When beech forests seed heavily, known as a mast, they can produce up to 15,000 seeds per square metre or 250 kg of seeds per hectare. This provides a bounty of food for native birds and insects. But it also feeds rodents, whose populations expand rapidly and in turn fuel an explosion of stoat numbers.
By forecasting a mast and measuring the seed present, we can be prepared to act. To forecast when a mast will happen, we use climate modelling. To predict the size of the mast, we count the seeds on sampled branches to estimate the amount of seeding.
Along with our tracking of predator numbers, we can use this information for predator control planning at sites where vulnerable native species need protection.
How we predict masts and their size
We use climate modelling to predict forest masts a year ahead. We then measure beech and rimu seed development during the summer months to check our prediction.
Predicting when a mast will happen
Each year we use NIWA’s summer temperature data to predict the likelihood of beech forest seeding or mast, in the following year.
When the summer temperatures during January, February and March are warmer than the previous summer, it is likely beech trees will flower the following spring. This means they will drop seed the following autumn. The difference in summer temperature is called the DeltaT. When the DeltaT is high, seeding is likely.
We map DeltaT over our beech forest areas across New Zealand each year. See the map of our mast prediction for 2022 and for previous years below.
Mast prediction for 2022
The difference between the previous two summer temperatures (DeltaT) shows a beech mast is very likely in southern Fiordland in 2022. Seeding may also occur in parts of North Otago, South Westland and North Canterbury, as well as in the central North Island.
The map below shows the predicted mast areas for 2022 using the difference in summer temperatures:
Map showing likely mast areas for 2022 | See larger (JPG, 365K)
Watch our 2019 video on forest sampling to learn more.
Measuring the size of a mast
We monitor flowering in spring using satellite imagery. Then in summer we measure how much seed there is in some forests to confirm where a mast is occurring.
We have been sampling beech and rimu forests since 2011 to measure the size of a forecasted mast event. To sample, DOC teams snip branches by helicopter. Then, back at the office, staff count any seed present.
There was no significant beech or rimu mast in 2020 or 2021 and no seed sampling was done in 2021. We will take some samples in summer 2021/22.
Mast predictions in past years
In 2019 DOC responded to the biggest beech mast in 40 years with predator control over a record 908,000 ha. Smaller but significant mast events also occurred in 2017, 2016 and 2014.
Past climate maps
The DeltaT maps below show the mast forecasts for 2019, 2020 and 2021:
See larger maps
Past sampling data
2020 sampling: Lull in seeding
Our sampling of beech and rimu trees in February and March 2020 confirmed there was no significant beech or rimu forest mast that year. This was based on samples from more than 1100 beech and 600 rimu trees at 46 sites across the country.
Results from seed sampling showed there was little or no beech seed across the South Island and only a moderate mast at the North Island sites in the Kaimanawa and Kaweka forest parks.
Rimu sampling also showed very little ripe fruit and new tips indicating no rimu mast in 2020 or 2021. Rimu fruit and seeds take 15 months to mature after fertilisation.
2019 sampling: Our 'mega' mast
In 2019, our scientists found beech forests around the country had particularly heavy seeding in the South Island and a mix of moderate to heavy seeding in the North Island. Tussock grasslands in the South Island also produced lots of seed.
Fruiting in South Island rimu forests was also heavy and exceptionally so on Stewart Island/Rakiura, in the Catlins and Taramakau valley on the West Coast. In the North Island rimu fruiting was more variable.