Mounts Ruapehu, ngauruhoe and Tongariro tower on the western skyline of State Highway One’s Desert Road, frequent targets of snapshots by passing travellers. Yet, lurking unseen in the middle distance, often obscured by swirling sandstorms in summer and hidden beneath snowdrifts in winter, lies one of the most distinctive and mysterious landscapes in the country.

Within the great upland Rangipo Desert is an area referred to by Maori—and recorded upon the oldest maps of the area—as Te Onetapu, the enchanted sands. Consisting of fine volcanic material driven from the flanks of Ruapehu by the relentless action of the Whangaehu River, it is a landscape of biological and geographic diversity continually shaped by the relentless processes of wind, water and volcanism over millennia and, more recently, by military shelling activity as well.

A desert within a desert, Te Onetapu presents an ever-changing kaleidoscope of desolate gravel ‘lag’, dotted with low vegetated dunes, interspersed in places by strands of shrubland and seamed by wide watercourses that are mostly dry channels studded with enormous boulders. The channels were formed by lahars, avalanches of volcanic debris that flow out of volcanoes, sometimes with deadly results, as in the Tangiwai rail disaster of Christmas Eve 1953, in which 151 travellers were killed.

It is also one of only a handful of volcanic dune systems on the planet, a place that English botanist William Colenso described in 1839 as “a fit place for Macbeth’s witches”. He noted burnt logs, polished to a glaze by wind-driven sand, the luxuriant tussocks, and the “undisturbed beauty” of Gentiana blooms that moved him to quote Thomas Gray’s elegy:

“Full many a flower is born to blush unseen, and waste its sweetness on the desert air.”

Yet, lying in the rain-shadow of the central volcanoes, this so-called desert is not a true desert at all—it receives two metres of annual rainfall in the north and a metre in the south, enough to support a forest. A short stroll onto one of the highest dunes reveals another world—the unmistakable feeling of the cool, humid interior of a rainforest. A pocket of mature mountain toatoa, bog pine and broadleaf is several centuries old and has the usual features of rainforest: ferny ground layers, shrubby understoreys and epiphyte communities, all in the middle of an otherwise barren landscape. The widespread absence of trees must be explained by reasons other than climate alone.

Nearly 2000 years ago, a tide of molten pumice from the most recent cataclysmic eruption of Taupo roared south between the central volcanoes and the Kaimanawa Mountains to the east, flattening and incinerating most of the existing beech forest in its wake and leaving the charred remains first noted by Colenso. More than 1000 years later, Maori arrived in the region, using fire to keep travel routes open and the forest at bay.

The area remains exposed, located in a wind funnel between two great natural obstacles. The soft, unconsolidated volcanic sediments are also highly erodible, and dust and sandstorms are a feature of the region during the dry summer and autumn months.

The high altitude, 900 m in the south rising to 1200 m in the north, means frost-heave occurs regularly during colder months, uprooting most seedlings that may have established. The ‘unseasonal’ frosts that are a feature of mountain climates, especially in late spring, can be devastating for soft new growth. Coarse sandy soils are renowned for their poor ability to retain water, and drought also plays a role in inhibiting the development of vegetation. Despite reasonable rainfall, this is an inimical environment for plant growth.

A sequence of changes in vegetation (termed ‘succession’ by botanists) in the desert is under study by Landcare Research. It begins with scabweed, which establishes on the gravel lag and traps wind-blown sand, slowly forming low mounds that are then colonised by shrubs—tauhinu, Ruapehu hebe or coin-leaved tree daisy. In places, red tussock—a huge, long-lived grass that in many ways mimics shrubs in its behaviour—appears to usurp their role. These ‘islands’ of vegetation grow in height and width by continuing to trap sediment, eventually allowing small trees such as mountain toatoa or bog pine to establish. If conditions allow, mounds coalesce to form a chaotic dune system of mounds and hollows, supporting islands of short forest. But they are just as likely to ‘blow out’ in the classic manner of dunes, exposing the root systems and slowly killing the developing forest. The system is then back to square one.

Why uninterrupted dune formation has occurred in some places but not others remains something of a mystery.

Analysis of vegetation composition and plant age—a surrogate for mound age—on a series of dune mounds of different heights (and presumably ages) will, it is hoped, reveal more about this system.

The New Zealand Army acquired most of the Rangipo Desert as part of its training area during World War II, and it has since become the major focus of the military’s shelling activities. Fire is a rare aftermath of shelling, as the odd swathes of burnt shrubland attest.

Due to the hostile environment, invasive plants that plague other regions of the country have failed to gain a foothold in the desert. North American lodgepole pine, capable of growing at much higher elevations than any native tree, was seeding from Karioi Forest, a plantation forest on the southwestern fringe of the desert, forming impenetrable stands that displaced native vegetation. However, a major weed-control programme undertaken in recent years by DOC and the Army has stemmed the green tide. Lodgepole pine is no longer cultivated in Karioi, but wildings continue to appear from time in time.

Other weeds, such as ling heather, which was planted at the Chateau on the north-western side of Ruapehu in the 1920s to provide grouse habitat and is now widespread there, has scarcely made an impact in the desert. Likewise mouse-ear hawkweed, also of European origin.

Provided that lodgepole pine is closely monitored, Te Onetapu will remain a unique natural area, a testament to the overwhelming power of nature to shape and reshape landscapes and to the remarkable tolerance of the diminutive flora and fauna that thrive there.