Beech is the dominant type of forest in the South Island and is also found on the mountain ranges of the North Island.
In “mast years” beech forests produce a very large amount of seed. Mast seeding is triggered by warm temperatures in late summer and early autumn.
Previously mast years occurred on average every 6-7 years. Since 2000 we have had a record string of warm summers in a row where above average temperatures have triggered beech masts – six mast years have occurred in the last nine years.Historically the large amount of seed in a mast year was eaten by birds and insects. Now the seed is eaten by mice and rats – and populations of these fast-breeding introduced pests explode thanks to the abundance of food. Numbers of introduced stoats also increase as the stoats feed on the high numbers of rats and mice. Once the seeds are gone, the huge numbers of rats, mice and stoats turn to eating our native birds and other native wildlife.
Historically, although native species were hit hard in mast years, their populations were able to recover between the relatively infrequent mast years. Now there is little chance for them to recover. This has taken a devastating toll on our native species:
In pre-European times mohua (yellowhead) were found throughout South Island forests but their numbers have been greatly reduced, largely due to predation by introduced pests. Their decline has accelerated in the last decade as predator numbers have increased due to frequent mast years.
Already mohua have been wiped out in many areas, and if we do not do more to protect them they will only remain on offshore pest-free islands and become extinct on the mainland.
Mohua are not the only species threatened by increasing mast years – others such as whio (blue duck), native bats, kiwi and kakariki are also severely affected.
These species are only just hanging on in areas of intensive pest control, particularly thanks to use of aerial application of 1080 at key sites under Operation Ark which was set up to respond to the critical threat posed by more frequent mast years. However it is not enough – just a temporary band-aid to keep the species barely