Mast Seeding Solstice Link Could Help Protect Biodiversity

Pinpointing summer solstice as a trigger for mast seeding events could help prevent predators from ravaging New Zealand’s native species.

Emeritus Professor Dave Kelly, from the School of Biological Sciences at Te Whare Wānanga o Waitaha | University of Canterbury (UC), is involved in a new discovery regarding mast seeding events (intermittent years of bumper seed production) in European Beech forests that could benefit conservation work in Aotearoa.

He is co-author of a paper in the journal Nature Plants showing that the summer solstice, or longest day, acts as a “celestial cue” that prompts beech trees to begin registering changes in temperature. Those temperatures then determine the size of next year’s seed crop. This explains why beech forest mast events happen in the same year across the European continent, despite growing in areas with very different maximum daylight hours.

Professor Kelly believes the research has important implications for New Zealand masting species, particularly the native southern beech, or Nothofagus, trees that make up nearly half of New Zealand’s remaining native forest. He says southern beech is of key interest for conservation as mast events in beech forests create perilous conditions for threatened native birds such as kākā, kākāriki, and mohua/yellowhead.

Significantly higher numbers of seeds (mast) are produced during a mast season and this abundance also causes population explosions in introduced rodents. Stoat numbers boom as they feed on higher than usual numbers of mice and rats. Unfortunately, they also prey on native birds.

Professor Kelly says it’s vital to know in advance when large mast years will occur so the Department of Conservation (DOC) can suppress the outbreaks of rodents associated with bumper seeding.

Current mast predictions are based on a joint UC and DOC programme that uses seed traps to collect and count beech seed. However, discovery of the summer solstice “starting gun” effect will enable scientists to hone these predictions more precisely.

“Carrying out a predator control programme over a million hectares of forest – you can imagine the logistics and cost,” Professor Kelly says. “Having these better indicators of timing will have massive flow-on effects.”

Professor Kelly has studied another mast seeding species – the native snow tussock, or Chionochloa — since the 1980s. When he analysed snow tussock data with his international collaborators, they found this species also appears to open its temperature-sensing window at the summer solstice.

“We now know the plants start paying attention to temperatures on the longest day — 21 December,” says Professor Kelly. “We still have to find out by trial and error when they stop paying attention, but knowing the start date increases the accuracy for measuring the weather cue that sets off flowering. This gives better predictions 18 months out of when predator control will be needed.”

He has seen first-hand the devastation large snow tussock mast years can cause for endangered birds.

“Every time there’s a big tussock seed year, you get mice increasing in the alpine, and then stoats also increase. That’s very bad news for endangered birds like rock wrens (pīwauwau) that live only in that habitat. Recent video shows that one of their main threats is nest predation by stoats, even though their nests are well hidden.”

He emphasises that many of these important conservation discoveries came from research into quite different areas of study.

“When I started researching snow tussocks in the 1980s, we didn’t know these things would become crucial for understanding the impact of global warming on these systems. It started as a purely academic interest, and it’s turned into something with very direct application. I think that shows the importance of ‘blue skies’ research.”

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