The Remote Ruahine

Forest and Bird, Issue 314, 1 November 2004, Page 12

The Remote Ruahine

SHAUN BARNETT

explores a rich enclave of

native plants and wildlife in the wilderness of the northwestern Ruahine Range.

here's a common perception that national parks contain our most pristine, spectacular and diverse landscapes. But other areas, such as scenic reserves and forest parks, can also feature some of the country’s most important ecosystems and striking landforms. One such place is the northwestern corner of the Ruahine Range, east of Taihape. A Department of Conservation botanist, Vivienne Nicholls from Palmerston North, describes the area as an ‘enclave of biodiversity. As well as a large number of rare species, including a bidibidi found nowhere else, the area also features red tussock grasslands now scarce in the North Island, as well as the only lake and lowland podocarp forest in Ruahine Forest Park. Perhaps the area’s most striking landscape occurs at Ruahine Corner, in the northwestern extremity of Ruahine Forest Park, where a snug hut overlooks the Mangaohane Plateau. Trampers who visit Ruahine Corner hut could be forgiven for thinking they were not in the North Island, but instead in a landscape of central Otago, or Kahurangi National Park. To the east, limestone cliffs drop away into the beech-clad Ikawatea Valley, a landscape boundary not

dissimilar to that of Kahurangi’s Thousand Acre Plateau. To the north, the Mangaohane Plateau features rolling, red-tussock grasslands, sinkholes, peat bogs, exquisite tarns, and limestone fashioned into ribbed outcrops. It is a striking forest boundary, however, that marks Ruahine Corner as unique. Here extensive pahautea (cedar) forests curl over undulating hills to end abruptly at the redtussock grasslands of the Mangaohane

Plateau. The scientist Les Molloy described it as ‘one of the most dramatic indigenous vegetation contrasts in the North Island’ It is perhaps not surprising then, to discover that this is not a natural margin, but a human-induced one. A DoC botanist, Geoff Rogers, has had a long association with the northwestern Ruahine Range dating back to the mid 1980s and wrote his PhD dissertation on the landforms and biogeography of the area. Later, in the mid 1990s he was employed by Landcare Research in Rotorua to write a Protected Natural Areas report for the Moawhango Ecological Region, of which the northwestern Ruahine is a part. Despite now living in Dunedin, he maintains an enduring interest in the place. Geoff Rogers says that the pahautea forests at Ruahine Corner are the most extensive of their kind in the North Island, a sizeable remnant of a 300-square-kilometre swathe of this ancient forest type that used to stretch from present-day Waiouru to Ruahine Corner. Fire, he explains, created the current forest boundary. Early Maori occupation of the area resulted in two significant conflagrations, both lit during the moahunting era, probably deliberately. ‘The first, which occurred about 570 years ago, burned much of the western pahautea and beech forest about Waiouru. A second fire on the Ngamatea and Mangaohane plateaux followed about 430 years ago. Geoff Rogers’s research on these burn-offs was the first to reveal that ‘our wetter montane forests went in huge conflagrations, rather than piecemeal fires’ Subsequent fires — and

there were many — gradually eliminated secondary shrubs such as inaka and mountain toatoa, and ‘selected’ for today’s red tussock grasslands. As the ‘fuel load’ remained low, these later fires did not substantially alter the pattern of relict forest. Later, in the 1880s, Pakeha settlers brought sheep and cattle to graze on the Mangaohane Plateau. Initially, stock was driven over the main Ruahine Range back and forth from Hawkes Bay but, as rail and road links improved, Wanganui became the main connection. Much of the lower-altitude red tussock was converted to rye and clover with Government subsidises in the early 1980s. The last big fire occurred in 1948, and the remaining red tussock grasslands on the plateau have been slowly changing back into shrublands ever since. Without further fires or other disturbances, more shrubs will establish, eventually allowing pahautea forest to regenerate. While human-induced fires provide an explanation for the current forest boundary at Ruahine Corner, far more complex processes have shaped other aspects of the unusual vegetation of the Mangaohane Plateau. The Makirikiri Tarns and Reporoa Bog are two peat bogs on the plateau that existed before the fires, enabling some special and wholly local plant species to survive. While both peat bogs lie within a few kilometres of each other, they have distinct ecologies, and both contain quite different plants. Other spots on the plateau also contain rare and locally specific plants. What many of these plants have in common though, is an uncanny link with South Island flora. For example, one species of eyebright, Euphrasia disperma, occurs in

the northwestern Ruahine Range but doesn’t crop up again until north Westland. Similarly, North Island records for Myosotis tenericaulis are restricted to the northwestern Ruahines and Waiouru, but this forget-me-not is more prevalent in the South Island. One creeping native herb, Tetrachondra hamiltoni, doesn’t occur again until Otago. The foxglove Ourisia modesta, known from just one damp location at Ruahine Corner, is also a rare plant in the South Island. Other plants in the area fit the same general pattern. These biogeographical anomalies confounded early botanists Norman Elder and Tony Druce who, in the 1940s, were amongst the first to study the area’s vegetation. Druce went on to describe the area’s special plants, and in turn spurred the interest of Geoff Rogers. Only as understanding of past landforms increased have plausible explanations of such unlikely plant distributions emerged. Rogers says these plants exist here because of a unique combination of factors: the soils, landforms, altitude, climate and geological history. ‘The habitats are just right for those species; here alone in the North Island; he says. Two theories can explain what are essentially unusual North Island occurrences of predominantly South Island plants. The first is to consider them refugees from North Island Miocene landscapes that existed before a more-recent period of inundation. In the late Miocene and Pliocene, some 8-2 million years ago, sea levels rose and flooded much of the lower North Island. During this period of submergence, marine deposits formed a

limestone cap in some areas, including the northwestern Ruahine Range (where limestone outcrops are still evident). While the inundation extinguished the "Miocene refugee’ plants from the lower North Island, the central North Island provided a refuge for them. After later tectonic uplift raised the Mangaohane Plateau, in the last 1-2 million years, the ancient plants colonised the area, but were subsequently lost from some of their former refuges in the central North Island. The second explanation suggests that, after the inundation, the Miocene refugees spread from their South Island strongholds, but survived only at localised spots like the northwestern Ruahine Range where their specialised habitats still exist. Other potential habitat elsewhere in southern North Island has been ‘too faulted, folded, and eroded’ for the plants to persist. The extent of such geological disruption is evident in the main Ruahine Range, which was upthrust during the last 1-2 million years. Some marine gravels exist on a few summits of the Ruahine Range, but the soft limestone cap has eroded off to expose the underlying greywacke that modern day trampers are so familiar with.

While both explanations are seductive, one plant doesn’t quite fit the biogeographical pattern. The bidibidi Aceana rorida seemed to occur only in the northwestern Ruahines and nowhere else. Geoff Rogers calls it ‘an enigma. However, botanist Kelvin Lloyd and colleagues have ‘possibly discovered’ the same plant in the Idaburn Valley of Central Otago. Even so, this would still mean the bidibidi has one of the largest disjunctions of any species in New Zealand — about 1000 kilometres. Vivienne Nicholls, from the Department of Conservation in Palmerston North, has been studying the local bidibidi Aceana rorida, carrying on from work begun by Geoff Rogers. This small bidibidi is restricted to the Makirikiri Tarns, a large peat bog just north of the Ruahine Forest Park boundary on Maori land owned by the Aorangi-Awarua Trust. Vivienne Nicholls describes the bidibidi as a flat-growing, no-hooks specimen, with leaves of a ‘lovely cloudy-pink colour’ Part of her study aims to monitor the potential threat posed to it by the introduced weed Hieracium (hawkweed). So far, so good, as the two plants seem to occupy slightly different niches. However, like other rare species, this little bidibidi maybe at risk from hybridisation with a more common species, the native bidibidi Aceana novae-zelandiae.

are birds as well as rare plants add to biological diversity of the orthwest Ruahine. At Ruahine Corner, North Island brown kiwi are infrequently heard at night, while during the daytime fernbirds, North Island kaka, falcon/ karearea and kakariki parakeets might be spotted. In fact, with the exception of kokako, just about every threatened North Island bush bird on the list crops up in the area. South of Ruahine Corner, while the limestone persists spasmodically, the pahautea forests end, and you find yourself instead in the beech forests so widespread in most of Ruahine Forest Park. In the headwaters of the Mangatera Valley, amidst a sea of such beech forest, an island of lowland podocarp forest exists — the only such area in Ruahine Forest Park. This forest surrounds the sole lake in the park, Lake Colenso or Kokopunui. It is named after the missionary-explorer William Colenso, who made several crossing of the Ruahine Range in the 1840s and early 1850s, although he never visited the lake himself. Archaeologists have discovered a number of sites around the shallow lake that suggest Maori parties occupied the area seasonally. Kokopunui served as a base during bird and eel-hunting forays.

While the birdlife today has probably declined from when these sites were occupied, an impressive number of species still survive, including New Zealand pigeon, North Island robin, whitehead, fantail, tui and bellbird. Blue duck whistle their lonely whio in nearby rivers. And birds are not the only winged creatures either; long-tailed bats inhabit the forests around Lake Colenso, while there are unsubstantiated records of short-tailed bats in nearby areas. Occupying a shallow depression above the Mangatera River, Lake Colenso lacks any significant in-or outflow. Consequently it remains free of introduced trout and weeds, and forms a haven for native freshwater fish. The scarlet mistletoe Peraxilla tetrapetala, the endangered Pittosporum turneri, and the root parasite Dactylanthus taylorii can also be found in the area. The native snail Powelliphanta marchanti also adds to what is a real hotspot of biological variety. Altogether the northwestern Ruahine Range displays a diversity of landforms, vegetation, and endangered animals rare for such a small corner of country. I doubt there’s a comparable portion of any national park that surpasses it. — SHAUN BARNETT of Black Robin Photography is writing a book about forest parks, which he believes have lacked the attention they deserve.