The eastern slopes of Tongariro, near Oturere Hut. The jagged teeth of Ruapehu's Pinnacle Ridge visible on right. Photo NC.

This view is typical of the landscape east of the volcanoes, and not only Tongariro and Ngaruhoe, but equally Ruapehu. The lower slopes of the volcanoes angle gently and merge progressively with the broad plain of the Desert Road.
Not a single, obvious factor can explain the sparse, desert-like vegetation cover of this whole area. Not precipitation, which between 1500 mm and 2200mm (59 in. and 86 in.) annnually is more than sufficient. Not altitude, since at the same height other areas of the park are covered in tussock or even forest. Not volcanism, since the northern and western areas of the park have also suffered from extensive destruction, yet have been recolonised by tussock or forest.
The most likely explanation is to be found in a combination of factors, among which: volcanic, orographic (strong drying winds in the rainshadow of the mountains), and climatic (a relatively high number of days with frost, thus shortening the vegetation season). But the human factor is also thought to have played an important part: the repeated fires, and more recently grazing by domesticated and other animals.
Once the vegetation is damaged, erosion sets in quickly in the loose soils of volcanic ashes and scoria, thus aggravating the process and creating the desert.