity-rich beech forests?
"We need to understand not only how the plants and the animals respond to warmer climates, but also how they interact," Kelly said.
"For example, ship rats in New Zealand have been spreading to higher altitudes in recent decades, which will alter local responses to southern beech mast seed crops.
"Overall, seeding strategies which worked well for forest trees in the past may increasingly fail under new climate conditions."
Previously, Kelly and fellow scientists have worked off two hypotheses.
One – dubbed the delta-T model, or DT model - was that plants would respond to the temperature difference between successive summers.
The other, called T1, speculated that plants might respond to the previous year's summer temperatures, plus having high resources last summer, which would more likely be the case if two years ago was cold so that one year ago had low flowering.
"These two models (DT and T1) are both broadly consistent with the observational data, but make different predictions under climate change," Kelly said.
Work that he and his colleagues have carried out into snow tussock suggested that gradual increases in overall mean temperature under climate change would not drive snow tussocks to try to mast every year.
"What drives masting is a relatively warm summer coming after a relatively cold summer," he said.
"So, masting will become more frequent if summer temperatures become more variable across years under climate change, which is possible."
Yet pest outbreaks in New Zealand weren't driven by tussock seed crops, he added, but those from beech trees.
"The two genera are quite highly synchronous across years, but we have not been able to run these transplant experiments in beech trees, so cannot verify whether they also use the DT mechanism," he said.
"However, I guess that would still be the most plausible working hypothesis at this stage."
Nevertheless, climate change was predicted to pose a wide range of problems to our already struggling biodiversity.
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.
There, they threatened some of the last strongholds of endangered native species, such as rock wrens, lizards, and invertebrates.
Whereas pest control could effectively manage rats and stoats in these areas, more work was needed to develop landscape control tools for mice.