Forest Vines to Snow Tussocks: The Story of New Zealand Plants
On the higher, steeper mountains, particularly of the South Island, herbfield gives way with increasing altitude to fellfield (Fig. 105). As a result of the severe environmental conditions in this zone, plants of this type of vegetation are both sparse and specialised. They are subjected to low average temperatures, heavy frosts, deep snow for part of the year, violent winds, and at times strong sunshine, which may raise the temperature of the extensive areas of exposed rock very considerably. Erosion by frost action and wind is quite rapid particularly on the greywacke sandstone of the Southern Alps and other axial ranges. The resulting angular fragments may form a thin layer of debris on ridge crests and sides; but gravity, assisted by wind and, more dramatically, avalanches, constantly moves the products of erosion downslope, so there is little opportunity for the development of even thin soils. Rock outcrops and bluffs in this zone provide more sheltered and secure sites for plants. Nevertheless a number of them do establish away from outcrops, especially in deeper debris on moderate to gentle slopes. Of these, some are small prostrate Hebe shrubs which are also to be found on rock outcrops. They are not of the whipcord type, but have small rounded leaves arranged in attractive patterns. Hebe haastii and H. epacridea (Fig. 106) are yellow or orangey-green; H. petriei is grey-green. Small herbs are more common — several diminutive grasses and sedges, and one to several species of Epilobium, Ranunculus, Celmisia, Gentiana, Myosotis and Parahebe. Small cushion plants also occur; these include Hectorella caespitosa, the softly pubescent species of Chionohebe, Phyllachne colensoi, and sometimes extensive mats of the silver grey Celmisia sessiliflora.
Figure 107 A rock in fellfield on the crest of the St. Arnaud Range, northern South Island. Frost action has formed fissures in the rock where small cushions of Colobanthus canaliculatus (left) and Raoulia bryoides (centre and right) have established.
Photo: J. W. Dawson.
Figure 108 Large cushion of the Marlborough vegetable sheep, Haastia pulvinaris. Mt. Cupola, Nelson Lakes National Park.
Photo: J. W. Dawson.
Figure 109 Close view of a portion of a Haastia pulvinaris cushion showing the branchlet tips closely invested by woolly leaves. Photo: J. W. Dawson.
Two spaniards (Aciphylla) are notable cushion plants in fellfield. A. dobsonii in South Canterbury and North Otago may form perfectly hemispherical cushions more than half a metre in diameter (Fig. 110), while A. simplex of Central Otago forms somewhat smaller cushions. Both species have thick, rigid leaves coloured bright orange-yellow.
The two flowering plants of fellfield which hold the altitude record are Hebe haastii and Parahebe birleyii, both having been recorded at 2900 m in the Mt. Cook region.144 At 2800 m Ranunculus grahamii is not far behind. Above these altitudes only lichens and mosses cling to steep rocky faces in the zone of permanent snow.