The Vegetative Cover of New Zealand

The Vegetative Cover of New Zealand

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This book considers the origins of New Zealand's vegetation and how it has been influenced by natural and anthropic factors, describes the compilation of the Vegetative Cover Map of New Zealand, and discusses the 47 Vegetative Cover Classes. Each of the Vegetative Cover Classes are considered in the context of their composition, appearance, area and distribution, ecological influences, and their likely future under present environmental conditions and management

The Vegetative Cover Map of New Zealand maps (North and South Island) are still in print and available, but the book is now out of print.

From: The Vegetative Cover of New Zealand, by P F J Newsome

CHAPTER 1

NEW ZEALAND’S VEGETATION HISTORY

IN THE BEGINNING

Some 300 million years ago, the section of the earth’s crust which was later to support New Zealand comprised a large, rapidly subsiding, undersea depression. This depression, known as the New Zealand Geosyncline, was located in antarctic latitudes off the eastern coasts of what were to become Australia and Antarctica, but which at that time were part of a much larger super-continent which researchers have named Gondwanaland.

For about 200 million years erosion debris from the nearby landmass of Gondwanaland was deposited in the New Zealand Geosyncline forming immensely thick accumulations of material. An arc of volcanic islands and undersea volcanic cones contributed volcanic debris to the basin. As the basin continued to subside the erosion debris were buried deeper and deeper and geological processes began to change these sediments into rocks such as sandstones and mudstones. With further increases in pressure and temperature, some were metamorphosed into schist and gneiss, and perhaps ultimately re-melted and solidified to form granitic rocks.

About 150 million years ago the the Rangitata Orogeny commenced, and continued for approximately 50 million years. This was characterised by a reversal in the behaviour of the earth’s crust from subsidence of the basin floor to reflexion. The earth’s crust began to buckle and evert and there emerged a new landmass, an ancestral New Zealand. The new land extended as far north as what we now call New Caledonia, as far south as Campbell Island, and as far east as the Chatham Islands. The climate was similar to that prevailing today. This primaeval, greater-New Zealand was colonised by plants and animals already present on neighbouring Gondwanaland and the volcanic islands of the former New Zealand Geosyncline. Among the animal migrants were the ancestors of some of New Zealand’s more distinctive elements; the Leiopelmid frogs (notable for having no free-living tadpole stage), the tuatara, and the ratite birds (notably the moa and kiwi). The plant life was no less distinctive and comprised ancestors of the modern conifers, including the podocarps and kauri (Agathis australis), the forerunners of our modern ground ferns arid tree ferns, and remnants of an even more ancient and now extinct group of spore-bearing herbs and trees.

The next 20 million years (from 100–80 million years ago) were characterised by a cessation of the geologic uplift coupled with a gradual lowering of relief as erosion began to wear down the land. Concurrently, there was continuing colonisation by plants from the ancient lands which then comprised Gondwanaland. This period was notable for the rapid worldwide evolution, speciation, and expansion of a new lineage of plants, the flowering plants or Angiosperms. The ancestors of the southern beeches first appeared in the New Zealand flora at this time.

During this period and in the following 20 million years (up to 60 million years ago), the primaeval super-continent of Gondwanaland began to disintegrate. This colossal geologic rifting between the earth’s crustal plates was followed by a gradual drifting away of sections of this greater landmass. The breakup of the Gondwanan super-continent occurred later in the Australasian sector than elsewhere and there was little impediment to plant distribution in this region until 70–80 million years ago. By this time the growing ocean gap between Australia/Antarctica and New Zealand was about half that of the present day. But, before these links were severed, there arrived recognisable ancestors of tawa (Beilschmiedia tawa) and rewarewa (Knightia excelsa), and quite probably a host of other flowering plants. In the ensuing competition for habitats, and in the face of continually fluctuating climates, many of the ancient lower plants and gymnosperms, and some of the angiosperms, became extinct.

Erosion continued to be a feature of the next 45 million years. During this time much of the land was gradually submerged beneath the sea, and the relief of the remainder became reduced to a low peneplain. The climate underwent a series of oscillations between warm temperate and cold temperate, with two pronounced warmings to subtropical conditions around 55 and 20 million years ago. The first evidence of an ice cap in the now polar continent of Antarctica dates from about 45 million years ago (Kemp 1978).

In ancestral New Zealand, now geographically isolated, the flora continued to adapt and evolve independently from its congenitors in what were to become South America, Australia, New Guinea and Antarctica. The great podocarps continued to dominate much of the forests but in association with a growing proportion of broadleaved species. Among the latter were recognisable ancestors of fuchsia (Fuchsia excorticata), hinau (Elaeocarpus dentatus), kamahi (Weinmannia racemosa), kohekohe (Dysoxylum spectabile), pukatea (Laurelia novae-Zealandiae), rewarewa, rata (Metrosideros spp), tawa, species of Coprosma, Leptospermum, Pittosporum, and many ferns, epiphytes, lianes and grasses.

Fleming (1979) recognises three geographic affinities in this flora: an Australian (typified by Leptospermum), a Paleoaustral (for example the beeches, the podocarps and rewarewa) and a Malayo-Pacific (including many ferns, nikau (Rhopalostylis sapida), kohekohe, tawa and mangrove (Avicennia marina var resinifera). Many of these species recognisably present in the fossil record from around this time would have been present in New Zealand prior to the ultimate separation from other Gondwanan lands. Others would have established by chance dispersal across the ocean, or by “island-hopping” along an archipelago which may once have extended northward along the Lord Howe Rise and southward along the Campbell Plateau. All would have undergone adaptation or speciation to compete in the array of niches in their new environment. Less competitive plants would have diminished in numbers and many would have faced extinction. It is noteworthy that during this time of speciation and adaptation in the New Zealand flora, other lands were witnessing the evolution of mammals, most of which were to become specialist herbivores. New Zealand, by contrast, remained populated with a relatively modest array of birds and insects. These, while exploiting a variety of habitats, were never to exert as great a level of environmental pressure on the vegetation as the mammals.

Approximately 13 million years ago there began another phase of mountain building the Kaikoura Orogeny. This was characterised by extensive faulting, folding and uplift, both of the existing land and also of the coastal sediments produced by the previous 100 million years of erosion. These geologic events coincided with a gradual climatic cooling which culminated two million years ago at the beginning of the Pleistocene glaciations.

The Pleistocene period lasted until almost 10,000 years ago, during which time the New Zealand flora was subject to the rigours of an increasingly severe climate. For the more tender elements of the flora, stranded on a land similar in size and shape to the New Zealand of today, there was no escape northward to more temperate latitudes. Forest species retreated to lower altitudes and northward, and into local refugia of favourable climate. Shrublands and grasslands expanded to take their place as far as their own limits of tolerance would allow. There was strong selection pressure for the evolution of a flora to fully exploit the increasing area of land between the depressed treeline and the zone of perennial ice and snow. This pressure for an alpine flora was satisfied in part by speciation and hybridisation among ‘genetically plastic’ lowland and montane genera, for example Celmisia, Coprosma, Dracophyllum, Hebe, Myrsine and Podocarpus. Others may have arrived by migration of elements of Antarctic and tropical mountain affinity, for example Epilobium, Gentiana and Ranunculus. But the plants which were adapted to the lowland habitat were in a more difficult predicament. With their retreat to warmer latitudes cut off, extinction for many species was inevitable. So this period saw the disappearance from New Zealand of a number of elements which had been prominent in the preceding 40 million years. Among those to disappear were a group of large-leaved beeches previously common in an arc from New Guinea, New Caledonia, Australia, New Zealand through Antarctica to South America but now only present in New Guinea and New Caledonia.

For the most part however, there was a more or less complete recovery of the vegetation during the interglacials and in the present post-glacial period. Forest occupied all but the most arid lowlands and extended up-slope to meet the now well developed highland shrublands, grasslands, and alpine herbfields.

THE POST-GLACIAL PERIOD

The post-glacial period was characterised by a gradual warming which culminated about 8000 years ago. Since this time, the climatic regime has oscillated around an average similar to, but slightly warmer and wetter than, the present day (Denton and Karlen 1973, McGlone and Topping 1977). These comparatively minor fluctuations in climate would have had an unsettling effect upon the vegetation resulting in constant small adjustments in plant distribution. But the great longevity of many forest trees and even the larger tussock grasses would have served to smooth out the effects of the more extreme climatic fluctuations.

More spectacular changes, although quite local in extent, are those initiated by sporadic volcanic events, or fire caused by lightning. Fire is known to have been widespread during this period (Molloy et al 1963) and is likely to have been most extensive in the drier eastern districts. Volcanism has been a feature of New Zealand for millions of years and has conferred upon the landscape such features as Banks Peninsula, the Taranaki volcanoes and those of the Volcanic Plateau, parts of the Coromandel Range, and many hills and domes in Auckland and Northland.

The New Zealand flora was well able to adjust to these and other types of disturbance, having as it does, a range of successional mechanisms to recover from natural disasters. But these natural events have usually been in the form of either gradual and extensive change or local and sudden catastrophe. Then, overnight in evolutionary terms, the character of disturbance changed to a form of repeated extensive and sudden catastrophes. This change was consequent upon the arrival of humans.

THE POLYNESIAN ERA

It is not known exactly when the first human set foot in New Zealand, but the earliest conceivable arrival of the Polynesians is thought to be about 750 AD (Cumberland 1981). Their manner of coming and their numbers are also a matter for conjecture. Current opinion reported by Cumberland (1981) inclines towards a theory of repeated accidental colonisation from large canoes carried off course on inter-island voyages, and under the influence of wind and current, making a landfall on the coast of New Zealand.

It is presumed that among the canoes’ cargoes were the tropical foods of the Polynesians: kumara, taro, yam and gourds, all of which require a mild climate and care in cultivation. In this new-found land, which was colder and wetter than they were used to, and largely covered with forbidding forest, the Polynesians found rich compensation in a prolific bird fauna, fertile soils and a range of edible plants.

Whilst the bulk of the Maori population remained concentrated in the north where the warmer climate allowed growth of their tropical crops, other parties began to explore the southern regions. These southern Maori, foraging for fern-root, fruits and shellfish and hunting the flightless moa and other native birds, evolved a parallel but separate culture to their northern kinsfolk. This gave rise to the myth of an earlier, more primitive, moa-hunter people.

In addition to their tools of bone, wood and stone, both the farming and the moa-hunting sub-cultures brought with them fire and used it in hunting, in communications, and in agriculture. Walsh (1896) relates

“… the firestick formed an important part in their travelling outfit and was constantly in requisition to clear away the dense growth of fern or tea-tree which impeded their movements.… Such fires were noticed by Tasman as he sailed along the western coast 250 years ago…”.

In northern regions, even with their higher populations, the area affected by fire was comparatively modest when compared with that of the central and southern regions. The northern Maori used fire principally as a tool to clear land for food crops. In these regions of heavy forest and a moist climate, it is probable that containment of burns was usually successful and repeated uncontrolled fire unusual.

In central, eastern and southern regions the effect of fire was more marked and more remarkable in view of the comparatively low density of Maori population. Fire was employed as a tool in hunting moa, to favour the growth of edible bracken fern (Pteridium esculentum), and to open up routes of travel southward and into the interior. For these purposes containment of burns was less critical but there were also climatic factors which made fire a much more devastating weapon here, than in the northern and western forests. Most of New Zealand’s weather derives from a succession of high and low pressure systems originating over the Tasman Sea and Australia. To a large degree the rain that they bring is intercepted by the western ranges giving a “rain shadow” effect to the eastern regions. Coupled with this naturally drier climate is a hypothesised climatic change to slightly cooler, drier conditions than was present 2000 years ago (Holloway 1954, McGlone and Topping 1977), although the evidence for this is by no means unequivocal (Burrows and Greenland 1979). For whatever reason the less dense, eastern podocarp-broadleaved and beech forests have a much more tenuous hold on the drought-prone hills and plains. Therefore, they were less able to recover from the repeated fires which raged through their domain. As a consequence, in the millennium before the arrival of the Europeans, forest was eliminated from much of the eastern lowlands from northern Hawke’s Bay to Southland. Figure 1 shows the reduction in forest cover over the country as a whole from approximately 75% of the total land area in about 700 AD to approximately 55% in 1800 AD (Cumberland 1981; Wards 1973).

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Figure 1: From 700 AD to 1800 AD there was a reduction in forest cover from approximately 75% of the total land area to approximately 55% (adapted from Cumberland 1981; Wards 1973).

The forest clearance was not synchronous throughout. As Maori populations increased and spread into less inhabited areas, there was an increase in the number and size of clearings made for village sites, gardens, and along routes of travel. Studies of the age of buried woods and charcoals in the eastern South Island (Molloy et al. 1963) show that there was a progression from earlier fires on the coastal plains, to later fires in the interior.

As time passed, the once plentiful moa became rarer on the coastal plains where the larger species had lived, and so they were pursued further and further into the inland valleys and basins. As the hunting became more difficult even the use of fire to drive the flocks from their retreats yielded fewer prey. Ultimately, not long before 1800, even the small bush moa had been hunted to extinction.

Consequent upon the decline in moa populations there was a decline in the prosperity and cultural strength of the southern moa-hunting Maori. This opened the way for tribes from the north to extend their influence in the south. And so it was, that the moa-hunter culture was eventually subjugated by the “classic Polynesian culture” from the north, to the extent that this was the Maori civilisation which prevailed when Tasman and Cook visited New Zealand in the 17th and 18th century (Cumberland 1981).

European contact was sporadic and local until almost the middle of the 19th century. However, even those early encounters were later to have enormous effect on the ecology of the land. For with them the early Europeans brought the first truly exotic plants and animals since the Maori introduced their own food crops and the Polynesian rat and Polynesian dog. These early explorers, whalers and sealers released pigs and goats and even established cultivated gardens.

Among these garden crops, and thought to have been introduced around 1770, was the potato. This immediately caused an agricultural revolution among the Maori (Cameron 1964). Here was a crop whose cultivation technique they readily understood and whose climatic range, soil tolerance, and productivity were far in excess of the kumara. Furthermore, potatoes and other foods had an increasing trade value to the rising numbers of whalers, sealers, and flax and timber merchants visiting the coast of New Zealand. This saw a resurgence in demand for arable land and a renewed assault on the forests and shrublands, particularly in the north. Many of the early writers described the shifting agricultural technique, and the land clearances effected. This account by Dieffenbach in 1843, reproduced by Cameron (1964), is typical of the period leading up to large-scale European settlement:

“It is evident that the forest has at some former period covered a greater extent of land in the neighbourhood of Taupo than it now does: it does not appear to have been destroyed by volcanic eruptions but by the fires kindled by the natives in order to clear the ground for purposes of cultivation …”

As the 19th century advanced the early whalers, sealers, missionaries, explorers and traders were followed by increasing numbers of settlers, and so were heralded the most far-reaching and rapid changes in New Zealand’s long history.

THE EUROPEAN ERA

Conventionally 1840 is taken to mark the beginning of the European era, although local settlement on a small scale had been known from the beginning of the century. The centres of population in the early days were in the north—around the Bay of Islands, Auckland and Tauranga—and later in the areas of Wellington, Nelson, New Plymouth, Wanganui, Canterbury and Otago. Other settlements were subsequently established as new immigrants arrived and as populations began to move from the coastal settlements into the interior.

In many areas, particularly in the North Island, and despite the forest clearances made by the Maori, a necessary prerequisite to making a living was to first build a dwelling, clear the forest and scrub, and sow the land with pasture grasses. To this task the settlers applied themselves with a dogged zeal. They would cut the standing trees and leave them to dry over the hot summer months and then return in the autumn to burn the felled forest. Timber mills were often established alongside the best stands of forest to serve the growing demand for sawn timber. Hundreds of small sawmills were established throughout the country and began a thriving trade principally milling kauri (in the north), totara, rimu and matai for both the domestic and export markets. But, in the face of a more immediate need for pastoral land, vast areas of prime forest, beyond the reach of the millers, were simply destroyed by burning.

Early writings of this period observe, often with sadness how, for days and even weeks at a time, the autumn sky would be darkened with smoke and the night lit with the glow of the forest burns (e.g. Reed 1948). The Rev. P. Walsh in 1896 expressed concern at the destruction of the northern forests in an address to the Auckland Institute:

“… since the axe and the saw have come into operation the living bush has been attacked throughout the length and breadth of the land, and not only is an increasing area annually deforested for farming purposes but the bush is gutted in all directions by timber-workings and roads and telegraph lines.”

In the drier eastern areas, particuarly those of the South Island, the pattern of development was somewhat different. Much of this country had been cleared of forest during the Polynesian era and now supported vast areas of short tussock and snow tussock grasslands. This was a landscape which the settlers of the old world felt they understood and to which they applied farming practices from their homelands, sometimes modified by experience gained in the Australian colony. Large pastoral runs (or “stations”) were taken up and stocked with sheep, often the fine-wool Merino breed. Unconfined by fences the flocks grazed widely from the plains and river flats to the alpine tussocklands of the mountains. To promote the growth of palatable young shoots and to assist in mustering the runholders would periodically burn the tussocks.

At first the results were encouraging and, as the settlers had experienced in their home country, fire promoted a flush of soft young palatable shoots. However, after a succession of fires a different effect began to show. After each burn the vigour of the tussocks decreased and fewer and shorter new shoots appeared. With the resulting decrease in ground cover, frost action began to severely inhibit seedling establishment and to loosen soil particles so that erosion became a serious problem. John Buchanan, then of the Geological Survey of New Zealand, wrote in 1868:

“Nothing can show greater ignorance of grass conservation than the repeated burning of the pasture in arid districts, which is so frequently practised.… Much of the grass land around Otago has been thus deteriorated, since its occupation, by fire, and it is no wonder that many of the runs require eight acres to feed one sheep …”

During the wheat boom of the 1870s much of the lowlands of the eastern South Island were converted from tussock to cropland. Today arable cropping in rotation with fat lamb farming on sown pastures, continues to be a prominent land use in this region. The expansion in the 1890’s of a frozen meat export trade saw a growing emphasis throughout the country on fat lambs and cattle farming and a lessening reliance on wool.

By the beginning of the First World War the pace of forest clearance in the northern and western districts was slowing down. Much of the cleared land had been sown to some form of permanent exotic grassland. In the years following the war there was more emphasis on ‘infilling’ in the rural sector, with larger station blocks being broken up into smaller farm units and more intensive development on hitherto poor pasture and scrubland. In some localities, soldier resettlement schemes saw a renewed development drive in marginal hill country. Some such schemes, particuarly those in the steep hill country of inland Wanganui-Taranaki-King Country, provided only years of heartbreak and ultimate failure because of high rates of soil erosion and scrub reversion, coupled with low stocking densities and very expensive roading. In the central North Island on soils derived from volcanic ash, the mysterious “bush sickness” plagued the early sheep and cattle farmers. Here and elsewhere as the depression years of the late 1920s loomed, many farmers were forced to abandon their hard-won gains of the previous decades.

From the mid-1920s an embryonic Forest Service began large-scale planting of exotic pine forests on the rapidly reverting lands of the Kaingaroa Plateau. These projects provided work for some of the thousands of unemployed during the Great Depression and later made available an alternative source of timber to the fast-dwindling areas of native forest. The momentum of these early efforts was to continue, and when joined by those of private companies, these plantations were to form the nucleus of today’s large exotic forest estate.

By the middle of the 20th century virtually all the plains and downlands, and much of the low hill country had been cleared of forest or scrub and converted to agricultural use. High-producing exotic grasses and clovers occupied a significant and increasing area of the country. More intensive farming and cropping on relatively small holdings became the norm, especially in the lowlands.

However, darkening the horizon of both the developed and the undeveloped land was a growing problem which had its roots in the very early days of the colonial period. The early settlers had brought with them, either by accident or design, an array of plants, fish, land animals and birds from their respective homelands or from countries in which they had travelled. Many of these introductions were absorbed into the New Zealand biota with little environmental impact, but others began to run rampant across the land.

The first ‘naturalised exotic’ to reach pest proportions was the rabbit. From the 1870s, through to shortly after the Second World War, these prolifically-breeding, voracious animals were responsible for the degradation of millions of hectares of extensively-farmed pasture and tussockland. In their effort to control the problem, farmers and the authorities tried shooting, poison, cats, dogs and ferrets and erected many kilometres of rabbit-proof fencing, but these measures served only to retard the rabbit’s advance. It was only after the Second World War and the technique of aerial dropping of poisoned bait that success appeared possible.

Among animal pests of similar magnitude are the possum and deer, and locally goats, pigs, thar, and wapiti. Because these animals established themselves largely in undeveloped or ‘wild’ habitats, and because they browsed at different levels in a forest structure, they have had an insidious and drastic effect on forest health and regeneration. These effects are manifest in the loss of vigour and death of palatable trees, increased susceptibility to disease, reduction or loss of understorey shrubs and seedlings, and, with the depletion of vegetative cover, a higher incidence of erosion and wind damage.

Of New Zealand’s naturalised exotic flora of over 1000 species, only about a score are gazetted as Noxious Plants (Upritchard 1985). The threat of some of the more aggressive adventive plants is familiar to most people, and concern has been expressed since the early days of botanical writing. Armstrong in 1879 remarked:

“The rapidity with which these introduced plants have spread over the province of Canterbury is indeed an extraordinary circumstance …”

and Townson in 1906 observes:

“… I was sorry to see that the country round the Junction (Inangahua Junction, North Westland) was overrun with blackberry, and I noticed that it had taken complete possession of several paddocks near the bridge …”

For the most part the degree of notoriety with which these ‘weeds’ are held is dependent on how much they interfere with human agricultural practices. Therefore, held in lowest esteem are plants like gorse (Ulex europaeus), sweet brier (Rosa rubiginosa), broom (Cytisus scoparius), Hieracium spp and blackberry (Rubus fruticosus) as well as thistles, ragwort (Senecio jacobaea) and a number of other agriculturally valueless or detrimental herbs or shrubs. Other plants are viewed with disdain and alarm by ecologists because of their potential to do irreparable harm to indigenous ecosystems and here must be included Pinus contorta, old mans beard (Clematis vitalba), Hakea spp and heather (Calluna vulgaris).

With all these plant and animal pests it was eventually realised that early policies of ‘extermination’ were impractical, and so the emphasis changed to a policy of ‘control’. The experience of recent years suggests that even with the more advanced methods available it will demand constant application to contain the menace which some of these plants and animals present.

The years following the Second World War saw further advances in land use technology and a maturing of New Zealander’s attitudes with regard to land use issues. Aerial topdressing provided an economic method for applying fertiliser and improved pasture seed to hill country, and offered an avenue for the control of crop diseases, noxious weeds and noxious animals via aerial spraying and poison drops. A more appropriate, and financially rewarding, use of high producing land was afforded with the cultivation of horticultural and orchard crops such as berryfruit, citrus, pip and stone fruit, grapes, kiwifruit and subtropical fruits. Meanwhile, on the less productive land, after a century of unrestricted and often ill-advised development, there was increasing concern that the cost in terms of soil erosion, deposition and flooding had long passed that which was acceptable. As a consequence, a soil conservation organisation was established with the responsibility for soil erosion control and repair, and to apply the principles of wise land use to help minimise future hazards. Recently the whole issue of environmental conservation has received wider public attention with a sharpened focus on land planning, district and regional schemes, moves toward a representative system of parks and reserves and most recently, with the establishment of government departments with a statutory responsibility for environmental and conservation issues. The natural end point of this trend, a land and people co-existing in harmony, is still far in the future. Meanwhile the Vegetative Cover Map provides a representation of New Zealand’s ever-changing landscape giving a hint of the turbulent past and offering to the perceptive some suggestion of where the future might lie.

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