Beech mega-masts and predator plagues – Expert Q&A

When New Zealand’s beech trees seed, they produce a bounty of food for native birds – and the mammals that feed on them.

These seeding events – called masts – don’t happen every year, but many species rely on them for reproduction.

This year has been called a mega-mast, as more than half of all beech forests are seeding across New Zealand. The amount of seed and fruit available has been boosted even further by the simultaneous flowering of rimu and tussock.

The SMC asked experts to comment on what stage of the masting cycle we’re in now, and how the mega mast will affect native bird and predator numbers over summer. Feel free to use these comments in your reporting. 

Associate Professor James Russell, University of Auckland, comments:

What stage of the current masting cycle are we in now? How is this likely to progress over summer?

“We are in the summer following the mast seeding event, when the rodents have survived winter with abundant food and not died off, continuing to increase in population size. Now with the mast seeding event over, the pests are beginning to starve and they are so desperate for food they are coming out in the day and turning their attention back to native birds and reptiles, just as those native species begin their critical breeding cycles.”

What are the flow-on effects from a mast year?

“Mast seeding creates a trophic fountain in the ecosystem – seeds and fruits feed mice and rats which in turn feed stoats. Not only does this take away food from native birds and reptiles, but when the mast seeding ends, the starving rodents and stoats switch their diet on to the birds and reptiles. Thus our native fauna are doubly impacted by mast seeding both losing food and gaining predators.”

What does this mean for predator control? What can we do during mast years to protect native species? How does this differ from non-mast years?

“When pest numbers are highest they cause the most damage, and they are high everywhere following mast years. Predator control needs to target the pests before the masting fruit and seed runs out and they switch to native birds and reptiles. In mast seeding years, this means undertaking more predator control than normal over winter and spring. Even better is the cutting-edge research being undertaken by groups such as Zero Invasive Predators to entirely eradicate rodent populations from areas of the mainland, leaving no rats to irrupt, and killing off all the stoats in the area at the same time.”

General comment

“Anyone who knows the New Zealand forest can see something is wrong at the moment and more 1080 is required. Mega-mast seeding years like this have historically only happened once every decade or two. In just one week around Kahurangi National Park in December 2019, I saw dozens of live mice, live rats, and live stoats chasing live mice and rats in full daylight. Our native birds and reptiles stand no chance in the face of such elevated predator numbers.”

Declared conflict of interest: James Russell is a scientific advisor to Zero Invasive Predators, Predator Free New Zealand, and from time to time undertakes consultancy work for the Department of Conservation.

Dr Adrian Monks, Ecology Researcher, Manaaki Whenua – Landcare Research, comments:

What stage of the current masting cycle are we in now? How is this likely to progress over summer?

“The current masting cycle started with the appearance of flowers in October-November of 2018, with fully developed seed appearing from late January 2019 onwards. The beech seed all fell from the trees by August 2019.

“By the end of November 2019, most of the viable seed will have germinated and become unavailable as food for rats and mice. Hence breeding rates will decline strongly as other foods become scarcer. Rats and mice are still increasing but have almost peaked.

“Work we are doing at Lake Alabaster in Fiordland, led by Jo Carpenter, has measured some of the highest rat densities recorded anywhere on the New Zealand mainland (c. 12 per ha) during this current mast cycle.

“Rodent numbers will plateau over summer, before declining toward winter. Over this period of decline, rodents mainly feed on fruit and seed from other forest species and some invertebrates. Rats also eat mice. Native birds and lizards are eaten opportunistically.”

There have been reports of a number of big masts in recent years, why is this year different?

“The primary cue for triggering a mast event is a warm summer in the previous year. This year (2019) was an extremely large mast event (extremely high densities of seeds) triggered by an extremely warm January and February in 2018. It is more significant than usual because it is very widespread, occurring in most areas with beech. In the past, ‘large’ mast seeding events have occurred in fewer regions and localities.”

What does the latest research tell us about how frequent mast years are? Will we see more mega-masts? Will climate change affect the frequency?

“This is still an active area of research. The evidence suggests that mast years are increasing in frequency, especially at high elevation. This has been attributed to climate change.

“In the short term, we will likely see an increase in the frequency of ‘mega-masts’ (i.e. large seedfall events that are very widespread) due to increases in the frequency of exceptionally warm summers. But over the longer-term, the outcomes of a warming climate for seeding patterns are likely to be more complicated.

“Large mast events require a lot of nutrients (both soil-derived nutrients such as nitrogen, and carbon from photosynthesis). Masting depletes nutrients and so beech trees generally cannot produce large seed crops in consecutive years. The effects of depletion may last over many years in infertile environments. Hence there will be a physiological limit to how frequently beech trees can produce mega-masts, even if the climate were signalling the trees to seed every year.

“One scenario is that seeding will become more frequent, but because of nutrient limitation, the crops will be smaller. Hence beech seed crops will become more evenly distributed across years, rather than really big years and really small years like the current situation. Another scenario is that trees could produce large crops every second year.

“We do not know to what degree trees will physiologically adjust to any future climate. If this does occur there may be little change to their seeding frequency. For example, beech trees at warm, low elevations currently mast just like trees at much colder high elevations in New Zealand. The difference is that low elevation trees produce slightly more regular seed crops, whereas high elevation trees produce larger, less frequent crops.”

What are the flow-on effects from a mast year?

“Mast years provide a big pulse of food (flowers and seed) into forests that in other years may have little food resources for animals. The first effect is an increase in invertebrates that feed on the flowers.

“These invertebrates, and the seeds that grow from flowers that weren’t eaten, are food for native birds and for rodents. In beech forests, some birds like kākāriki and kākā only breed in mast years. Stoats also increase in numbers because there are more rodents to eat. Unfortunately, more rats, mice and stoats mean that native birds (as eggs, chicks and/or adults depending on species) and invertebrates are more likely to be eaten.

“Currently our colder beech forests are refuges for our highly rat-sensitive bird species such as riflemen and mohua, because rats are at low density, except in mast years. If mast seeding becomes more frequent with climate change then these forests will no longer provide refuge, becoming more like northern warm forests which are continuously ratty.”

What does this mean for predator control? What can we do during mast years to protect native species?

“Large scale predator control will need to be applied more frequently with climate change if it results in more frequent, and more widespread mast events.”

General comment

“Don’t forget masting in the alpine environment. Chionochloa snow tussocks (found from sea level to circa 2000m above sea level) also mast, responding more or less to the same temperature cue as beech.

“Mice feed on tussock seed and the increased numbers of invertebrates associated with it. Masting of snow tussocks is associated with increases in mice. Mice in the alpine prey on the eggs of rock wren and feed on native alpine invertebrates. As with beech forest, the presence of a mice outbreak in alpine areas will also lead to the presence of more stoats. Stoats also feed on birds such as rock wren. We don’t know much about rats in the alpine.”

Declared conflict of interest: I have no conflicts of interest to declare.

Professor Dave Kelly, Biological Sciences, University of Canterbury, comments:

There have been reports of a number of big masts in recent years, why is this year different?

“Firstly it’s odd to have had three big beech/snow tussock mast years in six years (2014, 2016, 2019). We’d normally expect more like one every four or five years. Secondly, this year seems to be quite widespread, which is driven by how widespread the very warm 2017/18 summer was.”What stage of the current masting cycle are we in now? How is this likely to progress over summer?“In non-treated areas: mice are now peaking and running out of food. Rats (where present) are now at high levels. Stoat litters are now independent so stoat numbers are up greatly. Impact on native species is starting to rise. In 1080 treated areas, rodent numbers and stoat numbers are low and will remain low; birds are having a good breeding season.”

What does the latest research tell us about how frequent mast years are? Will we see more mega-masts? Will climate change affect the frequency?

“We have been doing a lot of research on this. There have been two hypotheses: the plants respond to the temperature difference between successive summers (the delta-T model, or DT model), which certainly gives a good statistical fit to observed flowering.”Alternatively, the plants may respond to last year’s summer temperatures (T1) plus having high resources last summer, which will 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.

“What we did was in snow tussock (Chionochloa), we carried out manipulations (changing temperatures by moving plants to higher or lower altitudes, and adding fertiliser), and recorded the flowering which resulted. We have also looked at gene expression studies of plants that flowered or did not. We found that adding fertiliser did not promote flowering, whereas changing temperatures did change flowering, in a way consistent with the DT model. We identified genes that increase to trigger flowering in snow tussock, and found that the apparent integrator gene increases in a way which fits the DT signals very well, and does not fit T1 as well.

“What that means is that in snow tussock, there is good support for the DT mechanism being how masting is actually controlled. That means that gradual increases in overall mean temperature under climate change will not drive snow tussocks to try and mast every year. What drives masting is a relatively warm summer coming after a relatively cold summer. So masting will become more frequent if summer temperatures become more variable across years under climate change, which is possible.

“Finally, it’s important to note that pest outbreaks in New Zealand are primarily driven by beech seed crops, not tussock seed crops. 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. However, I guess that would still be the most plausible working hypothesis at this stage.

“That work is all in a recently finished Marsden project, none of it is yet submitted to any journal, though we hope to do so by March 2020 or so.”

Declared conflict of interest statement: None current. Did get a research grant from Animal Health Board to study bird responses to aerial 1080 in 2012-2015 (see Van Vianen et al 2018 NZJ Ecol) but there were no restrictions placed on publication of the results.

Source: New Zealand Science Media Centre

Image source: Arthur’s Pass in Southern Alps. Credit:iStock

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