Southern beech (Nothofagus species) forests grow in harsh environments with steep slopes and shallow soils. Every decade or so vast swathes of beech forest are destroyed by a catastrophic event such as a storm or landslide. Beech species are adapted to cope with disturbance. The seeds and seedlings need light, and grow rapidly on open sites. Beech acts like a pioneer species. It starts the return back to mature beech forest, perpetuating itself.
Beech flowers and produces seed intermittently. Some years no beech seed, or very little, is produced. If a catastrophe occurs in a seedless year, the beech forest may fail to regenerate – instead, another type of forest will develop.
A brutal storm
On 2 February 1936 the North Island was hit by New Zealand’s most destructive storm of the 20th century. It laid waste to hectares of beech forest in the western Tararua Range, uprooting trees and snapping trunks. Sixty years later, broken tree trunks were still visible among the regenerating forest.
If mānuka and bracken fern are growing near a place where beech forest has been destroyed, they may colonise the area instead of beech. Once dense mānuka or bracken has become established, beech will not grow, as its seedlings do not thrive in shade. Instead, kāmahi is usually the first tree. One of New Zealand’s most widespread trees, kāmahi grows from lowland to subalpine regions in the forest’s understorey or its canopy. As it regenerates freely in shade, or in the small gaps left by fallen canopy trees, kāmahi can persist in an area for centuries.
In many mature stands of conifer–broadleaf forest, the largest trees are conifers, but there are few young conifer seedlings and saplings. For some years this puzzled and worried New Zealand’s forest scientists who talked of the ‘conifer regeneration gap’. What had caused the poor regeneration and what would happen to the forests if there were no juveniles to replace the adults?
Two early theories addressed the regeneration gap.
- Plant ecologist Leonard Cockayne thought broadleaved species would take over from conifers. In time, forests dominated by kauri or other conifers would be replaced by tawa, rātā, and kāmahi.
- Forester J. T. Holloway thought conifer regeneration had failed in the South Island when the climate became cooler and drier, around the 1500s.
Modern scientists have discarded both theories. The answer is simpler.
It seems that like beech, conifer regeneration is stimulated by disturbance. When a fire or storm leaves an opening in a forest, seedling and sapling growth of many species is prolific, and one or more species grows up filling the opening. This is known as gap-phase regeneration and occurs in New Zealand’s main forest types: kauri, conifer–broadleaf and beech forests.
For long-lived species like kauri (1,000–2,000 years) or other conifers (600–1,200 years), some disturbance is likely to affect the forest within their life spans. As the forest gradually grows back, broadleaved trees may dominate for 200–500 years – but this is just part of the life span of kauri or rimu. As long as some conifer seedlings establish themselves in openings created when broadleaved trees die, kauri or conifer–broadleaf forest endures.