Location and General Description
This ecoregion comprises the northern half of New Zealand’s North Island, the warmest portion of New Zealand. The Northland Temperate Kauri Forests ecoregion comprises two botanical provinces, Northland province in the north and Auckland province in the south (Wardle 1981). The hilly Northland province has a variety of substrates, ranging widely in age and composition. Volcanic offshore islands and ultramafic outcrops are known for their plant endemism. Further south, topography transitions to steeply rolling country near Auckland, and then into the gentle hills of the Waikato Basin.
Vegetation throughout the ecoregion was once predominately warm-temperate forest, dominated by kauri trees. Kauri forests occur only in New Zealand’s Northland region and the Coromandel Peninsula. They thrive in warm, humid areas, and their current range reaches no further south than 38ºS. Rainfall averages from 1,000 mm to 2,500 mm per year. Kauri trees grow on a variety of substrates, from volcanic soils to sandstone, however, they tend to dominate on ridge tops and areas with infertile soils, giving way to broadleaf forests in the more fertile valleys (Burns and Leathwick 1996). Kauris podolize the soil, and shed bark, creating a layer of mor humus on the forest floor that can be up to three feet thick. Kauri forests used to cover most of the Northland, but today only isolated stands remain. The largest extant stands are in the Waipoua State Forest near Auckland (Burns and Leathwick 1996).
Kauri trees can grow in monotypic stands, but they also occur in mixed forests. Both forest types are classed as kauri forest, due to the prominence of these enormous trees. The common broadleaf tree species in mixed kauri forest include: tanekaha (Phyllocladus trichomanoides), towai (Weinmannia silvicola), kanuka (Kunzea ericoides), rewarewa (Knightia excelsa), and hinau (Elaeocarpus dentatus). The common podocarps are: rimu (Dacrydium cupressinum), thin-barked totara (Podocarpus hallii), miro (Prumnopitys ferruginea), and kohekoe (Dysoxylum spectabile; Burns and Smale 1990). Mature kauri trees form an emergent layer over the broadleaf-podocarp canopy. The understory of kauri forests is fairly open as the kauri trees block light from reaching the forest floor. The silvery tree fern (Cyathea dealbata) is common in the understory, as is kauri grass (Astelia trinervia), and several other grass species (Fleet 1986).
Only 8 percent of the original wetlands habitat of New Zealand remain (Stattersfield et al 1998), but a large portion are concentrated in this ecoregion. Wetlands are common near the coast, with abundant swamps and lakes, as well as interdune lakes along the west coast. Sites with impeded drainage are often species-rich, and dominated by ferns and sedges. An important peat community is found at the Kopuatai peat dome (Frazier 1999), and kahikatea (Dacrycarpus dacrydiodes) swamp forest once occupied a large portion of Whangamarino swamp, before being overcome by flooding and peat growth (Wardle 1991). Locally threatened or regionally endemic plants include Hydatella inconspicua, Lycopodium serpentium, Thelypeteris confluens, and Cryptostylis subulatus (Given 1995).
This ecoregion contains more than 620 vascular species, with approximately 60 taxa endemic to this ecoregion. Plant endemism is highest in the northern Northland province, partially as a result of the varied substrates found here and also as a result of fluctuating sea levels, which have at times cut off and isolated some regions, such as Te Paki, from the rest of the North Island (Wardle 1991). Ultramafic outcrops in this northern region are local centers of endemism, with approximately 12 endemic taxa. Offshore islands, including the Three Kings Islands and the Poor Knights Islands also contain high numbers of endemic and threatened species. Located 56 km northwest of Cape Reigna, the Three Kings Islands harbor the monotypic Elingamita genus, and rare plants such as Pennantia baylisiana and Tecomanthe speciosa (Given 1995). A number of warm-temperate plants reach their southern limits in this ecoregion, including kauri and mangrove (Avicennia resinifera), while southern plants thrive only on summit vegetation.
The kauri is a New Zealand endemic found only in this ecoregion. It can live for 2,000 years, growing to more than 40 m tall with a girth of more than 13 m. Mature trees have smooth cylindrical trunks with no branches on the lower half and often first branches are 20 m off the ground. Despite the immense size of kauri trees, they have a comparatively shallow, exposed rooting system which makes them vulnerable to windthrow. There are additional endemic plants in kauri forests, such as taraire (Beilschmiedia tarairi).
Kauri forests provide important habitat for a variety of native taxa. The noctural North Island kiwi (Apteryx australis mantelli) can be found in these forests, and New Zealand’s forest birds were once abundant. Today, bird populations have declined precipitously, due to predation by introduced cats (Felis silvestris), rats (Rattus exulans, R. norvegicus, R. rattus), stoats (Mustela erminea), ferrets (M. furo), weasels (M. nivalis), and brush-tailed possums (Trichosurus vulpecula). Nevertheless, the endangered North Island kokako (Callaeas cinerea) persists in the Waipoua forest, and fantails (Rhipidura fuliginosa), North Island robins (Petroica australis longipes), whitehead (Mohoua albicilla), tuis (Prosthemadera novaeseelandiae), and kereru, or New Zealand wood-pigeon (Hemiphaga novaeseelandiae ) are all present (Hilton-Taylor 2000). New Zealand snipe (Coenocorypha aucklandica) were once widespread on the North Island, but had largely declined by Polynesian settlement, persisting only on Little Barrier Island until the 1870s (Stattersfield et al. 1998). Endemic invertebrates can also be found in the forests, most notably the large, carnivorous kauri snails (Paryphanta spp.). Flax snails (Placostylus spp.) are restricted to coastal forests in Northland. The only native mammals in New Zealand are two species of bats, both of which are found in this ecoregion: the New Zealand long-tailed bat (Chalinolobus tuberculatus) and the lesser short-tailed bat (Mystacina tuberculata VU). Two primitive species of frog are endemic to this ecoregion, Hochstetter’s frog (Leiopelma hochstetteri) and Archey’s frog (L. archeyi), which both utilize moist forest litter.
Wetland habitats harbor a number of rare and endangered plant species, in addition to providing valuable habitat for aquatic birds. The greater jointed rush (Sporadanthus traversii) is regarded as nationally threatened. Wrybills (Anarhynchus frontalis) winter in this ecoregion in large tidal harbors but are otherwise resident on the South Island while New Zealand dabchicks (Poliocephalus rufopectus VU) are normally found on coastal lakes (Heather and Robertson 1997). The Firth of Thames, one of New Zealand’s most important coastal habitats, is located in this ecoregion. Up to 74 shorebird species have been sighted here, with a peak of 40,000 migratory birds utilizing the Firth at one time, including the New Zealand dotterel (Charadrius obscurus VU) and more than half of New Zealand’s wrybill population (Frazier 1999).
Offshore islands also harbor a number of endemic and globally threatened species, some of which have been relocated from the mainland to protect them from rats, cats, and other introduced animals. The endangered takahe (Porphyrio mantelli), little spotted kiwi (Apteryx owenii VU), stitchbird (Notiomystis cincta VU), critically endangered kakapo (Strigops habroptilus), and saddleback (Philesturnus carunculatus) have all been released on offshore islands (Hilton-Taylor 2000, Stattersfield et al 1998). Other species found on these islands include tuatara (Sphenodon punctatus), Duvaucel’s gecko (Hoplodactylus duvauceli), chevron skink (Leiopisma homalotum). The 1.3 km2 Middle Island in the Mercury Group is known for its lizard diversity, with ten species present, as well as some notable invertebrates - the centipede Cormocephalus rubriceps and tusked wetas (Molloy 1994). A number of ecoregional endemics are largely restricted to offshore islands, including Whitaker’s skink (Cyclodina whitakeri), the robust skink (C. alani), Macgregor’s New Zealand skink (C. macgregori), Oliver’s New Zealand skink (C. oliveri), and the moco ground skink (Oligosoma moco). The Three Kings Skink (Oligosoma fallai) occurs only the Three Kings Islands (Pickards and Towns 1988).
Kauri forest once covered approximately 12,000 km2 in the Northland. Today, only 800 km2 remain. With its rich soils, long coastline, and abundant fishing this ecoregion was heavily utilized by Maori. European settlement introduced new threats, and logging, resin collection, and brush fires severely degraded native kauri vegetation (Burns and Leathwick 1996). There are currently patches of mature kauri forest scattered across the Northland and the Coromandel Peninsula. All of the remaining kauri forests on Crown lands are now under the protection of the Department of Conservation, and most of the kauri on private land are also protected. The largest tracts of kauri forest can be found in the Waipoua Forest Sanctuary and the Waipoua Kauri Management and Research Area; together these reserves protect a continuous block of approximately 130 km2 on the west coast of Northland (Burns and Leathwick 1996). The Trounson Kauri Park also contains mature kauri forest. There are also three Ramsar sites in this ecoregion: Whangamarino, Kopuatai Peat Dome, and the Firth of Thames (Frazier 1999). Wetlands throughout New Zealand were drained and converted to farmland or livestock grazing. Kahikatea swamp forest communities have especially been reduced.
Types and Severity of Threats
Past threats to kauri forest are largely mitigated: logging and gum-tapping have both ceased, and controlled burning regimes have reduced the range and intensity of brushfires. Today, the biggest threats to the kauri forests are natural senescence and browsing by brush-tailed possums (Trichosurus vulpecula) (Payton et al 1997). The extant kauri forests are well protected, although browsing by deer and possums is still a problem, and efforts to regenerate kauri forests are largely successful. The avifauna of the kauri forests is less secure, and still suffers from predation by introduced mammals. Kiwis are especially vulnerable to roadkill and dogs. The Department of Conservation needs to continue its predator control efforts and guard against brushfires to effectively conserve kauri forest communities.
Offshore islands are still vulnerable to introduced predators, and wetlands are at risk from hydrological control and contaminated runoff. Alien plant species are problematic throughout New Zealand, but especially in the warmer climate of the Northland region. Nearly all weeds have spread from gardens or farms. The main weed species being targeted in Northland are wild ginger (Hedychium garderianum), cord grass (Spartina alterniflora), mistflower (Ageratina riparia), and wandering Jew (Tradescantia fluminensis) (NZDOC undated, Wardle 1991).
Justification of Ecoregion Delineation
The Northland Temperate Kauri Forests ecoregion contains the ‘Northland’ Centre of Plant Diversity (Given 1995). This ecoregion is a combination of Wardle’s (1991) ‘Northland’ and ‘Auckland’ botanical provinces. Vegetation was originally nearly all warm-temperate forest, and many warm-temperate species reach their southern limits in this ecoregion.
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Burns, B.R. and M.C. Smale. 1990. Changes in structure and composition over fifteen years in a secondary kauri (Agathis australis) and tanekaha (Phyllocladus trichomanoides) forest stand, Coromandel Peninsula, New Zealand. New Zealand Journal of Botany 28(2): 141-158
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Prepared by: Winnie Roberts