Location and General Description
The sedimentary rocks of New Zealand were laid down in two main phases, each followed by a period of uplift and subsequent erosion. The rocks of the North Island’s rugged axial ranges are older, brittle, sedimentary rocks (greywacke and argillite) that broke up into a series of blocks (or ranges) along major fault lines as they were uplifted. The surrounding hill country and lowlands are formed from the younger, more malleable sediments (e.g., sandstone and mudstone) that have eroded into softer forms over 5 to 10 million years (Soons and Selby 1982). Then extensive volcanic activity started in the Taupo Volcanic Zone some 2 million years ago, and has persisted until the present. Pumice and ash deposits are found all over the southern part of the island and various eruptions and seismic events have defined most of the landforms of the central North Island. The climate is generally mild, though winter temperatures can plummet to several degrees below zero in the central plateau and on Mt Taranaki. The warmest area is the east coast where dry fohn winds regularly bring summer temperatures of more than 20oC.
Lake Taupo, the largest freshwater lake in Australasia, lies in the north-central portion of this region. It fills a massive volcanic cauldera that last erupted about 1,800 years ago. This was the most destructive eruption known to have occurred anywhere in the world in the last 7,000 years. More recent volcanic activity has occurred within Tongariro National Park and the result is a mosaic of vegetation types. In the eastern rain shadow of Mt Ruapehu, plants grow sparsely and are stunted by harsh climatic conditions, and ultra porous soils. To the south, there are dense podocarp forests and in the west, mountain beech (Northofagus solandri cliffortioides) or beech/podocarp forest grows along with great expanses of fire-induced red tussocks below the treeline (Department of Conservation 1994). Natural tussock herbfield communities are found at higher altitudes on the volcanoes and the adjacent ranges. The longest rivers in the region (e.g., the Waikato and Whanganui) have their headwaters in this part of the region.
Egmont National Park, which lies to the east of the region, is dominated by the symmetrical volcanic cone of Mt Taranaki (also known as Mt Egmont) and the older, outlying peaks of Pouakai and Kaitake. Mt Taranaki formed about 70,000 years ago, and last erupted in 1,750 AD. The area gets high rainfall, up to 8,000 mm, and has luxuriant vegetation. There are about 550 indigenous vascular plants, 5 of which are endemic to the mountain. Kamahi (Weinmannia racemosa) and rimu (Dacrydium cupressinum) grow at lower altitudes with rimu giving way to Hall’s totara (Podocarpus hallii) and kaikawaka (Libocedrus bidwillii) below a shrub layer dominated by leatherwood (Olearia colensoi). However, there is a surprising absence of many plants that are found in similar environments in the central North Island. This ‘gap’ is thought to be a result of the region’s history of volcanic disturbance, the relative isolation of Mt Taranaki from seed sources, and, in the case of beech, poor seed dispersal and poor competitive ability on relatively good soils (Clarkson 1986). Above the treeline, there are tussock herbfields (dominated by red and silver tussock) and mossfields. The mire vegetation, dominated by tussocks and sedges, is particularly rich with 260 different plant species. Most of the lowland areas (outside the park boundary) have been cleared and are now farmed.
Past eruptions from the Taupo cauldera and other northern volcanic centers formed a number of large upland ignimbrite plateaus. These were once dominated by the major popocarp species: rimu (Dacrydium cupressinum), miro (Podocrpus ferrugineus), matai (Podocarpus spicatus), totara (Podocarpus totara) and kahikatea (Dacryacrpus dacrydioides). Tawa/broadleaf forests dominated lowland areas. Some regions have kept a relatively intact cover but most have been logged and cleared for farmland or were replanted in exotic plantation species (mostly Pinus radiata).
Greywacke ranges run the length of the region, aligned from the northeast to the southwest. The forest cover is primarily beech (Nothofagus spp) at higher altitudes, with podocarp/broadleaf or podocarp/beech associations in sheltered sites. There are, however, distinctive ‘beech gaps’ in the southern Ruahines, northern Tararuas Ranges (Dawson 1988) and on the Wellington Peninsula, presumably a result of ice age extinctions and the poor dispersal ability of beech species (Department of Conservation 1996). The Tararua, Ruahine, Kaimanawa and Kaweka Ranges are the only ones with significant alpine zones and tussock grasslands and these areas are usually coated in snow each winter. A feature of the southernmost ranges is the almost impenetrable band of ‘leatherwoods’ at the treeline. Leatherwoods (e.g., Olearia colensoi and Brachyglottis rotundifolia) have tough leaves with felted undersides to minimize water loss in the prevailing windy conditions (Gabites 1993). The ranges to the north-east are lower and contain the largest contiguous tract of protected hill country in the region (2,127 km2), mainly within Te Urewera National Park.
Much of the western lowlands, from Taranaki south, has been cleared of indigenous cover and is now farmed. However, one large block of lowland forest (3,220 km2), including private land and the forests of Whanganui National Park, has survived more or less intact. The uniform canopy is dominated by broadleaf species, mainly kamahi (Weinmannia racemosa) and tawa (Beilschmiedia tawa), with some emergent podocarps (e.g., rimu and matai). Many species reach their southern limit here including mangeao (Litsea calicaris) and neinei (Dracophyllum laxifolium).
The Horowhenua floodplains, northwest of Wellington, still have dunes dominated by native sand-binders spinifex (Spinifex sericeus) and pingao (Desmoschoenus spiralis) as well as many estuaries and ephemeral inland wetlands. Forest once reached from the foothills of the Tararua Ranges to the dunes, but it was cleared by 1880, within 20 years of a tramline reaching the area. This area also had vast swamps lined with flax (Phormium tenax) which was harvested until the 1940s when the land was drained for farms (Park 1995). The south-eastern lowlands of Hawkes Bay and the Wairarapa are in the rainshadow of the axial ranges, and have a relatively dry climate (just over 800 mm of rainfall per annum). Very few forested areas have survived in this part of the region but there are several important wetlands and estuaries of note: the wetlands and associated forest remnants linked to Lake Wairarapa and Lake Onoke.
The most ecologically intact areas are in the north, northeast, and central parts of the regions. These areas are the last refuge for many species that are no longer found further south. There are, for example, several isolated populations (less than 400 pairs) of the North Island kokako (Callaeas cinerea wilsoni EN) scattered around these northern forest remnants (Hilton-Taylor 2000). The North Island kokako is an endemic wattlebird whose close relatives North and South Island saddleback (Philesturnus carunculatus rufusater and P. carunculatus carunculatus) and the huia (Heteralocha acutirostris EX) are now, respectively, limited to predator-free islands or extinct. The South Island subspecies of the kokako is also presumed to be extinct (Department of Conservation website). Two of the North Island’s largest populations of brown kiwi (Apteryx australis VU) and blue duck (Hymenolaimus malacorhynchos VU) can still be found in the central and eastern mountain ranges.
In general this region is not known for its floral diversity although there are some exceptions to the rule. The Moawhango area, for example, which lies between the Kaimanawa and Ruahine Ranges, has some 730 plant species recorded (Molloy 1994). Some, like the endemic Acaena rorida, are considered threatened. The southern parts of the region are also noticeably low in endemic species which may be due to either the severe condition prevalent here during the Pleistocene Ice Age, or to the area’s submergence prior to the last period of glacial fluctuations (Dawson 1988). However, it does have a variety of interesting parasitic plants which are either endemic or now rare in other parts of the country. Dactylanthus taylori is a member of the tropical Balanophoraceae family which depends entirely on their host plants for nutrients. It leaves a distinctive rose-shaped deformity on the root of its host that was prized by collectors last century which led, in part, to its current rarity. Several mistletoe species (e.g., Peraxilla tetapetala and P. colensi) which grow on beech trees are also present but are threatened by possums (Trichosurus vulpecula) which browse heavily on all mistletoe species.
In the Taupo volcanic zone there are at least 18 well defined geothermal areas with a very specialized and endemic flora; some species, like the prostrate kanuka (Kunzea ericoides microflorum), can grow only in geothermal areas. Others, like the distinctive umbrella fern (Dicranopteris linearis), are tropical in origin and would not normally be found this far south, but are protected from frosts and cold winds by the warm, steamy atmosphere (Burns 1997).
The region’s non-avian wildlife is interesting, and is perhaps most notable for the relative abundance of lizard species, especially in the southern part of the region. Wellington has 14 species including the only mainland population of Whitaker’s skink (Cyclodina whitakeri VU), as well as more common species like the forest gecko (Hoplodactylus granulatus) and the common green gecko (Naultinus elegens punctatus). Also of note is the threatened lesser short-tailed bat (Mystacina tuberculata VU), one of New Zealand’s two endemic land mammals. A volcanic plateau subspecies is found mainly in the central North Island and Taranaki. Short-tailed bats are the only surviving species in the Mystacinidae family (the other native bat, the New Zealand greater shorter-tailed bat (M. robusta EX) is extinct) and are listed by the Department of Conservation as a `species of highest conservation priority’ (DOC website). Snail diversity is not high but at least two endemic Powelliphanta species are still present in forest remnants along the western coast (Department of Conservation 1996).
Islands off the Wellington coast are important wildlife refuges. Over recent years many species have been released onto the islands as extra security in case mainland populations fail for any reason. Kapiti Island now has 10 percent of the national population of takahe (Porphyrio mantelli hochstetteri EN) as well as stitchbirds (Notiomystis cincta VU), North Island kokako (Callaeas cinerea wilsoni EN), long-tailed bats (Chalinolobus tuberculatus VU) and the country’s largest population of little spotted kiwi (Apteryx owenii VU), while Mana Island has Cook Strait giant weta (Deinacrida rugosa VU), the endemic McGregor’s skink (Cyclodina macgregori VU) and the gold-striped gecko (Hoplodactylus chrysosirecticus) (Department of Conservation 1996).
The wetland areas of the region support numerous waterfowl and native fish including the endemic brown mudfish (Neochanna apoda), which is considered a lower risk globally threatened species. The swamps, rivers, and estuaries associated with Lake Wairarapa and Lake Onoke make up the largest wetland system in the lower North Island but there are also important wetland areas along the western coast and in the north that support a wide variety of unusual plants and animals. However, many of these wetland areas are degraded by stock damage and nutrient run-off from farms and are threatened by on-going land development (Department of Conservation 1997).
Almost 170,000 km2 of land is managed by the Department of Conservation in this region, and there are other substantial areas of land held in Maori tenure that are still considered indigenous forest. However, after many centuries of occupation by Maori and subsequent settlement by Europeans, much of the region’s rich lowlands have been extensively modified for agriculture, silvaculture, and urban development. The majority of protected areas focus on upland forest habitat and alpine zones although Whanganui National Park does contain a large portion of relatively unmodified lowland forest. Te Urewera National Park has the largest area of remote forested hill country. Geothermal areas, lowland and coastal forest remnants, tussock grasslands, dune lands, and wetlands are, in general, not as well-protected.
Although the region has four national parks (Tongariro, Egmont, Whanganui, and Te Urewera), there were significant areas of indigenous forests that were poorly protected until the late 1970's. Many forests on the axial ranges and the central plateau were managed as ‘multiple use’ forest parks, and logging of indigenous timber was permitted. Widespread public protests in the late 1970's and 80's saw a move away from production logging of lowland native forests and, eventually, their protection as Conservation Areas.
Tongariro was New Zealand's first national park, created following a gifting of the sacred central North Island peaks to the nation in 1887. Subsequently, the park has been inscribed on the World Heritage List for its natural landscape and associative cultural landscape features.
Types and Severity of Threats
In the last 15 to 20 years the threats facing the region’s natural areas have shifted from habitat loss through human exploitation to degradation by alien species. Introduced animals alter habitat through browsing and predation while weeds compete with native plants. Foreign predators caused havoc everywhere they were introduced, starting with the Pacific rat (Rattus exulans) - now extinct on the mainland, followed by the ship rat (Rattus rattus), cat (Felis cattus), Norway rat (Rattus norvegicus), and stoat (Mustela erminea), that arrived with Euopean settlers after 1769. Today, those in the region that are most at risk are isolated populations of threatened species (e.g., Powelliphanta snails, Whittaker’s skinks, kokako). Most of the vulnerable (specialized) fauna are already locally extinct around main population centers and have held on only in isolated strongholds. Kokako, for example, are no longer present on the mainland to the south and east of the region, while blue ducks and the North Island brown kiwi have gone from southern ranges (Department of Conservation 1996).
Browsing by introduced species is a significant problem in the lowland forests. Goats (Capra hircus) and possums are the main culprits. Possums have caused the widespread death of kamahi and rata in lowland forests and the death of kaikawaka (Libocedrus bidwillii) and Hall’s totara (Podocarpus hallii) in the southern ranges (Department of Conservation 1997). Red and sika deer (Cervus elaphus and Cervus nippon) are present in most of the axial ranges. Deer and other browsers prevent the regeneration of favored plant species, which causes significant changes to the structure and composition of native ecosystems. At critical sites, non-replacement of canopy species can lead to canopy collapse. A more localized problem is the wild horse herd (Equus caballus) in the Kaimanawa Range. This population is now mustered periodically to restrict their range and minimize their impact on tussock grasslands and places with special values such as Moawhangao Ecological Area (Linklater et al 2000).
Weed species, like old man’s beard (Clematis vitalba) and banana passion fruit (Passiflora mollissima, P. mixta) are widespread and prolific in parts of the region, especially low-lying areas with milder climates like the Tarnaki/Wanganui lowlands. Pinus contorta and hawkweeds (Hieracium spp.) are a problem in the tussock grasslands of the volcanic plateau and higher reaches of the axial ranges. Scottish heather (Colluna vulgaris) is another invasive species that threatens low-growing communities above the treeline and the peat bogs in and around Tongariro National Park. In general, small forest remnants, disturbed areas, and open habitats are particularly vulnerable to weed invasion, as most weed species are intolerant of shade.
Fire is a threat because New Zealand’s forests are not fire-adapted like those in Australia. For example, fires were once frequent around Wellington on cleared land that had become covered with gorse (Ulex europaeus), a fire-adapted, introduced species. Gorse grew back easily, but the fires all but eliminated regenerating broadleaf species and removed valuable seed sources (Department of Conservation 1996). Other areas particularly at risk include the island refuges, with their concentrated populations of threatened species, and the tussock grasslands of the central plateau.
Habitat modification is now a concern primarily in the large forest areas still in private ownership and the smaller forest fragments along the coastal fringes. Wetlands and waterways are also at risk. Many of New Zealand’s fish species are migratory and need access to the sea to complete their life cycles. Hydro-dams, culverts, and fluctuating water levels can impede their progress and have contributed to the decline in abundance of these species. Native fish species are also threatened by riparian damage and siltation.
Justification of Ecoregion Delineation
This ecoregion is a combination of Wardle’s (1991) ‘Volcanic Plateau’, ‘Taranaki’, ‘Gisborne’, and ‘Southern North Island’ botanical provinces. There is good correspondence between this ecoregion and the ‘North Island of New Zealand’ Endemic Bird Area (Stattersfield et al 1998). While the northwestern portions of the ecoregion contain vegetation similar to that of the Northland Temperate Kauri Forests, there is considerable variation in physiography and topography. Before European arrival mixed conifer/broadleaf forests covered the higher elevations of axial ranges. The effects of rainshadows and a more temperate climate do influence vegetation as one moves southward.
Burns, B. R. 1997. Vegetation change along a geothermal stress gradient at the Te Kopia steamfield. Journal of the Royal Society of New Zealand 27(2): 279-294.
Clarkson, B. D. 1986. Vegetation of Egmont National Park New Zealand. DSIR Science Information Publishing Center, Wellington, New Zealand.
Dawson, J. 1988. Forest vines to snow tussock: the story of New Zealand’s plants. Victoria University Press. Wellington, New Zealand.
Department of Conservation. http://doc.govt.nz. viewed on September 2000.
Department of Conservation. 1994. Draft Conservation Management Strategy for Tongariro/Taupo Conservancy. Tongariro/Taupo Conservation Management Planning Series No 1. Turangi, New Zealand.
Department of Conservation. 1996. Conservation Management Strategy for Wellington. Wellington Conservancy Conservation Management Planning Series No 2. Wellington, New Zealand.
Department of Conservation. 1997. Conservation Management Strategy – Wanganui Conservancy. Wanganui Conservancy Conservation Management Planning Series No 2. Wellington, New Zealand.
Gabites, I. 1993. Wellington’s living cloak, a guide to natural plant communities. Victoria University Press. Wellington, New Zealand.
Hilton-Taylor, C. 2000. The IUCN 2000 Red List of Threatened Species. IUCN, Gland, Switzerland and Cambridge, United Kingdom.
Linklater, W., E. Cameron, K. Stafford, and C. Veltman. 2000. Social and spatial structure and range use by Kaimanawa wild horses. New Zealand Journal of Ecology, 24 (2): 139-152.
Molloy, L. 1994. Wild New Zealand. New Holland Publishers. London, United Kingdom.
Park, G. 1995. Nga Ururoa. Victoria University Press. Wellington, New Zealand
Soons, J. and M. Selby, editors. 1982. Landforms of New Zealand. Longman Paul, Auckland, New Zealand.
Stattersfield, A. J., M. J. Crosby, A. J. Long, and D. C. Wedge. 1998. Endemic Bird Areas of the World. Priorities for biodiversity conservation. BirdLife Conservation Series No. 7. BirdLife International, Cambridge, United Kingdom.
Wardle, P. 1991. Vegetation of New Zealand. Cambridge University Press, Cambridge, United Kingdom.
Prepared by: Sonia Frimmel