Sometimes large areas flower and seed at the same time as occurred in 2014 and is happening again this year.
This produces masses of seed, which feeds the mice and rats.
The well-fed rodents breed through the winter when they normally wouldn't and numbers rise exponentially.
The stoats eat the rodents and produce up to a dozen kits -- more than usual -- the following spring.
This is how beech seeding triggers a rodent and stoat plague and it's bad news for our native birds, bats and giant land snails.
By spring the seed is gone -- eaten or germinated -- and the rats turn to prey on our native birds when they are nesting and most vulnerable.
Young stoats emerge from their dens in early summer and also kill birds and chicks.
Both predators will also raid bat roosts where the baby bats are being raised.
Before rodents and stoats arrived in New Zealand, forest seeding was a boom time for the birds.
Flowers and seeds provided food for insects and birds, which then bred prolifically.
Today, when the beech forest seeds, our native bird and bat populations take a hit, with the most at-risk pushed into further decline.
Q. Conservation Minister Maggie Barry has described the more than 30 million rats as a plague of a Biblical proportion. Is it true this number is far more than we'd encounter on average?
A. We don't know exactly how many rats there are after beech seeding and DoC is doing some research into this.
But scientists estimate they could number in the ten of millions across our beech forests after a big seeding event.
Rats can breed every two months and produce six or more young each time.
So rat numbers can increase 20-fold over eight months.
Beech forests generally don't have a lot of rats unless they are seeding.
DoC uses tracking tunnels to measure rat levels.
In many forests the normal tracking rate for rats is less than five per cent.
Yet after seeding, this can rise to above 80 per cent by early summer.
Generally our forest birds are at serious risk when tracking levels get beyond 20 per cent and this triggers the need for pest control.
Q. The target areas for this operation will be Fiordland, Otago, South Westland, North Canterbury, Kahurangi, the lower North Island, Taranaki and Tongariro. Why is that and what particular species are under threat?
A. Battle for our Birds targets pest control to the most at-risk populations of native species.
Our target species are kiwi, kaka, kea, whio/blue duck, mohua/yellowhead, kakaraki/orange-fronted parakeet, rock wren, long and short-tailed bats and giant snails.
These areas are home to the remaining populations of these endangered species.
Most areas are beech forest and a number have well-established pest control programmes in place.
These birds, bats and snails are all at risk of significant decline if predators are not controlled.
We could lose some populations of these species from some areas
For example, mohua disappeared from the Marlborough Sounds after the beech mast in 2000.
Q. How and when exactly will these areas be targeted? And how much of the operation will likely consist of 1080 aerial drops as opposed to ground operations?
A. We haven't yet confirmed detailed sites for this year's Battle for our Birds pest control programme as this depends on rodent levels, which we are still monitoring.
But planning is underway to treat up to 800,000ha of priority conservation areas with 1080 applied by air.
This method is proven to be the safest and most effective means of knocking down rats, stoats and possums over large areas of rugged terrain.
This is supported by ground-based pest control methods.
A combination of stoat traps and 1080 treatment is used where there are especially at-risk populations of species such as orange-fronted parakeets, mohua and bats.
Poison bait stations are used in some places where there is road access but is much more expensive than aerial application.
At some sites traps alone are used to control stoats such as to protect takahe in the Murchison Mountains in Fiordland.
Q. In its scope and size, how will this operation compare with the other big efforts that have come with the Battle for our Birds?
A. In 2014 DoC launched Battle for our Birds and successfully carried out aerial 1080 pest control over about 690,000ha of priority conservation areas.
The bulk of this -- 600,000ha -- was in South Island beech forests aimed at knocking down rat and stoat plagues.
The programme being planned for this year is slightly larger in scale at 800,000 ha.
Q. How successful have the previous operations been in knocking back pest populations?
A. Field monitoring showed that DoC's large-scale pest control programme in 2014 successfully knocked back rats and stoats and helped protect our most at-risk native species over that breeding season and beyond.
Research showed that mohua, rock wren, kea, robin and rifleman all had better nesting success and raised more young in areas treated with 1080 than in areas without.
For kea, rock wren and rifleman, improved nesting was measured over the second summer after pest control, showing ongoing benefit.
Both species of bat did well and bat numbers increased. Past monitoring has shown significant benefits for kiwi and whio/blue duck.
Q. We've already seen one large masting season since the "battle" kicked off in 2014 and now there's another. Are the frequency of these events likely to increase?
A. We don't know if forest masting will increase due to climate change.
Beech seeding appears to be triggered by the difference in summer temperatures between any two years.
If average temperatures increase each year, we may therefore see more frequent seeding.
If weather becomes more variable, as climate scientists suggest, then there may also be a higher chance of a summer being significantly warmer or cooler than the one before it. This might also increase the frequency of seeding events.
Q. The operation will draw renewed criticism by opponents of 1080, who argue the poison is ineffective and also kills native wildlife. What is your response?
A. Our research shows that 1080 is very effective at killing rodents and stoats and losses of native species are very small.
In all cases where monitored birds such as kea have died, the gains to the species from reduced predation more than outweighs the losses.
The greatest threat to our native species is from introduced pests.
Most of the endangered native species we are trying to protect are not susceptible to 1080 poisoning at all.
This includes kiwi, mohua, orange-fronted parakeets, bats, kaka and whio/blue duck.
Our research shows that no monitored robin, rifleman or weka died during our 2014 pest control programme.
