Mast years

What is a mast? You might be forgiven for thinking it had more to do with ships than conservation!

The term comes from the ancient English word ‘mæst’, which was used to refer to years when forest acorn production increased significantly and pig farmers were allowed to drive their pigs into the forest to fatten them up.

Today, the term ‘mast’ is a general term for when plants (predominantly trees) flower together en masse every few years, producing large amounts of seeds.

Most plants in New Zealand flower and seed each year, but these are mostly small flowering events. Some years see larger than normal flower and seeding events. In New Zealand, masting refers to the intermittent heavy flowering of populations of trees and other long-living plants. When scientists refer to masting, they are referring to synchronised mass flowering and seed production in populations of plants like a beech forest, not individual plants.

Mast events pose big challenges for our conservationists who are working to protect our native flora and fauna.

Why do mast years occur?

Scientists are working to better understand why masts occur, and it is likely that masting as a reproductive strategy is due to one of several factors.

The key common feature is that the plants get some benefit from making occasional large seeding efforts rather than regular smaller ones. The two most common ways plants can benefit are wind pollination success and predator satiation. Large mass flowering will increase the chances of successful wind pollination, and predator satiation is when plant populations provide a glut of seed every few years to make it much harder for consumers of the seed (birds and mammals) to eat it all so that some seed will remain available for new plants to grow.

A large number of plant species in New Zealand show masting, but the reasons for this remain a mystery. It has been hypothesised that because humans and mammalian herbivores were absent from New Zealand until very recently, this might have enabled plants like grasses to evolve the long life spans that are necessary for masting to be a viable reproductive strategy.

Learn more about plant reproduction in the articles Seed dispersal, The seed-flower life cycle and Pollination and fertilisation.

What plants mast in New Zealand?

In New Zealand, beech and podocarp forests mast. Podocarps are cone-bearing plants that include trees and shrubs. Podocarp trees that mast include rimu, kahikatea, mataī and tōtara. Like beech trees, all these podocarps are wind pollinated.

Unlike most other countries, New Zealand also has masting in long-lived non-woody plants, including grasses like snow tussock, harakeke (flax) and taramea (wild spaniards).

Mega-mast events occur when a number of masting populations synchronise and flower en masse — for example, the 2019 mega-mast was the result of many species, including beech and podocarp forests, snow tussocks, harakeke and taramea flowering heavily throughout most of New Zealand.

What are the impacts of mast years?

Mass seeding events mean that there is an abundance of food in our forests. While this is great news for our native birds that feed on the seeds and fruit, it’s also great news for other animals – like introduced mammalian predators!

The abundance of food leads to explosions in the numbers of mice and rats that feed on the seeds. In turn, the increase in rats and mice leads to an increase in other predator populations — like stoats. And of course all these predators like to feed on our native birds, chicks and eggs, lizards, bats and insects.

A mast event in 2007 resulted in a halving of the nationally critical takahē population, and an earlier one in 2000 resulted in the complete elimination of the mohua (yellowhead) population on Mount Stokes in the Marlborough Sounds.

Further, the loss of birds impacts the pollination rates of other native trees and plants.

Being able to predict when mast and mega-mast years are about to happen enables our conservationists to prepare for the increase in predators in order to put measures in place to better protect our native animals.

Predicting mast years

Population ecologist Dave Kelly led research into climate data to predict mast years. The predictive model from this work, known as delta T, shows mast years can be predicted with climate information – a cool summer in year 1 followed by a warmer than average summer in year 2 will lead to a mast event in year 3. This is very useful as conservationists get 12 months’ warning of a mast year.

How to combat increased predators in mast years

To mitigate the effects of a mast year, conservation organisations need a good head start to get additional financing and logistics in place to carry out larger-scale operations to track and eliminate predators. Using the delta T model, the Department of Conservation (DOC) was able to commence work on planning for the 2019 mast year from March 2018. This work includes organising aerial drops of the poison 1080 over a million hectares of land from August to December 2019.

Predator numbers need to be well under control by the bird breeding season between September and December to ensure native bird populations can breed successfully to maintain healthy population growth for long-term survival.

Tracking predators remains important, as although DOC makes plans on the basis of climate signals, it doesn’t go ahead with pest control operations until it is sure the pests have increased to dangerously high numbers.

To date, the most effective measures to control predator populations are ground-based trapping and bait stations over small areas and aerial drops of 1080 over large remote areas.

Take a closer look at some of the other work being done in New Zealand to protect our native birds.