The pōhutukawa are in full bloom and making headlines.

A leading conservationist has described the flowering of pōhutukawa in Northland as the most "spectacular" he's seen in 20 years.

And he's not just looking through rose-tinted glasses.

Botanists call this a "mast seeding" event.

It begins with prolific flowering followed by the production of huge amounts of seeds.

It's likely happening across a range of species, but the crimson flowers on large, coastal pōhutukawa are particularly noticeable.

Known as New Zealand's Christmas tree, pōhutukawa occur naturally in the upper half of the North Island, although can be found growing nationwide.

Both pōhutukawa and rātā belong to the genus Metrosideros and hold a prominent place in Māori mythology.

University of Canterbury plant ecologist professor Dave Kelly, has researched mast seeding since the 1980s and is considered a global expert on the subject.

What is it?

Many New Zealand plant species show this variable seeding but it's commonly associated with beech trees, Professor Dave Kelly says.

When beech trees seed, they produce a bounty of food for native birds - and the mammals that threaten them.

In 2019, beech trees experienced a "mega-mast", resulting in more food for insects, birds, and lizards, but also mice, rats, and stoats.

Conservationists have known about this phenomenon for decades and ramp up pest control measures in response.

Mast seeding happens across tree species in many parts of the world, but it seems to be more common in New Zealand, Dave says.

It not only occurs among trees but also harakeke or flax and snow tussocks found in the Southern Alps.

There are sites for trapping the seeds of common tree species, to track mast events nationwide.

But there's no data on pōhutukawa or rātā, for the simple reason the seeds are too small to collect and count.

"People just notice when it's a good year," Dave says.

The synchronised flowering often against a coastal backdrop is hard to miss.

Is it rare?

It depends on the species, Dave says.

Cabbage trees, for example, "tend to be on, off, on off, and occasionally they miss a year and get out of step".

Some horticultural plants such as avocados have a good year, followed by a not-so-good year.

Most native plants have three or four years of low flowering, followed by a big year.

Causes

Unlike some folklore suggests, the plants aren't signaling a good summer to come.

Rather, mast seeding is in response to previous weather cues.

"[Mast events] are driven by weather - retrospectively," Dave says.

The obvious cue is rain, right?

But Dave says synchrony is important for mast events and rain is often too patchy to affect plants equally.

Temperature is more uniformly experienced.

"Typically, you'll get a big seed year after a hot summer, which was typically after a cold summer."

If it sounds complicated, well, it is.

With some species, such as the European beech in England and Scotland, studies have shown mast seeding is triggered by temperature.

With others, such as the snow tussock, the trigger is the difference in temperature, year-on-year.

Climate change is causing issues for those in the former category.

"It's a case-by-case basis, figuring out what will be upset by a half-degree increase in temperature caused by climate change."

Consequences

So, what will a pōhutukawa mast seeding event mean for New Zealand?

Dave says it's unlikely to cause a pest outbreak, thanks again to the tiny seeds: "They're not great food for rodents."

Though if other species with bigger seeds are also experiencing mast events, that could have an impact.

"The birds and bees will be happy," Dave says.

And the native, short-tailed bat, he adds, found on Te Hauturu-ō-Toi or Little Barrier Island in the Auckland region.

"They go crazy on pōhutukawa."

- Katie Kenny/ RNZ