In February 1839, pioneer European exlorer and botanist John Bidwill journeyed o the centre of the North Island. Here e encountered “frost flats” for the first time, something he described in Rambles in New Zealand: “As we went on, the land became more and more barren and level, till it became a mere moor, without a shrub, and almost without vegetation; a few bushes of the miserable-looking Dracophyllum being all that occurred to break the monotony of the plain.”
New Zealand’s most famous short-story writer, Katherine Mansfield, after a coach trip in November 1907 from Napier to Rangitaiki and Galatea, across the desolate high plains of the Kaingaroa plateau, wrote in The Woman at the Store:
All day long the heat was terrible. The wind blew close to the ground; it rooted among the tussock grass, slithered along the road, so that the white pumice dust swirled in our faces, settled and sifted over us and was like a dry-skin itching for growth on our bodies. The horses stumbled along, coughing and chuffing… Hundreds of larks shrilled; the sky was slate colour, and the sound of the larks reminded me of slate pencils scraping over its surface. There was nothing to be seen but wave after wave of tussock grass, patched with purple orchids and manuka bushes covered with thick spider webs.
A hundred years later, the landscape presents a vastly different prospect to the traveller. The rutted pumice track has become a modern highspeed state highway. The native tussock and manuka have been largely replaced by a firebreak fringe of scrawny grass, backed by mile after mile of sombre radiata pine relieved at intervals by groves of feeble eucalypts, an attempt by the late New Zealand Forest Service at roadside beautification. In other places, sheep graze on sparse pasture, and, increasingly, dairy herds on lush swards of ryegrass and clover. Only the heat is still there, and the skylarks. On clear summer days, this is about as continental a climate as one finds in the North Island.
Yet, remarkably, just a short distance off State Highway 5, close to the northern foothills of the Ahimanawa Range, a piece of the landscape encountered by early travellers and described so vividly by Katherine Mansfield has survived nearly a century of land development in the region. This is Rangitaiki Conservation Area, a corner forgotten after its abandonment by farmers in the early 1920s until its rediscovery by scientists of the Forest Research Institute (FRI) in Rotorua in the mid-1980s.
John Nicholls, responsible for promoting the idea of a nationwide network of reserves within the then state forests, was poring over early black-and-white aerial photographs of the region one autumn afternoon when he identified what appeared to be a great tract of short scrub around the upper reaches of the Rangitaiki River. Wondering if it was an area that had escaped the effects of the decades of forestry expansion over so much of that part of the country, he collared a fellow scientist and drove down there a few days later. To the two men’s amazement, as they crossed the low saddle that separated the area from the highway, they were greeted by a vast expanse of red-brown monoao (Dracophyllum subulatum) shrubland, the largest and best surviving example either had seen of frost flats, an ecosystem that once stretched over thousands of hectares of the volcanic plateau but by then had all but disappeared.
In the 1940s, Forest Service forester John Ure (later Conservator of Forests, Rotorua) used to cycle around the vast Kaingaroa plains, then under their first rotation of exotic conifers, measuring the heights of the 20 tallest trees at various spots and recording indicator species among the vestiges of natural vegetation in nearby firebreaks in order to classify the area into site types for plantation forestry. (Tree height at a given age is a universal measure of site quality, being independent of competition effects.) Ure published his findings in the New Zealand Journal of Forestry in 1950, producing a suite of maps showing the distribution of site types, which formed a continuum from benign to extreme. The most extreme site type, on “gently undulating or flat country between 1500 and 2400 feet” (460 and 730 m), where manuka (Leptospermum scoparium) was rare or absent, and typified by “low, broken cover” of silver tussock (Poa cita) and monoao, is classic frost-flat terrain, and covers broad tracts towards the high southern end of the plains.
Frost flats on a much smaller scale were scattered widely about the volcanic plateau. Although most have since vanished, ploughed or planted out of existence in the frenzy of land development that followed WWII, lesser but still significant areas survive at places such as Pureora and Whirinaki, protected by Crown tenure within largely native forest parks. However, wherever she can, nature is doing her bit to annihilate them. The less extreme sites—where the cold season is shorter and the frosts are less severe, and which have supported frost-flat communities of sorts since the original forest cover was burnt by Maori—are slowly being colonised, first by manuka, then by seedlings of frost-hardy trees such as lancewood (Pseudopanax crassifolius), kohuhu (Pittosporum tenuifolium) and broadleaf (Griselinia littoralis), and later by matai (Prumnopitys taxifolia) and totara (Podocarpus totara) seedlings, foreshadowing an eventual return to tall conifer forest.
The story of the frost flats of the central North Island is neatly summed up by three Fs: frost, fertility and fire. The flats were created after vast quantities of gas and near-molten pulverised rock swept across the landscape in a pyroclastic flow immediately following the titanic eruption of Taupo about 1800 years ago. In places like Rangitaiki, pumice—a light, porous, acidic rock formed from rhyolite (magma rich in silica) expelled by the eruption—was washed off the surrounding hills into basins by rain, forming low-lying flats, and some was later re-deposited by rivers, forming terraces. Being on an elevated inland plateau yet surrounded by higher ground, these flats were foci of cold-air ponding and so attracted year-round frosts.
After a summer night at Atiamuri in 1839, sleepless because it was “so intensely cold”, Bid-will complained that the temperature inside his tent was still only 4°C at 9 o’clock in the morning. Researchers trying to find ways of establishing exotic conifers on frost flats in the 1970s recorded up to 230 frosts per year in southern Kaingaroa Forest. Summer temperatures were regularly as low as -5°C, while winter temperatures occasionally dipped to -18°C, cold exceeded in New Zealand only in the basins of central Otago. Such conditions are far too harsh for most common native trees and shrubs; only the toughest, such as mountain toatoa (Phyllocladus alpinus) and bog pine (Halocarpus bidwillii) can survive in them (their scarcity being the result of other factors). Daytime temperatures can soar in summer, so the temperature range on any given day can extend to 30°C or more.
Pumice is naturally infertile, lacking many nutrients, such as phosphorus and magnesium, essential to the health of most plants and animals. This proved to be critical in the shaping of the European history of the plateau. Early attempts at livestock farming were largely unsuccessful, owing in particular to a want of the trace element cobalt in the soil and vegetation. So marked was the effect of this deficiency on grazing animals that it acquired the name bush sickness.
Before the cause of bush sickness was discovered and remedied in the late 1930s, vast expanses of the volcanic plateau were planted in exotic conifers, the land being apparently useless for anything else. Both the state and private companies carried out this work, using the cheap labour freely available during the Depression years of the late 1920s and early 1930s. Thus were created the largest plantation forests in the Southern hemisphere and the basis of New Zealand’s exotic-timber industry. Although most frost flats were afforested at that time, the plantings on many of them failed. It wasn’t until FRI scientists came up with ways to protect seedlings from frost, implemented from the 1970s onwards, that the majority were planted successfully.
The crucial part played by fire in the frost-flat story became clear only when researchers started counting monoao growth rings and thus established the age of these shrubs, the only widespread plants of any size. As “turpentine bush”— the common name for Dracophyllum species—implies, monoao burns readily and even promotes fire, but it is completely killed in the process and is therefore unable to re-sprout. However, fire also clears the ground of lichen and moss mats, so providing a seedbed for another generation of monoao. Seeds blow in from surviving stands on the winds that sweep across the plateau. As the new cohort ages and eventually senesces, moss and lichen become increasingly prevalent, and these must be burnt off before yet another monoao stand can establish.
With warm, dry summers and flammable vegetation of many kinds growing on it, the volcanic plateau has a long history of burning by humans: early European explorers like Bidwill commented on the widespread and apparently indiscriminate use of fire by Maori to clear land for crops. Natural fires also occur but on a much smaller scale, usually caused by electrical storms and soon dowsed by the downpours that tend to follow them. The last such fire at Rangitaiki was in February 1994, and the site has been closely monitored since to follow vegetation changes.
Like many harsh environments, frost flats are notable for the few species of “higher” (i.e. seed-bearing) plant they support. There is only a handful of really common ones. Monoao, the archetypal frost-flat plant, is, like all heaths, peculiarly adapted to the very infertile soils in which it grows. Specialised fungi on its roots enable it to extract what little phosphorus is available in the soil. It also grows extremely slowly: at Rangitaiki, monoao in century-old heathland is only knee-high, its growth rings so close together they can be distinguished only under a microscope. A further adaptation is to be found in its leathery, needle-like leaves, which have a specialised layer of cells with thickened walls, called sclerophyll, which prevents moisture loss in the windy, dehydrating environment.
The other common higher plants are another, much smaller, heath, called patotara (Leucopogon fraseri); Pimelia prostrata, a low-growing shrub at home on poor soils all over the country; two daisies—the miniature mountain daisy Celmisia gracilenta and the ubiquitous introduced catsear (Hypochoeris radicata); and three grasses—silver tussock and the much smaller common danthonia (Rytidosperma gracile) and mountain oat grass (Deyeuxia avenoides). Many lower plants, including lichens, mosses and liverworts, are also at home on frost flats, some rare liverworts having been recorded. Coral lichen (Cladia retipora) is the main frost-flat lichen, forming snowy-white patches on the ground that contrast dramatically with the dark red-brown of monoao and the pale straw-yellow of silver tussock. It is an early coloniser following fire. Woolly moss (Racomitrium lanuginosum) forms dark olive-brown carpets that smother more and more of the ground as heath-land ages, spreading under old monoao bushes as they decline and fall apart. Birds of open country, such as pipits (Anthus novaeseelandiae), fernbirds (Bowdleria punctata), banded dotterels (Charadrius bicinctus), southern black-backed gulls (Larus dominicanus) and harriers (Circus approximans), are also at home on frost flats.
Perhaps the most intriguing question about frost flats for scientists is the inevitable one about their past. Have places like Rangitaiki always supported low shrubland? Or has the vegetation on them changed dramatically as a result of the increase in fires following the arrival of humans? Pollen spores preserved in the airless, acidic conditions of peat bogs provide unique clues to past vegetation, and in the centre of Rangitaiki is a large wetland that begs to be sampled for such pointers. It seems likely that small conifers—mountain toatoa and bog pine—were once far more common in the area than they are today. Both burn readily and depend on birds to distribute their limited seed output, and their presence has been reduced to a scattering of single plants in the open and the occasional small grove in confined places like river gorges, where they enjoy some protection from fire. The question arises: Should we try to re-create the kind of plant cover that characterised frost flats before man arrived? Or should we be content to preserve these areas in their altered, but still valuable, state?
Other questions arise as a result of this consideration. Without regular fires to make way for new monoao seedlings, what will happen when existing stands of monoao reach the end of their natural lifespan? Old plants can linger on; as branches collapse outwards, they often “layer”, producing new root systems where they meet the ground, sometimes even producing feeble “new” plants. In addition, a few seedlings manage to establish at the base of silver tussocks. It seems likely, however, that woolly moss will become more and more adundant and that the shrublands of today will mostly end up as mossfields tomorrow, more open and desolate than ever.
As with many greatly reduced natural systems, there are ominous clouds on the horizon for the few areas of frost flat that remain. Amongst the more threatening are weeds, in the shape of aggressive introduced plants that oust natives. Lodgepole pine (Pinus contorta), introduced as a timber tree from the Rocky Mountains of North American, can tolerate extremes of cold far in excess of anything frost flats ever experience, and also has the unfortunate habit of producing seed at the extraordinary age of only three years. If left to its own devices, it displaces native vegetation completely, nothing surviving in its dense shade.
Mouse-ear hawkweed (Hieracium pilosella), the bane of South Island dryland farmers, has gained a foothold on the plateau and seems to be expanding from seed and runners. It establishes happily amongst coral lichen and grows underneath the lichen carpet, eventually sloughing this off and killing it. Biological-control agents now being introduced may limit the vigour of hawkweed.
Gorse (Ulex europaeus) and broom (Cytisus scoparius) not only smother native vegetation but also fix nitrogen from the air, changing soil fertility. Ling heather (Calluna vulgaris), introduced to provide partridge habitat in Tongariro National Park, spreads steadily along roadside gravels and makes a regular appearance on the edges of frost flat. Fortunately, its beautiful mauve flowers make identification and removal in autumn easy. Yet other exotic species tend to capture the vacant growing space created by fire more quickly than natives.
More subtle are the changes in fertility wrought by various uses of adjacent land. Top-dressing drift allows dense swards of European grasses such as browntop (Agrostis capillaris) and Chewings fescue (Festuca rubra) to establish, displacing natives. Nutrient pulses released after the clear-felling of pine plantations on surrounding hills can boost weed growth at the margins. Rabbits and hares redistribute nutrients both vertically and horizontally, concentrating them at the surface and allowing weedy grasses such as Yorkshire fog (Holcus lanatus) to establish. Although native, black-backed gulls at Rangitaiki bring in yet other weeds, including sheep’s sorrel (Rumex acetosella). Most destructive of all intruders, humans damage the fragile plant communities directly with the indiscriminate use of off-road vehicles and the uncontrolled lighting of fires.
Like many unique and special places, frost flats are now firmly on the conservation radar screen. At Rangitaiki, the Department of Conservation is carrying out a programme of weed control. Land-management practices more sympathetic to the integrity of frost flats are being promoted on adjacent property. Ecosystems which, like frost flats, are naturally rare are part of an eight-year research programme being undertaken by Landcare Research and funded by the government-supported Foundation for Research, Science and Technology. Hopefully, Katherine Mansfield’s raw encounter with nature in one of her harsher guises will be an experience available to others for a long time to come.