Masting: Understanding feast and famine in NZ forests and grasslands

Friday, 24 July 2020

University of Canterbury researchers Samarth, left, and Alan Woods sample tussocks for gene expression at Mt Hutt. Samarth uses one name.

At Arthur’s Pass township last year, there was a mouse plague. Locals struggled to rodent-proof their houses. This plague was expected. It happens every few years, when a big seed crop throughout Nothofagus (southern beech) forests provides abundant food for the mice. Such rodent outbreaks threaten native wildlife, but luckily, recent scientific discoveries are making it easier to protect rare species.

When rodent numbers increase, predators like stoats also increase. Both stoats and rats then attack native birds including mohua, kākāriki and whio. The Department of Conservation responds with widespread aerial pest control to protect at-risk native species.

In three recent outbreaks (2014, 2016 and 2019) DOC has run pest control on more than half a million hectares costing more than $10 million each time. This treatment works: research by Graeme Elliott and Josh Kemp showed improved nest survival for birds like mohua, rock wrens and robins.

But how will climate change affect this system? Based on historical data, we would expect three to four years between large seed crops (called mast years). So why have we had three mast years in the last five six years? That matters both for conservation and for budget planning.

A high flowering year for snow tussocks in the same place, Mt Hutt, 1999.

A low flowering year for snow tussocks at Mt Hutt in 1998.

* Starving rats and mice prey on birds and reptiles after mega-mast food runs out

Composite photo showing low and high mast years (2001 and 1999 respectively).

* Curious kea at risk in 1080 operations ahead of mega mast rodent buffet

* DOC to launch huge predator control programme in response to 'mega mast'

* $30.5 million hit planned for 'mega mast'

* Opinion: 1080 is not perfect but for killing vermin it's the best we have got

To predict when mast years will occur, we need to know what mechanism controls them. Mast years are remarkable because plants over large areas are synchronised in their seed production, despite not being able to communicate with each other. Synchrony happens because all plants respond independently to the same weather cue.

In 2013 a University of Canterbury team found the weather cue for Nothofagus is a two-year temperature sequence: a cold summer, then a warm one, leads to a mast year the third summer. This allows DOC to predict mast years a year in advance, helping with budget planning. Notably, this cue requires plants to compare temperatures one year apart, a difficult task.

A just-completed Marsden Fund project by PhD student Samarth, suggests that removable markers, which prevent some DNA sections being read, allow plants to store information from one year to the next. Samarth uses one name.

So, will climate change make mast years more frequent? If so, rodent and stoat outbreaks might change significantly. But if the weather cue came more often, the plants might run out of reserves to make seeds with – a big seed crop depletes essential nutrients like nitrogen.

Two new studies help predict what might happen. First, a 39-year study on masting of English beech trees in the journal Nature Plants, led by Polish ecologist Michal Bogdziewicz with University of Canterbury input, found that a warming climate did lead to more frequent masting. This altered the relationships with beech seed predators, in that case a European moth. The result was that all the benefits of increased seed production went to the moth, not the trees.

Second, the Marsden Fund project studied New Zealand snow tussocks, which mast in step with Nothofagus but are easier to run experiments on. We tested the relative importance of nutrients and weather cues, and found that nutrients had little effect. This suggests that the weather predictor DOC uses should continue to be a reliable indicator of future mast years.

Thus, excellent science leads to better management. There are also two pointers to how science contributes. Firstly, long-term seedfall data – originally collected for other purposes – are vital to answer new questions on global warming. Secondly, New Zealand is unusually rich in mast seeding species. That biological heritage has made this country a global leader for solving questions about masting which are relevant worldwide.

Dave Kelly is a plant ecologist at the University of Canterbury who has researched mast seeding for over 30 years.