In praise of North-west Nelson

Forest and Bird, Volume 23, Issue 1, 1 February 1992, Page 22

In praise of North-west Nelson

Peter Williams

The north-west corner of the South Island — less than 3 percent of New Zealand — is an area with an astonishing variety of rock types, soils, landforms and vegetation. This region of bewildering ecological complexity contains New Zealand’ oldest rocks and half of our plant species including 50 that occur in no other part of the country. It is also the home of two-thirds of the world’s species of giant land snails. Last June the New Zealand Conservation Authority set in train an investigation for the establishment

of a national park in north-west Nelson.

looks at why north-west Nelson has long been

considered a natural region of exceptional importance.

HOUSANDS of square kilometres of coasts, valleys and mountains form the bulge at the north-west corner of the South Island, between the Tasman Sea, Golden Bay, and in the

south, the Buller River. The area is remote and sparsely populated with roads only at the margins. Beginning at one corner of the area proposed for the national park, just inland from Karamea, lies the broad Oparara basin. Here, in an area encircled by hills

that prevent the outward flow of cold air, are some of the densest rimu stands in the South Island. Beneath the ground are archways and caves with fossil bird bones and the giant north-west Nelson cave spider. In contrast, the Heaphy basin to the

north opens to the sea allowing the cold air from the mountains to flow out at night. The forests are therefore more influenced by the warm, moist coastal climate and are richer and more diverse. They contain dense stands of rimu, miro, matai, beech, rata, nikau and, most sig-

nificantly, pukatea. This mighty buttresstrunked tree reaches its southern limit as a conspicuous component of the forest only a little further south. Birds such as South Island kaka and yellow-crowned parakeets abound in these Heaphy and Oparara forests. In the evening a blue

duck may whistle upstream, while deeper into the night a great spotted kiwi may call. Long ago, moas collected limestone pebbles here as gizzard stones then deposited them further inland in non-limestone areas, inadvertently providing evidence of their passage.

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Between the Heaphy River and Kahurangi Point is the northernmost breeding fur seal colony in New Zealand. On rocky outcrops are the common coastal plants of this part of the world. These include succulents related to the familiar garden fat-hen that have adapted to the full blast of the salt-laden sea. Slightly further back from the tideline the shore hebe displays lilac flowers, protecting beneath its tangled branches the coastal spleenwort, tiny filmy ferns and the prostrate Peperomia urvilleana, which is near its southern limit.

NLAND from the coastal limestone, clear patches stand out from the beech-covered, granite hill country. These are the downlands, covered by red tussock, umbrella fern and wire rush. The Mackay Downs are the lowest, while Gouland, Gunner and Gorton downs are at higher altitudes. Many plant species are found on the downlands: amongst the tussock, in the bogs, by little creeks, and in the forests covering the tongues of limestone that protrude out into the tussock. A few species grow nowhere else but on the

Gouland Downs, such as the little yellowflowered Bulbinella talbotii. There are other surprises on the downlands, especially on warm foggy nights. These conditions suit the giant land snails, Powelliphanta, and perhaps evoke the climates of the ancient land mass of Gondwana where they evolved along with the tuatara and kiwis. Each of the many Powelliphanta snail species lives as if on an island, separated by mountain chains and also, more recently, by the fragmentation of Nelson’s once almost-continuous forest cover. Several species are confined to areas of limestone, like those at Mokihinui or in the Oparara, partly because the snails require the limestone to make their shells and eggs. Many species have striking concentric and radial markings, and warm colours of tawny fawns or rich chocolate browns. It now appears that the wonderful variety of these snails is the result of a very long evolutionary history rather than the widely-held misconception that they rapidly radiated into many colour forms during recent mountain-top isolation.

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T THE MOST northern point of the South Island, curving 25 km into Golden Bay, like the claw of a giant crab, lies Farewell Spit. The spit comprises 9,500 hectares of quartz sand, derived from rocks mined by mountain erosion then joggled and ground at sea on its journey along the barren beaches. Now, after fire and the introduction of grazing animals, many of the native sand plants such as pingao and silver sand grass have been replaced by the introduced marram grass and scattered tauhinu shrubs. Between the dunes are meandering sand slacks, populated by the star-flow-ered herbs, Lilaeopsis, and several little sedges found nowhere else in Nelson. The vast tidal flats on the south side are greened with fields of eel-grass waving underwater, then collapsing to a squelchy lawn at every low tide. Out here the probing beaks of 50,000 wading birds search in the mud for food. Most numerous in the summer are migrants from the northern hemisphere: up to 25,000 knots, 20,000 godwits, and birds whose names evoke distance and mystery — Siberian tattler, Asiatic wimbrel, Hudsonian godwit and Mongolian dotterel. But not all have come so far to the feast, for many thousands of birds from within New Zealand spend their non-breeding time at Farewell Spit — the oystercatchers, pied stilts, banded dotterels and black swans. In spring most will head for the braided riverbeds but black swans remain to breed, alongside a recently established gannet breeding colony — only the third mainland colony in New Zealand. ERHAPS IT IS TIME to reflect on the complex reasons that north-west Nelson is such a treasure trove of vegetation and fauna. The whole north-west Nelson land mass is moving north along the Alpine Fault, as it has for millions of years. Only in western Southland, at the other end of the fault, are there similar rocks of comparable antiquity and variety. But whereas most of the high parts of western Southland were

capped with ice during the Pleistocene glaciations, much of north-west Nelson remained ice-free. The lack of a complete ice sheet also meant the survival of both ancient peneplained landscapes and overlying remnants of young, covering rocks. Today’s climate plays its part too, and several species common in the northern lowlands reach their southern limits in the mild climate of north-west Nelson. The differences in cloud cover and rainfall from west to east also contribute significantly to the diversity of vegetation and plant species: twice as much rainfall and half the sunshine falls on the western mountains compared with those further east. One consequence is that soils in the west are more heavily leached of plant nutrients. Many western species do not extend right across the region and, conversely, many eastern species do not extend to the far western sector. E NOW TURN SOUTH to the endless mountains of the Wangapeka, Arthur and Matiri districts. An aerial view of the mountainous interior reveals the

sombre greens of vast forests (mostly beech), the browns of a band of subalpine scrub above the forest and, beyond that, the tawny hues of the tall tussocks and carpet grasses, and a multitude of colours in rock and scree. Only in winter and early spring is there snow on these mountains for any length of time although small patches will survive into summer. Deep river valleys slice through the forests. Trampers know the larger valleys well: the Wangapeka, Little Wanganui and the Heaphy. The largest river of them all, the Karamea, roars down after

rain, past mist-shrouded hills foaming and thundering, thick with silt. Come late summer and the waters tinkle with a mineral glitter over granite boulders, or glide hushed through bottomless pools where speckled stones lie like trout quietly reflecting the sun. Massive eels inhabit these depths too, perhaps chasing a common bully darting for cover. At sundown caddis flies will dance the surface, drawing to the pool a native long-tailed bat or, rarer still, a lesser short-tailed bat. Aside from a few pockets of podocarps on river terraces, the beech forests reign supreme with only a scattering of rimu, miro and Hall’s totara. At lower altitudes red beech and hard beech, together with southern rata on dry sites, are the most important, while silver beech increases greatly on higher slopes. Mountain beech occupies extreme sites at the tree line while silver beech dominates the tree line in the west where it favours more fertile, moderately drained sites. In the Anatoki Range overlooking Golden Bay there are tree lines at 1550 m, as high as any in New Zealand. Many stretches of tree line in northwest Nelson are abrupt and, where low saddles and cols occur at or near the limit

of tree growth, a dense scrub of beech may form a tight stunted mass. In most extreme sites beeches will grow horizontally as well as vertically. Gripping the bare earth and half buried for their length, trunks protrude knee high, the foliage on their knobbly branchlets hedged to reflect the prevailing direction of cold air and wind-blasted sand. Subalpine scrub grows more upright in such places. Furry-backed leatherwood grows abundantly with Dracophyllum uniflorum in areas of high rainfall but fades out half way across to Takaka. Boulder fields are marked by species of

Hebe endemic to north-west Nelson. Rock bluffs that spawned the boulders from above are festooned with layers of bluish Dracophyllum pubescens, another endemic, and dark clusters of Helichrysum intermedium shrubs. Should water seep down the face all sorts of plants will appear, including the densely flowered and glossy-leaved Celmisia bellidioides, or the long-stalked Forstera mackayi. On drier rocky sites masses of edelweiss with yellow and white furry flowers appear. Tall tussocks, some 100 years or older, are the main cover of the high mountains although carpet grass — with shiny and tightly rolled leaves to reduce moisture loss — often creeps out onto the shallow soils of exposed ridges. Mid-ribbed snow tussock is the most abundant of the tall tussocks, while broad-leaved snow tussock, favoured by deer, prefers sunny fertile sites. Red tussock defines the wet sites more accurately than any hydrologic instrument. Amongst the tussocks are alpine herbs, many occurring only in north-west Nelson. Larger plants include the north-west Nelson endemic Astelia skottsbergii, with its flax-like, yellow-green leaves, and Astelia nervosa whose clustered silver leaves guard bunches of bright orange fruit. No sight in the mountain grasslands rivals, however, the pale orange and green heads of female wild speargrasses in a flowering

year. In late summer they stand a full two metres tall with thousands of plump car-rot-flavoured seeds bursting forth from between the wicked spines. Only the giant weevil dares penetrate the bases of these plants to suck at the sap. The true scree plants of the eastern South Island are rare in north-west Nelson due to the paucity of active scree here. Only around the drier eastern margins of the Arthur and Wangapeka districts are screes developed enough to support scree species such as Stellaria roughii and Lobelia roughii. Coloured greygreen or purplish they huddle amongst the rocks, sheltering not from the searing sun but apparently from the searchings of alpine grasshoppers. Beneath the stones may be found the giant scree weta, the most spectacular of our native insects, while keas, falcons, gulls, pipits and the occasional rock wrens frequent the tops. On flatter sites, where drainage is slowed by impermeable rock, the remains of innumerable plants accumulate to form peat. Small tarns often form on the bogs, their shimmering black faces reflecting the dark peat beneath the water. Grasshoppers scull the surface in search of safety. Between the tarns are mounds of comb sedge and compact cushions of Donatia and Phyllachne starred with small, white flowers. Between them are the larger blue flowers of the lily, Herpolirion novae-

zealandiae, and creeping plants of Nertera and Coprosma bearing orange berries, a favourite food of the kea. The kakapo, too, was fond of these. This unique ground parrot was thought to survive in the most remote parts of north-west Nelson until very recently. HE FULL DIVERSITY of the landscapes and flora of the northwest Nelson mountains can only be understood with reference to the underlying rocks which run northsouth in four wide contrasting belts. Their different chemistry and modes of weathering contribute to the diversity of habitat that makes the region so rich in plant species. In the west, and again in the south-east above the Buller, are grey granite mountains that leach to an infertile soil. Many plants that grow there reflect the infertility by their small harsh leaves, while plants from more fertile regions are absent. Because granite does not shatter readily, huge boulders and outcrops survive for long periods of time, heaped like blocks in a giant’s playground. Their cracks and fissures offer refuges for plants from browsing goats and deer. Alongside and east of the main area of granite, and stretching from Golden Bay to the Buller, lies a 20-kilometre-wide

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belt of quartz-rich sandstone and siltstone. These strata form the breathtaking Anatoki Peaks and Dragons Teeth on the Douglas Range. On their broad northern slopes red tussock on poorly-drained soils alternates with mid-ribbed snow grass on better-drained soils. Both are studded with Astelia and Aciphylla. Further east, the rocks are extremely diverse with some that shatter to produce mobile rocky screes, while dark grey volcanics or pale crystalline limestone protrude stubbornly from ridges. This zone harbours almost the full botanical splendour of north-west Nelson. Plants endemic to the region include the large Astelia skottsbergii and the tiny Gentiana filipes, with its intricate rosettes and minute seeds that sail on the wind to seek the tiniest patch of limestone. The conglomerate rocks in areas such as the slopes behind Lake Peel above the Cobb Reservoir are packed together like a superhard, colourful, concrete. It breaks into heaps of angular blocks, providing stable landscapes for shrubs such as Hebe cockayneana and Brachylottis adamsii to colonise. Many rocks here are rich in iron, so they weather to reddish colours visible even from the top of the Takaka Hill, 22 km away. Extremely iron- and magne-sium-rich soils develop on rocks from serpentine and asbestos such as those in the Cobb and Takaka valleys. A small group of scrubby plants are quite at home in the hostile chemical environment these rocks produce as well as a species of red tussock, and several small herbs that are found virtually nowhere else but on these soils. The most-eastern of these rock groups is the sedimentary belt of rocks that forms the skyline peaks of the Arthur Range.

These lime-rich ancient rocks dissolve in the acid groundwater rather than crumble, and so there are numerous holes in the landscape. The deepest of them lead to some of the world’s most spectacular cave systems under Mt Arthur and Mt Owen. The shallow holes and trenches provide wonderful places for plants to grow, especially as deer and goats cannot reach them. They are well drained, fertile and sheltered, like giant inverted planter boxes. Their walls are studded with glossy-leaved buttercups and senecios, and even sensitive ferns that perhaps survive because the constant down-draughts of cool air protect them from frosts. The tussock lands have been reduced in quality and extent by past grazing but plant diversity is very high with profusely flowering willow herbs and many other alpine plants abundant on the limestone screes. Further to the south-west, almost to the Buller, are rocks and landscapes of an entirely different nature. The Garibaldi Ridge above the Karamea and the Matiri Tops are plateaux of young Tertiary rocks that avoided the worst of the glaciations. Nowhere else in New Zealand are such landscapes to be found above the tree line. These limestone plateaux look down on debris-filled valleys and lakes dammed by earthquakes. These are very special places. The plateaux are covered in red tussockland and more broad-leaved snow grass than grows in most of the rest of north-west Nelson. About 500 plant species are found here, amounting to about half the total flora of the region, and between 20 to 25 percent of the total New Zealand flora. From the ramparts of Garibaldi Ridge, we can see further ranges to the north. Smoke curls up as a leatherwood fire quickly boils the billy. Evening brings the

native mountain trout, or koaro, from beneath the tussock-studded tarn banks. Cold air settles now, drawing sun-stored warmth from the rocks while the moon throws glistening light across the stilled waters. Time now to lie beneath the stars and wonder about this vast region that has something of everything and much more of what our spirits need. % Peter Williams is a botantist with the DSIR in Nelson.