Response to disturbances
Earthquakes, landslides, gales, fire, snowstorms and drought can kill stands of trees. These leave gaps in the forest that trigger dynamic responses.
Small gaps are covered by nearby tree crowns, but larger gaps will be filled by beech seedlings. Dense patches of shade-tolerant seedlings establish themselves after mast years (when there are large numbers of flowers and seeds), but their growth is restricted. The seedlings can persist in the understorey for decades. When large trees die, extra light and nutrients become available, and the seedlings can grow into trees.
In forests of mixed beech, or beech with other trees, responses to gaps can be complex. For example, in mountain–silver beech forest, faster-growing mountain beech seedlings initially fill new gaps, but the more shade-tolerant, longer-lived silver beech gradually takes over. The forest patchwork continually changes, but in the long term the relative proportion of species is maintained.
Many leaf-eating insects live in beech canopies. Among the most important are caterpillars of the moth Proteodes carnifex. They periodically defoliate thousands of hectares of mountain beech forest in outbreaks that last several years.
Insects can increase the damage caused by natural disasters and weather. Wood-boring pinhole beetles (Platypus) are attracted to moist, dead wood in damaged forests. They carry the Sporothrix fungus, which kills trees and prompts a new cycle of pinhole beetle invasion.
Increase in predators
In mast years, when large numbers of male beech flowers fall, caterpillars have plenty to eat and their numbers increase. Seed- and insect-eating birds also have a good breeding season. But rats and mice also prosper, competing with the birds for food. With more rodents come more stoats, which eat the rodents and then prey on the birds. This cycle may have caused the decline in kākā (parrot) numbers, and the near-extinction of mohua (yellowheads) and kākāriki (orange-fronted parakeets).
An ark for birds
In 2003 the New Zealand government launched Operation Ark, where 11 key areas of South Island beech forest were monitored to identify a possible plague of rats or mice after mast years. Once the rodents were trapped and poisoned, the cycle that leads to the death of native birds was broken.
Honeydew and wasps
Beech forests in the South Island are infested with sooty beech scale insects (Ultracoelostoma), which produce sugary honeydew, an important food for native insects and nectar-feeding birds such as tūī, bellbirds and kākā. For five months each year, introduced wasps eat at least 90% of the honeydew. They also prey on insects, spiders and other invertebrates, further reducing the food of native animals.