At low altitudes on warm, moist, fertile sites, mixed conifer–broadleaf forest dominates. At higher altitudes, where the weather is colder and wetter, growing seasons are shorter and soils are less fertile, beech forest replaces mixed conifer–broadleaf forest.
Similarly, as latitude increases, beech forest gradually replaces the mixed conifer–broadleaf forest.
Pure beech forest
Pure beech forests, made up of a single beech species, grow where conditions best suit a particular species. There are large areas of mountain beech forest on the eastern mountains, particularly in the South Island. Silver beech forest dominates the wetter, western mountains, and grows from sea level to the treeline in Southland and Fiordland.
Pure forests of other beech species have a more limited distribution. Black beech forms lowland to mountain forests on eastern slopes from Canterbury to the East Cape, and is the only beech in the Taranaki region. Red beech forests grow in deeper, more fertile soils, and at lower altitudes than other beech forests.
Forests may be made up of a mixture of beech species. All four of New Zealand’s beeches grow together in the north-west South Island and on the East Cape mountains. Red and silver beech co-exist on mid-slopes in the north-west of the South Island. Forests of silver and mountain beech are also common.
Beeches form forests with other broadleaf trees and conifers. For example, there is hard beech–kauri forest in Auckland and the Coromandel Peninsula, beech–tawa forest in the central North Island, and beech–rātā–kāmahi forest in the western South Island. Hard beech grows with other broadleaf species and conifers (such as rimu) in the North Island and in the north-west South Island.
Some areas have suitable climates and soils, but lack beech forest. There is a large region free of beeches in central Westland, and beech does not grow on Stewart Island. Other 'gaps' are Mt Taranaki (Mt Egmont), in the northern Tararua and southern Ruahine ranges (known as the Manawatū gap), and central Canterbury.
These gaps may have been created when glaciation or volcanic activity destroyed forests. The beech gap in central Canterbury may have been caused by drought. In the past, all forest in the area was destroyed by fire – naturally caused or lit by early Māori. The beeches then failed to regenerate because the climate was unsuitable or there was competition from other plants. Beech is particularly slow to spread because its seeds are dispersed by wind or water, and its seedlings' roots have to establish a relationship with specialised fungi to survive.