Looking east towards the Kaimanawa Ranges from Tama Saddle area. Photo NC.

The sparse and stunted state of the vegetation here is the result of the harsher condition created by altitude (approx. 1400 m/4593'), but no doubt also of the strong westerly winds funneled through the Saddle between the big volcanoes.
Like the rest of New Zealand, Tongariro National Park is subject to a weather pattern in which westerly winds are dominant and which brings depressions and anticyclones in succession. Because they lie high across the path of the weather, the volcanoes have very few windless days per year, and their western sides receive high rainfall (2800 mm/110 in. at 900m (2952') near Whakapapa Village, more so higher up).