Forests

In pre-human times, New Zealand’s land mass was blanketed by forests. Where trees could grow, they did – once covering nearly 80 per cent of the total land area of this country. These forests had been in a process of continual change over countless millennia due to climate cycles, new species arrivals and disturbance events, such as volcanic eruptions. However, the most rapid changes have occurred in the last 700 years of human occupation. Today, indigenous forests cover just over seven million hectares, or approximately 25 per cent of the total land area, and have been extensively modified through land clearance, logging and as a result of the impacts of invasive species.

New Zealand’s land mass is home to some unique forest ecosystems; including kauri forests, kahikatea swamp forests, pōhutukawa forests and mixed broadleaved forests. These support a diverse array of flora and fauna that evolved in the almost complete absence of land mammals. Most of New Zealand’s native land birds inhabit forest or shrub land.

Logging and land clearance have completely destroyed much of the country’s native forest and its associated biodiversity. Forest clearance has had the greatest impact on lowland forests, significantly reducing some forest types. Today the greatest threats to native forests come from damage caused by invasive species, the fragmentation of forest habitats, and the lack of intact forest buffers.

Forest processes

Under natural conditions the vegetation within indigenous forests, and consequently the species that inhabit these ecosystems, changes over time. Processes such as disturbance and succession (continual change) resulting from natural events including landslides, falling mature trees and forest fires, lead to bare areas of soil. The first plants to colonise an area of bare soil are lichens, mosses and small herbs. These plants are important for the forest, as they build up the fertility of the soil, which in turn supports smaller hardy trees and shrubs. Succession within a forest system can be primary, which begins with bare rock exposed by a geological activity, or secondary, which is where the soil is open and exposed following a disturbance event in pre-existing forest such as a fire or storm. This process can also occur at multiple scales, from single tree gaps through to catastrophic events.

Trees

Forests in New Zealand are typically made up of mixtures and pure stands of broadleaved, conifer and beech species. The two most common forest types are conifer-broadleaved forest and beech (Nothofagus) forest.

Conifer-broadleaved forest in temperate, lowland areas, are the tallest and most complex of New Zealand’s forests. At higher altitudes they tend to be shorter and floristically poorer than the lowland equivalents. There is a huge diversity of species found within this class of forest.

Beech forests cover just under three million hectares of New Zealand and consist of one or more beech species. Beech is the dominant forest cover on the main mountains of the North Island and much of the western South Island, but is absent in most of Northland, near the Manawatu Gorge and in central Westland. Beech forests are generally found at higher elevations where the climate is colder and wetter, growing seasons are shorter, and soils are less fertile. In these areas beech forest replaces conifer-broadleaved forests.

Beech trees also grow alongside non-beech species forming mixed beech and conifer-broadleaved forests. These forests cover over 1.4 million hectares and include the hard beech-kauri forest in the Auckland area and beech-podocarp forest in the central North Island.

Other flora

Beneath the forest canopy a tier of smaller trees, including tree ferns and nikau palms (which are the southern-most palm species in the world) are found. These then give way to a layer of saplings, shrubs and groundcovers. New Zealand has a very rich diversity of mosses living in the forest habitats. Approximately one fifth of mosses are endemic, including Epipterygium opararense, which has only been found growing on a single boulder in the Kahurangi National Park in the north-west of the South Island.

Historically, fungi have not been well studied, but in terms of species their numbers significantly outweigh plants. Over 900 species have been recorded growing with the four species of native beech. New Zealand has more than 500 species of liverwort and 13 species of hornwort. The nationally critical liverwort Frullania wairua has to date only been found living on the twigs of a threatened rata species Metrosideros bartelettii. Large numbers of liverworts enjoy the moist conditions offered by native forests.

Ferns are mostly found in moist forest areas where there is plenty of water. For a temperate country, New Zealand has an unusually high number of ferns, with approximately 200 species. It is thought that around 40 per cent of these are found only in this country. The silver fern (Cyathea dealbata), also known as ponga, is a national symbol of New Zealand. The ferns found in the forest ecosystems can range in size from just a few millimetres long to huge tree ferns.

Large leaf-like lichens are also common in forests as they like to grow on the damp environments found in tree trunks. A large foliose lichen, Menegazzia pertransita, grows on the trunks of beech trees in the wet areas of Fiordland. Lichens are also found growing on decaying logs and stumps on the forest floor, as well as on the leaves of trees as epiphylls.

Lianes are various climbing and woody vines that are commonly found within New Zealand’s forests. Epiphytic plants grow on another plant or object, obtaining moisture from the air, or from the dampness on the surface of their hosts. Common epiphytes within forest ecosystems include the perching lily Collospermum hastatum and the bamboo orchid Earina mucronata.

Birds

New Zealand’s native land birds are most commonly found in forest or shrub land. In some areas exotic forests provide important habitat for native birds. The availability of good quality native forest (including exclusion of predators) is important to the survival of many of the uncommon and threatened bird species of this country, including the kōkako and mōhua. Native forest habitat offers important sources of food and shelter, particularly during winter when the availability of food is at its lowest. The most valuable forests for native birds are the tall lowland conifer and broadleaved forests, as well as the lowland beech forests, because of the complex food webs and diverse habitats offered by these ecosystems. 

Native birds are a critical part of forest communities, providing important pollination and seed dispersal services. All podocarp species primarily rely on birds for dispersal. Large forest birds, like kererū distribute the fleshy fruits and berries from trees such as tawa, pūriri, miro, and matai found in the conifer-broadleaved forests. Other native species that are important for seed dispersal are tūi, bellbird and silvereye. Increasingly, introduced birds are also playing a key role in seed dispersal.

Invertebrates

Native forests provide habitat for a wide diversity of invertebrates, which are a fundamental part of the forest ecosystem. Invertebrates play an important role in breaking down organic material, recycling nutrients, pollinating plants and providing food for native birds, reptiles, frogs, fish and bats. There are many threatened invertebrate species living within native forest habitats which rely on this biome for their survival.

Reptiles and frogs

Geckos and skinks are the only lizards found in this country. While they inhabit a wide range of elevations and environments, many of them are found in the moist conditions offered by native forest habitats. The endangered striped skink Oligosoma striatum lives in epiphytes that grow in trees and rotting logs. Tuatara, whilst they appear to be lizards, are from the Sphenodontia order which lived amongst the dinosaurs. They like to eat native invertebrates such as weta and worms. The native frogs found in New Zealand (pepeketua) also belong to an ancient genus of frog, Leiopelma. The critically endangered Archey’s frog Leiopelma archeyi, found only in two sites in the Coromandel and west of Te Kuiti, is one of the world’s most primitive frogs and is described as a ‘living fossil’.

Ecosystem services

New Zealand’s native forest ecosystems play an important role in delivering ecosystem services (described in Chapter 1). There are a number of values and benefits that are derived from indigenous forest ecosystems, including:

  • Improving water quality
  • Reducing soil erosion
  • Retaining natural nutrient cycles
  • Decreasing air pollution
  • Regulating local air temperatures
  • Supporting pollinated species
  • Supporting biological pest control species
  • Regulation of atmospheric carbon levels and temperatures
  • Sequestration of atmospheric carbon

Native forests offer a significant potential for storing carbon. As the trees grow, carbon dioxide is absorbed and stored in the forests. A significant amount of carbon is also stored in the soil. Indigenous forests are a far greater carbon store than mono-culture plantation forests; compared hectare for hectare, native forests store around three times as much carbon as pine forests.

Threats

Prior to human settlement, New Zealand was largely forested below the climatic tree line. The arrival of Māori led to significant areas of lowland forest being burned to encourage the growth of bracken fern that was used as a food source,to make cross-country travel easier, and as a strategy for hunting moa. Forest clearance rapidly increased following the arrival of European settlers.

Burning open land for future clearing in the 1960s (Crown Copyright: Department of Conservation: Te Papa Atawhai, n.d.)

Early New Zealand landholders were required by law to improve their land, and many achieved this through burning the forest. Primary forest clearance continued into the mid-20th century, and during the 1950s increasing amounts of forest in the mountain ranges was converted to farmland or fast-growing exotic plantations. The net result of exploitation of New Zealand’s indigenous forest was the loss of approximately three-quarters of the forest, reducing it from 82 per cent to 23 per cent of the land surface area.

Forest clearance has had the greatest impact on lowland forests, reducing some forest types such as kauri to less than one per cent of their former extent. There is a bias in the protected land network towards upland forests and grasslands, which leaves remaining lowland forests vulnerable.

Whilst the widespread clearance of native vegetation has now stopped, the most pressing issue facing the protection and restoration of native forest biodiversity is the damage from invasive species. Other threats include fragmentation of forest habitats and the lack of intact forest edges.

Last updated at 1:55PM on February 25, 2015