When we think of land, we think of soil. But soil is a relatively recent feature of the Earth's surface, the child of rocks and living things formed by the fusion of mineral sediment and organic matter. For more than 90 percent of our planet's 4,600 million year history, vegetation and organic soil did not exist. Life was confined to the oceans, rivers, and lakes while the land consisted of bare rock, sand, volcanic ash and dust. It was not until 460 million years ago that photosynthesising sea organisms colonised the shoreline and set the scene for the evolution of soils.
Seaweeds (large algae) and lichens (fungi which harbour microscopic algae) were the first coastal land-dwellers. In time, one line of green algae gave rise to the plant kingdom. The first land plants were bryophytes (liverworts, hornworts and mosses) whose transition from algae to plants probably occurred in tidal marshlands (Niklas, 1994; Palmer, 1995; Stevens et al., 1995). Because they had simple leafless stems, no roots and no vascular systems to transport water and nutrients within their tissues, the bryophytes were confined to moist areas, forming green matlike carpets on damp sediment and wet rock faces. Fungi, particularly lichens, also colonised these areas, forming symbiotic relationships with the plants.
As each generation of plants and fungi died, they formed matted layers of humus. Over millions of years, these layers of organic matter mixed with eroded rock sediment to form something new on the planet-organic soil. Today the mosses and lichens still occupy damp areas and they still play the role of colonisers on rocks and bare ground where other plants cannot take root.
Around 410 million years ago, a new group of plants evolved from these primitive forms-ferns and similar plants. Because they had vein-like vascular systems, the ferns and other fern-like plants could absorb moisture and nutrients from the soil through their rhizomes (underground stems) and transport the nourishing sap throughout their tissues. They were thus able to grow taller than the bryophytes and intercept more sunlight.
Soon even taller plants, with true roots, had evolved-the first trees. By 360 million years ago, evergreen gymnosperm forests were widespread. The tree roots accelerated soil formation by breaking up rock, loosening sediment, and binding the soil. Soil and vegetation, therefore, evolved together within the last 9 percent of Earth time. As tree seeds were dispersed by wind and water, a green and brown carpet spread slowly over the planet's barren places, establishing a life support system for the land animals that were to follow.
When the first soils and trees were evolving on Earth, New Zealand did not exist. It was just a long strip of sea floor off the coast of the supercontinent, Gondwana, near the region that would later split up into Antarctica and Australia. Whereas parts of Australia contain 3.8 billion year old rocks, the oldest New Zealand rocks were formed less than 600 million years ago from sediment washed into the sea by Gondwana's rivers. For millions of years the sediment accumulated, with each layer compressing the ones below into sandstones and other soft sedimentary rocks. Marine animals died and became fossilised in the sediments, leaving a visible record of their life and times. Some of the lower sedimentary layers were compressed into hard metamorphic rocks while volcanic igneous rock was added to the mix by underwater volcanoes near the edge of Gondwana's tectonic plate.
Between 120 and 140 million years ago, the New Zealand region was thrust out of the water by violent buckling of the tectonic plate (Stevens et al., 1995). The new land mass stretched from New Caledonia in the north to Campbell Island in the south, with arms extending eastwards to the Chatham Islands and westwards along the Lord Howe Rise (see Fig 8.1). It was just off the Gondwana coast and formed land bridges with both the Australian and Antarctic parts of the supercontinent. For about 50 million years, this exposed new land mass was rained on, creating silts and sediments which were colonised by various Gondwana plants and animals-ferns, kauri and podocarp trees, dinosaurs, tuatara, large walking birds, primitive spiders, insects and other invertebrates. Mammals had not yet evolved. Nor had snakes.
About 125 million years ago, a new plant group began to spread throughout Gondwana the angiosperms, or flowering plants. Unlike the sombre gymnosperms, these plants relied on animals to pollinate and disperse their seed. They evolved the ability to attract birds and bees using brightly coloured flowers, strong fragances, and sweet pollen, nectar and berries. They proliferated into blossoming trees, flowering shrubs and herbs and grasses. Although they now dominate the plant world, the angiosperms did not become so dominant in New Zealand because they had barely reached the eastern edge of Gondwana when the continent began breaking up. Its underlying tectonic plate, the IndoAustralian plate, began moving east, shunting the New Zealand land mass out to sea.
By 80 million years ago, the New Zealand-New Caledonia fragment was on its own, a raft of ferns and dark gymnosperm forests with just a few species of opportunistic angiosperms-notably the high altitude beech forests. The distance from Australia was still very small and a chain of islands still linked New Zealand to Antarctica. These land masses remained in close proximity until about 55 million years ago, allowing a continuing stream of plant and animal colonists to swim, float or fly here. The ancestral seeds of the redflowered pohutukawa trees, for example, were carried here by birds or wind currents 5365 million years ago (Stevens, et al., 1995).
The pohutukawa seeds arrived in the wake of a global catastrophe which has not been equalled since. As much as half the world's species became extinct in a geological instant 65 million years ago (see Chapter 9). The most notable victims were the dinosaurs, but many other animals also died out, both on land and in the sea. Plants were less severely affected, but many of these disappeared also. The mass extinction is now generally attributed to gross climatic disturbances caused by the impact of a large asteroid (Ward, 1994).
It took 20 million years for global biodiversity to recover as new species of plants and animals slowly evolved from the survivors. By then, the New Zealand land mass was becoming too remote for new colonisations, though some flying species (birds and bats) continued to arrive. They did so only to face another, more protracted, catastrophethe Oligocene drowning (Cooper and Millener, 1993).
Textual description of figure 8.1
The landmass of New Zealand precariously straddles the boundary between the Indo–Australian and the Pacific plate. The boundary between the two plates cuts through the south-west to the north-east of the South Island, along the Alpine fault line and then north past the east coast of the North Island up towards Samoa.
Underneath New Zealand these plates are sliding under and over each other in opposite directions. The North Island, which is on the leading edge of the Indo-Australian plate, is moving to the north-east, as the Pacific plate drives below it.
As the IndoAustralian plate pushed it further out to sea, the New Zealand land mass began to sink. It continued to do so until 25-30 million years ago. At its lowest point, barely a fifth of present day New Zealand may have been above sea level. The remaining plants and animals survived on mountains which became a string of islands. These islands remained separate for millions of years so that their populations had time to evolve into different species-giving rise to 11 or more moas, four or more kiwis, numerous lizards, many invertebrates (e.g. land snails, flatworms, spiders, insects) and plants (e.g. mosses, liverworts, beech trees, and herbs). During the Oligocene drowning many unknown species were probably wiped out completely.
The sinking stopped about 25-30 million years ago as the floating landmass became jammed in the subduction zone between the Indo Australian and the Pacific plate. Too big to be swallowed, the land mass ground to a halt, precariously straddling the boundary between the two plates. Pushed from the west and blocked from the east, the landmass was squeezed upwards to form ranges of towering mountains. It has remained in this position ever since, with new tectonic movements periodically forcing the land higher while heavy rain works constantly to erode it back down. The latest mountain building period began about six million years ago.
Throughout all this, the shape of the land has constantly changed, sometimes forming a single elongated land mass, sometimes forming several large islands. Ice ages during the last two million years have added a further dimension to these changes. As the land was being forced up, the sea levels around were independently rising and falling in response to the formation and melting of the polar ice caps. Meanwhile vast glaciers of ice were gouging deep valleys and lake basins among the South Island's mountains and the plunging snowline was fragmenting the alpine herbfields, grasslands and scrub into evolutionary islands.
In the midst of the latest ice age, 22,590 years ago, one of the world's largest volcanic eruptions blasted a huge hole in the centre of the North Island, forming Lake Taupo and temporarily obliterating all life in the central and eastern North Island (Carter, 1994). When the ice age ended, around 12,000 years ago, the rising sea flooded the low-lying land bridge between North and South Islands, forming Cook Strait and dividing the land into the shape we know today.
Below us, the plates are sliding under and over each other in opposite directions. The North Island, which is on the leading edge of the IndoAustralian plate, is moving to the northeast, as the Pacific plate dives beneath it. This subduction zone is marked by a line of trenches in the ocean floor running north from the Kaikoura coast up past the east coast of the North Island and beyond the Kermadec Islands (see Figure 8.1).
As the Pacific plate burrows beneath the North Island, it has caused the east coast and the Marlborough region to buckle and fracture into rugged hill country broken by faultlines. It also generates volcanoes and geothermal eruptions in the centre and west of the North Island as its diving edge melts under the intense pressure and friction more than 200 kilometres below the surface. At the other end of the country, the tectonic power struggle is reversed. Beneath the ocean surface, south west of Fiordland, another line of trenches in the sea floor marks another subduction zone. Here, the Pacific plate, on which most of the South Island rides, is moving to the south west, forcing the Indo Australian plate beneath it. Volcanic activity associated with this is confined to the ocean.
Between the two subduction zones, the opposing forces twist and fracture the landscape, creating the long Alpine Fault in the South Island, and multiple fault lines in the upper South Island and in the lower and eastern North Island (see Figure 8.2). These are the Marlborough, Wellington and Wairarapa fault systems. Fault lines also slice up into Hawke's Bay and Gisborne on the east coast and through the centre of the North Island from Wanganui to the Bay of Plenty. The grinding and shoving of the tectonic plates is also forcing the mountains east of the Alpine Fault to rise. In the Haast area they have risen some 120 metres in the past 16,000 years-a rate of seven to eight millimetres per year (Simpson et al., 1994).
On either side of the Alpine Fault line, the South Island is being pulled in different directions. The west coast is being carried northeast by the IndoAustralian plate, while the rest of the South Island is travelling southwest with the Pacific plate. The extent of the movement is revealed by the red rocks of Nelson (west of the fault line) and those of Southland (east). Once connected, these rocks are now separated by of some 480 kilometres (Wallace, 1995).
Hundreds of earthquakes occur every year, though most are imperceptible, and very few cause even minor damage. They are most frequent in the east and south of the North Island and in the top half of the South Island. They are least frequent north of Auckland and south of Banks Peninsula (see Figure 8.2).
Shallow earthquakes, which occur within the 35-40 kilometre thick crust, are more dangerous and more common than the deep ones. They cause mass movement erosion, landslides, and damage to roads and buildings. Repeated shallow quakes have fractured the soft sedimentary rocks in much of the eastern North Island and Marlborough, making the hills in those areas more susceptible to erosion. Deep earthquakes occur below the crust, down to depths of 200-600 kilometres. They are caused by the subducting plate as it dives beneath the over riding plate. The deep quakes are frequent in the volcanic zone of the North Island and off the southwestern corner of the South Island, but are rarely felt at the surface.
Compared with other parts of the earthquake- prone Pacific rim-such as the Philippines, Japan, California, Mexico and the Pacific coasts of Central and South America-New Zealand has a moderate frequency of earthquakes, with most no greater than 4 on the Richter scale. A shock of Richter magnitude 6 or above occurs on average about once a year, a shock of magnitude 7 or above once in 10 years, and a shock of about magnitude 8 perhaps once a century.
Although one of the fatal Napier earthquakes of 1931 reached 7.8 on the Richter scale, the largest earthquake since European settlement had a magnitude of 8.1. This occurred in 1855 along the Wairarapa faultline, and was centred in Cook Strait. It was felt throughout most of New Zealand and raised the land on the southern coast of the Wairarapa by some 6.5 metres and around Wellington harbour by 1.5 metres (Ansell and Taber, 1996). The Rimutaka range between Wellington and the Wairarapa was badly damaged by landslips which carried away almost a third of the vegetation and much of the soil. Based on the geological evidence of previous earthquakes, scientists estimate that the Wairarapa Fault has a major rupture every 2,000 years while the central Wellington Fault experiences such shakes every 450–670 years. The Wellington Fault's last big movement was 350 years ago. The 7.1 Wellington earthquake of 1942 was not a major rupture.
Textual description of figure 8.2
New Zealand lies over both the Indo-Australian and Pacific tectonic plates. The Alpine Fault runs along the Southern Alps, diverging to the east side of the North Island and through the North Island to the Bay of Plenty and north-east of New Zealand. The area between the North Island fault lines is a disturbed boundary zone.
Volcanoes are located in the North Island primarily around the major fault lines. The Taupo Volcanic Zone is the main geothermal area.
Earthquakes are generally situated along fault lines throughout New Zealand and in the surrounding area, particularly in the disturbed boundary zone.
Source: NZ Meteorological Sevice
Many earthquakes have been recorded with magnitudes of between 6 and 7, with widely varying impacts. The 1987 Edgecumbe quake, in the Bay of Plenty, had a magnitude of 6.3. It created ruptures up to seven kilometres long and caused land northwest of the town to drop two metres. The property damage came to more than a billion dollars-four times the cost of New Zealand's most expensive storm, Cyclone Bola, which hit the East Coast the following year (Ministry of Civil Defence, 1994). Conversely, the magnitude 6.6 earthquake which occurred in 1994 near Canterbury's sparsely populated Arthur's Pass, caused slips and road damage, but relatively little property damage.
Earthquakes have occurred more frequently since 1990 than in the previous decade. This does not reflect a worsening trend but merely a return to normal following the relatively low incidence of large earthquakes during the 1970s and 1980s (Statistics New Zealand, 1995).
Active volcanoes are densely clustered in the central and western North Island. Many of the soils and rocks in this area are therefore of volcanic origin. The active volcanoes include Ruapehu, Tongariro, Ngaruahoe, White Island and Mount Tarawera. Others, such as Mount Taranaki (or Egmont), and Rangitoto may be considered dormant at present, although they are still regarded as significant hazards. Auckland sits above a volcanic field whose surface is pockmarked by the domes of more than 60 small extinct volcanoes.
New Zealand's volcanoes have been relatively quiet in the past 1,000 years. However, large eruptions still occur. Within the last 150 years seven eruptions have led to loss of human life (Statistics New Zealand, 1995). The largest eruptions this century have been from Mount Ruapehu in 1945 and again in 1995 and 1996.
The largest eruption since European settlement was that of Mount Tarawera in 1886 which killed more than 100 people and obliterated New Zealand's greatest tourist attraction at the time-two spectacular slopes of waterfalls, pools, and unusual rock formations known as the Pink and White Terraces. All plants and animals, including insects, within an eight kilometre radius of the eruption were destroyed, which led hopeful Tauranga fruit growers to speculate-fruitlessly as it turned out-that the codlin moth may have been exterminated (Grayland and Grayland, 1971). Ash from the eruption spread over 1.4 million hectares, and mud from it now forms the surface soil over a 12,000 hectare area. Despite the immediate effects, vegetation began to regenerate just a few years after the blast and now, a century later, recovery is complete.
A far larger eruption from Lake Taupo about 1,875 years ago (around a.d. 115) flung pumice, ash and rock over a wide area. Though much smaller than the eruption which formed the lake 20,000 years earlier, it devastated approximately two million hectares of forest throughout the central and eastern North Island from Hawke's Bay to the Bay of Plenty. Until recently, it was assumed that most of the area remained deforested for many centuries after the blast. Most maps of New Zealand's prehistoric forest cover still show a large barren area between Taupo, the Bay of Plenty and Hawke's Bay. Recent studies of fossil pollen, however, indicate that the entire area was reforested within 300 years, with tall matai and totara forests taking root in the new pumice soils (Stevens et al., 1995).
The constant tectonic activity has produced a restless landscape of rising mountains and steep hills interspersed with swift rivers, lakes and flood plains. The steepness of the slopes and the softness of much of the underlying rock, leads to high erosion rates in many parts of New Zealand. Three quarters of New Zealand's land is more than 200 metres above sea level and 14 percent is in the alpine zone, above the forest line. The 18 tallest peaks in the Southern Alps exceed 3,000 metres, with the tallest, Mount Cook, reaching 3,754 metres. The tallest North Island peaks are volcanoes, three of which exceed 2,000 metres.
The erosion of the uplifted mountains by rain and rivers has laid down thick deposits of sediment near river mouths and over large areas of floodplain. Major plains are found in Canterbury, Southland, Hauraki, Hawke's Bay, Bay of Plenty, and Manawatu. However, flat land (i.e. land on a slope of less than three degrees) makes up only 15 percent of New Zealand's total area. 'Rolling' lands (with slopes of from three to 12 degrees) make up a further 15 percent. Nearly all the remaining land, (around 18 million hectares, or twothirds of New Zealand) is hill country and the vast majority of it is classed as steep land (see Table 8.2).
Both terrain and soils owe much of their character to the rocks which make up the underlying land mass. The three main rock types are:
- igneous rock which originates in molten magma from within the Earth and takes the form of intrusive rock (e.g. granite and gabbro) where the magma has cooled and hardened beneath the surface, and volcanic rock (e.g. rhyolite and basalt) where the rock has formed at the surface following eruptions;
- sedimentary rock (e.g. sandstone, greywacke, argillites such as mudstone and siltstone, and limestones such as dolomite and chalk) which accumulates from the erosion of other rocks; and
- metamorphic rock (e.g. schist, gneiss and marble) which is created when intense pressure, heat or chemical activity changes the structure and texture of a rock, turning limestone, for example, into marble.
|North Island||South Island*||New Zealand|
|Types of terrain||Area (hectares)||%||Area (hectares)||%||Area (hectares)||%|
|Flat and rolling land (under 12°)||3,741,000||32||4,151,000||27||7,892,000||29|
|Hilly land (from 12° to 28°)||3,550,000||31||2,073,000||13||5,623,000||21|
|Steep land (more than 28°)||4,096,000||36||8,757,000||57||12,853,000||48|
|Lakes and riverbeds||109,000||1||329,000||3||534,000||2|
|Land below 300 metres||6,474,000||56||4,731,000||31||11,205,000||42|
|Land between 300 and 900 metres||4,404,000||38||6,004,000||39||10,408,000||39|
|Land between 900 and 2,100 metres||618,000||5||4,484,000||29||5,102,000||19|
|Land higher than 2,100 metres||-||191,000||1||191,000||<1|
* South Island figures include Stewart Island (177,000 hectares) but exclude Chatham and other off-shore islands
Adapted from Molloy (1980)
Almost three-quarters of New Zealand rocks are sedimentary. The South Island has older rocks than the North Island and a greater proportion of metamorphic rock. Some of its oldest rocks date back to the preGondwana seafloor, more than 570 million years ago. These rocks are found in Fiordland, Westland and Nelson and are relatively resistant to erosion. Although old in human terms, they are very young geologically when compared to the 3,800 million year old rocks found in parts of Australia. Most New Zealand rocks are less than 350 million years old, consisting of sedimentary rocks, such as greywacke and argillite, throughout eastern areas, from Marlborough to East Cape. Metamorphic schist occurs over a large part of Otago.
North Island rocks are mostly sedimentary, and are very erodible in some areas. On the east coast the soft sedimentary rocks have been severely crushed and shattered by repeated earth tremors and fault line movements. This makes them a very unstable base for the soil above. Rocks in the central plateau, and areas around Auckland, the Coromandel Peninsula, and Northland, are of volcanic origin. They, and the young soils they bear, also tend to be erodible.
The combination of sedimentary rock and the erosive effect of water has produced some attractive and bizarre landscape features, such as the karst limestone terrains popular with tourists in many parts of New Zealand. These include glow worm caves in such places as Lake Te Anau in Fiordland and Waitomo in the King Country, the West Coast's Pancake Rocks at Punakaiki and a variety of sculpted pools, caves, arches, and unusual rock formations in various parts of the North and South Islands. The karst landscapes are formed when water dissolves calciumcontaining rocks such as limestone and marble.
Minerals are an important component of our rocks, sediments and soils. They affect the chemistry of our waters and soils and also provide economically useful materials. Textbooks usually define minerals as inorganic substances that have formed naturally in rock or soil. Industrial definitions tend to include anything that can be mined, including fossilised organic substances such as coal.
The most economically important minerals are aggregates of sand, rock, and gravel (used for road and building construction), gold, silver, coal, and ironsand. Limestone is also important. It is used to make fertiliser, cement, building blocks, and even the backing for 'all wool' carpets. A number of other economic metals exist in small to moderate quantities (e.g. copper, lead, zinc, tungsten, manganese, mercury, uranium, aluminium, antimony, arsenic, chromite, nickel, monazite and rutile). Among the minor nonmetals that have been mined at some time are the clays, bentonite (used for filler) and halloysite (used for ceramics), silica sand (used to make glass), sulphur and phosphate (fertilisers), serpentine, feldspar, pumice, woollastonite, magnesite, asbestos and diatomite.
The soils which evolved wherever New Zealand's rocks met forests, tussocks and wetlands varied with the terrain, climate, parent rocks and vegetation. The hundred or more soil types can be broadly lumped into three main categories:
- pumice soils are derived from volcanic rhyolite and are widespread in the central plateau and geothermal zone of the North Island;
- ash soils (e.g. red, brown, and yellow-brown loams, brown granular clays) are derived from volcanic basalt and are common in Taranaki, Waikato, parts of Northland and also western Southland; and
- sedimentary soils (e.g. yellow-brown and yellow-grey earths, sand and recent silt) are derived from sandstones, siltstones and mudstones, and are widespread on plains, rolling hill country and coastal areas throughout both islands.
|Land classes||Able to sustain the following productive uses:||Area (hectares)|
|I, II, III||Multiple use farming (i.e. cropping, orcharding, and other uses based on regular cultivation) ; |
or Pastoral farming;
or Native vegetation (e.g. forest, scrub, wetland.)
|3,828,000 (14% of land area)|
|IV,V, some VI||Pastoral farming (i.e. permanent pasture in which cultivation is only for pastoral renewal) ; |
or Native vegetation (e.g. forest, scrub, wetland)
|4,537,500 (17% of land area)|
|VI, some VII||Restricted pastoral farming (i.e. pasture with erosion controls, such as tree planting, temporary land retirement etc.) ; |
or Native vegetation (e.g. forest, scrub, tussock, duneland)
|7,577,600 (28% of land area)|
|VII||Erosion control forestry (i.e. forestry requiring specific management systems to minimise erosion) ; |
or Native vegetation (e.g. forest, scrub, tussock, duneland)
|3,612,500 (13% of land area)|
|VIII||Native vegetation (e.g. forest, scrub, tussock, duneland, herbfield)||6,219,600 (23% of land area)|
|Not classified||Towns, lakes, rivers, islands, and undefined areas.||1,274,800 (5% of land area)|
Adapted from Eyles and Newsome (1991) and Eyles (1993)
Despite their diversity, most of our soils tend to be thin and prone to acidification, with moderate to high carbon levels (reflecting their forest origins) and low nutrient levels (reflecting the geological youth of their parent rocks). From an agricultural or commercial forestry perspective, they are inherently deficient in nitrogen, phosphorus, and sulphur and, to a lesser extent, potassium and boron.
Often lime is needed to reduce acidity, and sometimes trace elements are needed for plant and animal nutrition (e.g. molybdenum, selenium, cobalt). The most widespread group of soils, covering nearly half the country, are those on steep land. Although their structure and chemistry vary, steepland soils are all erosion prone, particularly when their vegetation cover is removed.
In 1952, following the example of the U.S. Soil Conservation Service, the Water and Soil Division of the Ministry of Works and Development adopted the Land Use Capability (LUC) classification system. This classifies land according to its risk of soil erosion under different land uses. The classification is based on climate, rock and soil type, slope, erosion type and degree, and vegetation cover. The land use classes range from the fertile Class 1 and 2 land, which has a low erosion risk under cultivation and covers a mere 6 percent of the country, to the steep, low fertility, Class 7 and 8 land, which covers 44 percent.
Using survey data from the New Zealand Land Resource Inventory 197579, the land use capability classification has been further developed by Eyles and Newsome (1991) who took into account the potential difficulty of erosion control and the assessed rate at which land would naturally revert to indigenous vegetation. From this, they were able to estimate the areas of New Zealand which could sustain particular activities (see Table 8.3). They concluded that barely 32 percent of New Zealand is capable of sustained pastoral use (i.e. grazing) without significant erosion control measures being applied. Some of the erodible pasture land is in eastern parts of the South Island, but most is in the North Island.
New Zealand's vegetation cover has changed considerably in the past 700 years, with the most dramatic changes occurring in the past century. Each of the major vegetation types is described briefly below, but the primary distinction is between the indigenous vegetation, which evolved here over millions of years, and the exotic vegetation which was introduced very recently by human beings.
The indigenous vegetation is predominantly rainforest, but a variety of other vegetation types exist too, resulting in a diverse range of land-based ecosystems. This is recognised in the Department of Conservation's Protected Natural Area Programme which divides New Zealand into 268 'ecological districts', localities where the geology, topography, climate, and biology, as well as the broad cultural pattern, inter-relate to produce a characteristic landscape and range of biological communities (Myers et al., 1987).
Only 2.5 million hectares (9 percent) of New Zealand was originally above the treeline (McGlone, 1989). Perhaps half of this alpine area carried tussock, herbfields and scrub. The rest was bare rock and ice. Below the treeline, pockets of tussock and scrub also occurred in areas where the forest cover had been permanently inhibited by such factors as temperature, infertile soil, salty conditions, low rainfall and poor drainage, or temporarily removed by windfall, volcanoes, earthquakes and lightning strikes. In these zones low, profusely branching (divaricate) shrubs and a variety of herbs and grasses were dominant. These non-forest ecosystems took many different forms, often occupying the buffer zone at forest edges or forming highly adapted plant communities in extreme zones near the ice caps and on the coastline.
In total, the grass, herb, and low shrubland communities seem to have covered no more than 5-10 percent of New Zealand's land area, flourishing mostly in the wettest, driest or highest areas, such as river terraces subject to regular flooding, frost-prone valley floors, steep cliffs, active sand dunes, leached shallow soils, and areas temporarily deforested by wind, fire, volcano or earthquake. A working figure for this report assumes an original land coverage of approximately 5 percent tussock grassland and 5 percent scrub, though this may be an overestimate.
Many of the non-forest plants were ecological opportunists, specialising in the temporary occupation of disturbed sites after landslips, dune movements or forest fires, but others were more stolid, forming the permanent vegetation cover in areas that were hostile to forests. Despite their relatively limited area these smaller vegetation communities contain a large proportion of New Zealand's plant biodiversity.
Away from these marginal zones, the land was covered by an unbroken blanket of evergreen forest and tall shrubland. The actual percentage of New Zealand that was under forest before humans arrived has been variously put at 78 percent (Wendelken, 1976; Molloy, 1980; King, 1984; Froude et al., 1985), 85-90 percent (McGlone, 1989) and 90 percent (Flux, 1989; Anderson and McGlone, 1992). The lower estimates date from a time when it was thought that the deforestation of the central North Island by the Taupo eruption had been more extensive and long-lasting than it was. For this report, a working figure of 85 percent is used (23 million hectares), though the actual forested area may have been anywhere between 80 percent (21.5 million hectares) and 90 percent (24.3 million hectares).
The exotic vegetation cover that has replaced large areas of forest, tussock, and wetland now extends over 45 percent of the country. It includes some 9.6 million hectares of exotic grasslands, 1.6 million hectares of exotic forests, and almost a million hectares of crops, horticultural land, suburban lawns and gardens, golf courses, sports fields, public parks and road verges.
Although almost 2,000 exotic plant species (out of some 25,000 introductions) are known to have become established in New Zealand, with the possible total running as high as 6,000, only a few of these are widely grown (Halloy, 1995). Generally, the area of exotic plant cover has low biodiversity, with fewer than 50 species dominating more than 95 percent of the domesticated land area.
The vast majority of exotic plant species exist either as weeds or as cultivated plants in gardens, nurseries and scientific collections. As weeds, they can sometimes reach higher levels of biodiversity on conservation land than on farmland, even outnumbering native species. For example, the Whitiau Scientific Reserve at the mouth of the Whangaehu river, has one of the most diverse native dune plant assemblages in the region (120 species) but an even greater diversity of exotic weeds (134 species) (Ogle, 1996).
The most widespread exotic species, covering most of the pasture grasslands, are ryegrasses (Lolium spp.) and clovers (Trifolium spp.) sown in a roughly 80 percent to 20 percent ratio. In some places, depending on climate, soil and terrain, these may be accompanied or replaced by any of 20 other exotic pasture species. Outside the pasture land, the dominant exotic species is radiata pine (Pinus radiata), which covers 90 percent of the planted forest area. Beyond the pasture and forest areas, 95 percent of the remaining exotic area is dominated by just 19 plant species, of which the most widespread is barley ( Hordeum vulgare) (Halloy, 1995).
The following is a brief description of the main types of indigenous and exotic vegetation cover in New Zealand.
Before human settlement the natural tussock grasslands were very limited in area. They probably extended over no more than 1.5 million hectares (roughly 5 percent of the country), mainly in the high country of the South Island. The total area is hard to quantify because much of the 23 million hectares potentially available for tussock also bore low shrubland, alpine scrub and bare scree.
Most of the pure grassland was in the subalpine zone above 1,200 metres, where snow tussocks (Chionchloa spp.) predominated, some species growing as tall as 1.5 metres. At lower altitudes, various short tussocks, growing up to half a metre, were predominant. They included hard and alpine fescue tussock ( Festuca spp.), blue and silver tussock (Poa spp.), and bristle tussock, or danthonia (Rytidosperma spp.). These short tussocks occurred in small patches in lowerlying dry or waterlogged areas, such as the Cromwell Basin and the banks of regularly flooding rivers like the Upper Ahuriri in the McKenzie Country, the Travers Valley of Nelson Lakes National Park, and the gravels and lapilli fields formed by volcanic debris in the Tongariro National Park.
In very dry seasons or areas (e.g. Central Otago), large patches of scrub and forest were burnt off at least once or twice every thousand years by lightning strikes and (in the North Island) volcanic eruptions. Tussock grassland quickly colonised the burnt out areas. Over subsequent centuries the forests would slowly regenerate and replace the tussock only to be eventually struck down again by fire.
This natural cycle has been interrupted over the past 600-700 years by repeated human-lit fires which allowed the tussock to expand greatly at the expense of scrub and forest. Snow tussock became widespread below the 1,200 metre contour, extending down to 900 metres. Below this, fescue and silver tussock became dominant, though the tall red tussock (Chionochloa rubra) became widespread in the wet soils of Southland and the central North Island. Following European settlement, sheep and rabbits entered the grasslands, fires were lit even more frequently, and the snow tussocks began to retreat. In many areas they were replaced by short tussocks which, in turn, are now being replaced by exotic grasses, ground-hugging rosette herbs called hawkweeds (Hieracium spp.), and native alpine cushion plants called scabweeds (Raoulia spp.).
In summary, most of the tussocks below 1,200 metres are colonisers that took advantage of forest fires lit by humans. Outside the driest parts of Central Otago, only human fires and grazing animals have prevented reversion back to scrub or forest. If allowed, native trees such as Hall's totara, kanuka and beech would slowly reestablish in many South Island areas. Similarly, most of the red tussock grassland of the central North Island is fire-induced and is now developing into scrub. Despite their cultural origin, however, the expanded tussock grasslands have great ecological importance, providing secure habitat for unique plants and animals formerly restricted to very small areas (Ashdown and Lucas, 1987).
Dunelands consist of dry sand ridges (dunes) and the damp hollows between them (slacks). New Zealand has over 300,000 hectares of sand dunes but only a small area of this has natural duneland vegetation. Most of the dunes are covered in pasture or exotic forest, except for some 50,000 hectares of foredunes (i.e. dunes directly adjacent to the sea), which are dominated by sand-binding dune grasses, and a further 40,000 hectares of backdunes which are covered in scrub (Hunter and Blaschke, 1986; Newsome, 1987).
At the time of Polynesian settlement, the coastal foredunes may have had an area of 60,000 hectares (King, 1984). Behind them, the extensive backdunes were covered in scrub and native forest. There is evidence of widespread movement of the North Island sand dunes both before and after Māori colonisation (McGlone, 1983; McFadgen, 1985). Climatic factors, such as tropical storms, played a significant role (McFadgen, 1985, 1989; Anderson and McGlone, 1992) but human firing of forests and dune vegetation appears to have contributed as well, leading to dune destabilisation and sand drift (McGlone, 1983, 1989; Anderson and McGlone, 1992).
The foredunes were dominated by the endemic sandbinding sedge, pingao (Desmoschoenus spiralis) and the native dune grass, raumo or spinifex (Spinifex sericeus). The back dunes, which sometimes extend hundreds of metres inland, were occupied by other indigenous species such as the sand grass Austrofestuca littoralis, sand convolvulus (Calystegia soldanella), pohuehue (Muehlenbeckia complexa), clubrush (Isolepsis nodosa), sand coprosma (Coprosma acerosa) and tauhine (Cassinia leptophylla). These plants restrict wind action on the sands and build up soil humus levels until the dunes are stabilised. At that point, the back dunes would be colonised by scrub and forest in wet areas, and native grasses in dry areas (Partridge, 1992).
Between the dunes, the presence or absence of water has a marked effect on the vegetation communities which form. In dry areas, where the sand is windblown, vegetation is often absent or restricted to such species as pimelea (Pimelea arenaria), a sand daphne, and scabweed (Raoulia australis), a cushion plant. In dune slacks (moist hollows) wetland plants such as Selliera radicans, Leptocarpus similis, or harakeke flax, take root. In wetter areas dune lakes form, often fringed with the native reed, raupo.
Among the specialised dune animals are several native moths and butterflies, some of which are restricted to the dune environment, as well as the sand dune hopper, whose paddle-like legs are adapted to sand digging, the speckle-coated sand beetle and the nocturnal sand scarab beetle. The dune slacks and lakes often have aquatic insects and may even contain rare fish. For example, the Northland dune lakes are the only habitat of the dwarf inanga (G alaxias gracilis) a threatened species of indigenous fish.
Today, most of the dune ecosystems have been replaced by introduced pasture grasses and exotic pine forests (Newsome, 1987).
In most of the 52,000 hectares that remain, introduced marram grass (Ammophila arenaria) is now the dominant sandbinder, assisted by the nitrogenfixing lupin, Lupinus arboreus. The roots and stems of the marram grass are more effective sand-binders than pingao, and were deliberately planted in many areas to stabilise dunes whose original vegetation had been burnt off or grazed by livestock.
The 'pure' alpine and subalpine herbfields cover nearly 200,000 hectares, though many of the herb species within them are also present over two million hectares of high altitude tussock and scrub. They are dominated by various species of alpine daisies, notably those of the genus Celmisia, as well as buttercups, cushion and mat shaped herbs and grasses, rosette shaped herbs, lichens, and, in areas of poor drainage, rushes, sedges and umbrella ferns.
Despite their small area, the herbfields have considerable plant diversity, with more than 600 species. Although many are closely related, more plant species are found here than in the forests (Dawson, 1988). Their harsh environment has created many opportunites for evolutionary divergence from the ancestral populations. The inhospitable conditions have also provided some protection from human, stock and weed invasions. Despite this, the larger herbs have been depleted by introduced deer, goats, chamois, tahr, and hares which run wild in these mountains.
Shrublands, commonly called scrub, are dominated by ferns, bushes and small trees. They have always been an important part of New Zealand's vegetation cover, harbouring much of our plant biodiversity, though their original area was probably no more than 1.5 million hectares. Sometimes shrublands are the dominant vegetation in marginal subalpine and coastal environments. In other cases they are a successional stage in the regeneration of indigenous forest, particularly on disused or neglected farmland.
In the North Island and moister areas of the South Island, the scrub communities are dominated by mixed broadleaved shrubs, manuka (Leptospermum) and kanuka (Kunzea) trees and bracken. In the drier parts of the South Island, matagouri (Discaria toumatou) dominates. These days, introduced species, such as gorse (Ulex europaeus), broom (Cytisus scoparius), and sweet brier (Rosa rubiginosa) also dominate in some localities.
Before human settlement, scrub occurred mostly in sub-alpine and coastal zones, around wetlands and lake edges, on the Canterbury river flats, and in areas of forest regeneration following natural events such as windfall and lightning strikes. With deforestation and the establishment of extensive grasslands, scrub has increased considerably in cutover forest, on forest margins, in unimproved or abandoned pasture, and in disturbed dunes and wetlands. Introduced browsing animals, namely possums, goats and deer, have also turned many areas of mature forest into scrub and fern ecosystems. These animals eat young plants and shoots, preventing new young trees from replacing old dying trees. They also kill some mature trees outright.
Around 7.5 million hectares of scrub-associated vegetation were mapped in New Zealand a decade ago, though in much of this scrub was not the dominant component of the vegetation. On around 4.2 million hectares the scrub occurred on pastoral grassland. On almost one million hectares it was scattered among subalpine tussock. On a further 1.3 million hectares it was part of regenerating forest vegetation. Only about one million hectares were classified solely as scrub communities (Newsome, 1987).
The forests which covered about 85 percent of New Zealand contained over 100 identifiable forest types, distiguishable to the expert eye by the mix of tree species within them (Nicholls and Herbert, 1986). A simplified classification for non-experts identifies three major forest classes: kauri dominated forests in the north, beech forests at higher altitudes and much of the south, and podocarp hardwood forests at lower altitudes and stretching from one end of the land to the other. Being conifers, kauri (Agathis australis) and podocarp trees are distant relatives of the northern hemisphere pine trees (Pinus radiata) which now dominate our commercial forest industry.
The kauri forests flourished at the top of the North Island from the Bay of Plenty, through Auckland to Northland. Although they sometimes occurred in pure stands, the kauri trees were mostly intermingled with podocarp hardwood forests at densities of about four trees per hectare (Fleet, 1984).
Few trees in the world reach the imposing size of the kauri. Growing from 30 to 50 metres tall, with a tremendously thick cylindrical trunk, kauri became a prized timber tree, particularly sought after by boat builders because it is naturally durable and easily worked. Left to themselves, the trees are very long-lived. Some of those felled in the past century had started their lives more than 2,000 years ago.
The podocarp-hardwood forests are New Zealand's major forest type, occurring in many variations up and down the country. They contain an unusual mixture of conebearing trees (the podocarps or 'softwoods') and many species of flowering trees (the 'hardwoods' or 'broadleafs'). Although podocarps occur throughout the southern hemisphere, the 20 or so species found here are unique to New Zealand. With the associated hardwoods, they made an unusual type of forest which looked more tropical than temperate.
The podocarps include the rimu, or red pine (Dacrydium cupressinum), often the dominant species in a forest, and the kahikatea, or white pine (Dacrycarpus dacrydioides), which pre-dominated in areas of poor drainage. These trees can reach heights of 50 metres, with some kahikatea reaching 60 metres. Other prominent podocarp species are totara and Hall's totara ( Podocarpus totara and P. hallii); matai and miro ( Prumnopitys taxifolia and P. ferruginea); pink pine (Halocarpus biformis); silver pine (Manoao colensoi); yellow-silver pine (Lepidothamnus intermedius); tanekaha or celery pine, toatoa and mountain toatoa (Phyllocladus trichomanoides, P. toatoa and P. alpinus).
The hardwoods are, in many cases, related to tropical families. They include pohutakawa and southern and northern rata (Metrosideros excelsa, M. umbellata and M. robusta); tawa and taraire (Beilschmiedia tawa and B. tarairi); hinau (Eleaocarpus dentatus); kamahi (Weinmannia racemosa); black maire (Nestegis cunninghamii); pukatea (Laurelia novae-zelandiae); puriri (Vitex lucens); and rewarewa (Knightia excelsa).
Podocarp-hardwood forests are sometimes referred to as 'lowland' forests because they tend to predominate at lower altitudes (i.e. below 800-900 metres in the North Island, below 650 metres in large parts of the South Island, and below 450 metres in the southern South Island and Stewart Island). The hardwood tawa was often the dominant tree in the North Island at lower altitudes and the podocarp rimu was dominant at higher altitudes. In the South Island, rimu was generally dominant. Some areas, however, were dominated by hardwoodsthe red-flowered rata and the white-flowered kamahi. Even where these hardwoods were not dominant they were well represented. In fact, kamahi is often regarded as the most abundant native tree in New Zealand.
When the Europeans arrived, the podocarp-hardwood forests had a luxuriant, impenetrable, junglelike appearance particularly in high rainfall areas. Below the tall tree canopy, a profusion of shorter trees, shrubs, vines and ferns vied for space, and below them lichens and mosses spread over the ground and the trunks of fallen trees, except in the wet kahikatea forests, where they vanished into dark, still water.
The beech forests generally grow in cooler environments than the other forests, though in many areas they intermingle with the podocarps and hardwoods. Pure beech forests occur in the North Island mountain ranges from the Raukumara Range to the Huirau Range, and in the South Island, along the main ranges from north-west Nelson to southern Fiordland and Te Waewae Bay. They have never grown on Stewart Island.
The beech trees are hardwoods, members of the angiosperm group. The New Zealand ones belong to the genus Nothofagus which evolved shortly before the breakup of Gondwana. Species of Nothofagus also occur in Tasmania, Australia, New Guinea, New Caledonia, and Chile. Four species exist in New Zealand: red beech (N. fusca); hard beech (N. truncata); silver beech (N. menziesii); and black beech (N. solandri ), which also includes the subspecies mountain beech (N. solandri cliffortioides). Beech forests are generally sparser than the podocarphardwood forests. Their understorey may contain only young beech saplings, ferns, and mosses. The lacy quality of light from filtered sun gives these forests a different ambience to the more densely packed podocarp-hardwood forests.
All the New Zealand forests were characterised by a general absence of bright colours, with the small hardwood flowers dwarfed by subdued shades of green foliage. The main speckles of colour came from the redflowered rata and pohutukawa, and several other angiosperms with tropical relatives (i.e. kakabeak, puriri, rewarewa, mistletoe, kowhai and, on the forest edges, flax). The young Charles Darwin found New Zealand's forests fascinating, but rather gloomy, when he visited the country in 1832 (P.H. Armstrong, 1993).
The mainstay of New Zealand's pastoral farming system has been the combination of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens), or, in drier areas, ryegrass and subterranean clover (Trifolium subterraneum). These species require high levels of nutrients and low soil acidity. As a result, considerable effort and expense has gone into 'developing', 'improving', and maintaining pasture soils through the regular addition of phosphate fertiliser, lime, and various other nutrients and trace elements (Goh and Nguyen, 1991).
Ryegrass needs large amounts of nitrogen. This is provided by the clover, which is a legume whose roots contain Rhizobium bacteria. These bacteria convert the air's nitrogen into soil nitrates which can then be absorbed by the ryegrass. Clover, however, requires high amounts of phosphorus and cannot tolerate soils which are too acidic. As a result, the sustainability of the ryegrass/clover association depends on widespread applications of phosphate fertiliser and, in many areas, lime. The lime reduces acidity and also supplies calcium as a nutrient.
Dessication can also be a problem for the pasture grasses. In dry parts of the South Island, irrigation is necessary. In recent years, new mixes of grasses (such as cocksfoot) and legumes (such as lucerne) have been tried in various pasture mixtures, and plant breeders continue to develop new strains of pasture grasses that will thrive in different soil conditionswet, dry, acid, or alkaline.
When properly maintained, the exotic pasture grasses form dense swards with extensive root systems that are good for the soil. The matted roots are shallow but provide organic matter and a greater degree of erosion protection and runoff control than do unimproved grasses. On plains, easy slopes, and in moist areas, the ryegrass-clover association is the dominant pasture vegetation.
In hill country and drier areas, however, these species are often joined by various other grasses, such as cocksfoot (Dactylis glomerata), timothy (Phleum pratense), browntop (Agrostis capillaris), sweet vernal (Anthoxanthum odoratum), dogstail (Cynosurus cristatus), and Yorkshire fog (Holcus lanatus). In the northern North Island, where the temperature is higher, the dominant species may include paspalum (Paspalum dilatatum), the native carpet grass (Chionochloa australis), Kikuyu grass (Pennisetum clandestinum) and ratstail (Sporobolus africanus).
In many areas the pasture grasses are perennial and do not enter an annual seed phase. They are replenished by oversowing of new grass seed. However, in semiarid parts of the South Island, and on steep, dry, north facing slopes, annual grasses do better than perennial ones. They cope with dry conditions by putting their limited resources into seed production for the following season rather than into tissue growth. These species include rapidly establishing, bulky grasses and clovers, such as the short rotation Manawa and Italian ryegrasses (Lolium spp), red clover (Trifolium pratense), subterranean clover, brome grasses (Bromus spp.), hairgrasses (Vulpia spp.), lotus (Lotus subiflorus), and lucerne (Medicago sativa). In less fertile land native grasses may also be present (e.g. danthonia, and short tussock).
New Zealand has about 3.8 million hectares of Class I, II and III land physically capable of regular cultivation. However, at any one time, most of this is in pasture and the total area in crops (peas, grain, fodder, orchards, vineyards, and market gardens) covers less than 500,000 hectares. The standing crop area has actually declined since the early days of this century when large oat crops were needed for horses.
In recent years, the area devoted to horticulture has expanded by 40 percent (from 88,000 hectares in 1990 to 124,000 in 1995). This is reflected in our trade figures. Although we import half our wheat, we export a wide range of horticultural products. The horticultural sector includes fruit production (e.g. apples, oranges and lemons, peaches, apricots, nectarines, kiwifruit and other berries etc.), market gardening (e.g. green vegetables, tomatoes, potatoes and other root vegetables etc.), wine production (i.e. grapes), and floriculture (for the cut flower market).
While the standing crop area shows the amount of land used for cropping at any one time, it dramatically understates the total amount used over longer periods. This is because many pea, grain and fodder crops, and some vegetable crops, are grown in rotation with pasture. Under this mixed cropping system, a field will produce crops for several years and then be returned to pasture to replenish the soil. A new field will then be converted from pasture to cropland. After several crop rotations, the total area used for cropping is several times larger than the standing crop. In parts of the South Island, rotations of nine to ten years are often favoured, but in very dry areas, rotations are as short as two to four years.
About 40 percent of the standing cropland area is devoted to rotational crops of grain (barley, wheat, maize, and oats) and peas. A further 20 percent is devoted to crops of fruit and vegetables (principally kiwifruit, apples, grapes, potatoes, onions, and squash). The remainder is divided between fodder crops (e.g. hay, rape, turnips) which provide food for livestock when winter pasture growth in the South Island is insufficient, and fallow land which has been left to recover after a period of repeated cropping.
The main cropping areas are in the fertile coastal plains. Wheat, barley, and oats are grown mostly in Canterbury, Otago, and Southland, though significant amounts of barley are also grown in the Manawatu. Maize is grown mostly in Waikato and the Bay of Plenty, and to a lesser extent in the Manawatu-Wanganui region. Areas of significant fruit production occur in Northland, Auckland, Bay of Plenty, Gisborne, Hawke's Bay, Nelson, Marlborough, Canterbury and Central Otago, though orchards, berry gardens, and vineyards exist in other parts of the country as well (e.g. vineyards in the southern Wairarapa).
Crops, orchards and market gardens, as well as dairy and fat lamb pastures, are generally classified as intensive forms of land use, requiring high inputs of fertilisers, mechanical energy, labour, pesticides and herbicides, and often putting considerable pressure on soils. Indigenous biodiversity is virtually nil in these systems. Of all the forms of cropping in New Zealand, market gardening probably keeps the land in most continuous production and requires the most careful soil management.
Because they grow faster and taller in New Zealand than anywhere else, including their native California, radiata pine trees (Pinus radiata) are the dominant timber tree here. They make up 90 percent of the 1.6 million hectares of exotic forest. The rest of the exotic tree stock consists of 5 percent Douglas fir (Pseudotsuga menziesii), 2 percent eucalypts (Eucalyptus spp.), and a variety of special purpose species, such as blackwood (Acacia melanoxylon), black walnut (Juglans nigra), macrocarpa (Cupressus macrocarpa), ponderosa pine (Pinus ponderosa) and Corsican pine (Pinus nigra).
Most exotic forests are planted as single species crops. The fast-growing pine crops are harvested in short rotations of 25-35 years. Wide-spaced planting and intensive pruning and thinning are now widely adopted as the standard management practice (Purey-Cust and Hammond, 1995). Herbicides are often used in site preparation and during the first few years, mostly to reduce competition from aggressive exotic weeds, such as blackberry, bracken, broom, gorse and grasses (Davenhill, 1995). Young forests in wet humid areas are occasionally sprayed with copper-based fungicides to combat the growth-debilitating needle blight fungus (Dothistroma pini) which reached New Zealand several decades ago (Gadgil et al., 1995). Occasionally, fertilisers are applied to plantation soils where nutrient levels are low, most commonly nitrogen, phosphorus and boron (Mead, 1995).
Exotic forests were first planted on a small scale in the 1890s. Large scale planting began in the 1920s when it became apparent that native timber supplies would eventually run out. The first plantings were mostly on low fertility pumice land in the central North Island (Purey-Cust and Hammond, 1995). Tourists travelling through the centre of the island are often impressed by these extensive tracts of dark pine forest.
A second planting boom, which began in the 1960s, was more scattered. Many plantations were established throughout the country, often replacing cutover native forest and regenerating scrub. A proposal to convert extensive areas of Crown-owned beech forest to pine plantations led to a prolonged confrontation between foresters and environmentalists during the 1970s until the proposal was finally dropped.
The third planting boom is a 1990s phenomenon, triggered among other things, by high export returns from pine logs and low returns from sheep. As a result, most new forest planting has been on pasture, about a third of it on hilly and steep land that was cleared of native forest in earlier decades. The direct replacement of native forests by exotic plantations had virtually halted by the 1990s, due mainly to economic constraints, but also to agreements between environmentalists and the forest industry. However, in some regions, indigenous scrub was still being converted to exotics, mainly by smaller forestry companies (McLaren, 1995).
Because they are not indigenous and are managed and tended as monocultural crops, exotic forests are sometimes seen as 'biodiversity deserts'. However, a number of studies have found moderate to high levels of indigenous plant, insect and bird diversity in pine forests (Clout and Gaze, 1984; Ogle, 1976 and 1989; Allen et al., 1995; Ledgard, 1995; McLaren, 1995; Spellerberg and Sawyer, 1995). In most cases these are opportunisitic species, such as weeds and exotic birds, that invade the site shortly after harvesting and then become rarer as the site ages.
However, on fertile and wet sites where trees are older than 25 years and are well spaced out pine forests often have a dense, varied, understorey, sometimes including rare native plants and animals. A number of native birds have colonised pine plantations. Grey warblers, fantails and silvereyes are common. Robins, whiteheads, wekas, harrier hawks, kingfishers and shining cuckoos can also be found. However, birds which eat nectar and berries or which nest in holes and old logs are rarely present.
A dramatic example of a native bird colonising a pine forest is the North Island brown kiwi in Waitangi Forest, Northland (Colbourne and Kleinpaste, 1983). Medium to high densities live and breed there. After logging, they retreat for several months to forest remnants in gullies and swamp margins and then re-establish territories in adjacent pine stands. Sensitive forest management is attempting to accommodate the birds (Steven, 1995).
Clearly, older pine forests are not biodiversity deserts. On the the other hand, they are far from biodiversity havens (Rosoman, 1995b; Spellerberg, 1996). Plantation forests have two major limitations as habitat for native animals (Steven, 1995). First, most species have evolved behaviour which is especially adapted to the native forests. Pine plantations have fewer suitable eating, nesting and breeding sites. Second, the short harvest cycles of plantation forestry mean that any native understorey which does develop provides only temporary habitat.
Although the biodiversity of pine plantations is less than that of indigenous forests in similar locations, foresters often point out that much of New Zealand's high altitude beech forest consists of even-aged single-species stands that have little understorey and few other vascular plants present (McLaren, 1995; Rosoman, 1995b). Of more significance, though, is the fact that exotic forests support more indigenous species than the pastures they are now replacing. While it is true that some of the areas being planted today would have reverted to native forest and scrub if left alone, many other areas would have remained in pasture had they not been converted to pine forests. Many environmentalists now see pine forests as playing a positive role in that they take the logging pressure off native forests and also enhance soil and water quality on farms by stemming erosion and reducing run-off.