For motorists taking State Highway I through the central North Island, one of the most memorable sections is surely the stretch between Turangi and Waiouru known as the Desert Road. Although Mt Ruapehu's bulk is often concealed by cloud, the plains of rippling brown tussock which make up the volcanic plateau provide a strangely compelling vista. An army of power pylons stretching into the distance completes the alien aspect of this austere landscape. Yet the actual Rangipo Desert, which gives its name to this stretch of highway, is poorly known and little visited.
The desert encompasses an area of approximately 100 square kilometres between the Desert Road and Ruapehu. Of this, a little over half is Army land, and the rest is part of Tongariro National Park. The Army uses its part of the desert, along with a large area of land on the eastern (Kaimanawa) side of the Desert Road, for training and exercising.
From the road, Rangipo Desert appears as a barren, stony wasteland bereft of trees. For the curious few who venture into the interior, however, a harsh beauty is revealed, particularly in the gentle light of dawn and dusk. My own exploratory journeys into this region have left two lasting impressions: the power of erosion, and the tenacity of the plant life which survives here.
As would be expected of a desert, water is in short supply here. Yet it is not through lack of rain: Rangipo receives more than 2000 mm annually-twice as much as Wellington. The problem is in retaining the walter. The "soil"-an unusually coarse mixture of pumice, sand, scoria and ash from past eruptions, containing next to no organic matter-has little water-holding capacity. Much of this distinctive and unprofitable substrate has been carried down from the summit of Ruapehu by lahars (mud slides). The shape of the upper cone of the mountain means that the face adjoining Rangipo Desert is much more prone to this sort of devastation than other slopes are.
From below, gravity sucks the water away through the porous debris. From above, the strong winds which bring moisture one day carry it off by evaporation the next. The soil can offer only limited resistance to these two relentless forces, and, along with scorching summer hcat, becomes bone-dry for much of the year.
The area is frequently lashed by wind. Funnelled by the mountains, the prevailing north-westerlies whistle across the desert, while southerlies roaring up between the Kaimanawa Mountains and Ruapehu also touch the landscape with their chill fury.
The winds not only dry the desert but blast exposed plants, causing wind-burn at any time of the year and often freeze-burn during the cold months. Over winter, plants have also to contend with the effect of severe frosts. These frosts break apart the top layer of the soil with what is known as "frost heave." As the moisture in the ground freezes, it forms columnar ice crystals up to 5 cm long which prise apart and splinter the earth, leaving the surface loose and ready to be carried off by wind or rain.