The forests of the Grey River headwaters are ecotonal between the mixed podocarp and large-leaved hardwood forests which prevail further south in mid- Westland, and the Nothofagus
forests which prevail further north. At low altitudes, in the south and west of the survey area, the dominant species are kamahi and Quintinia acutifolius
, with some rimu, miro, kahikatea, Hall's totara, southern rata and mountain cedar. At high altitudes mountain cedar, pink pine, Dracophyllum traversii, Olearia ilicifolia
and O. lacunosa
dominate. In the north and east the dominant species are red beech, silver beech, and mountain beech. The sub-alpine shrublands are of more uniform composition and are dominated by Dracophyllum longifolium, D. uniflorum, Phormium colensoi, Podocarpus nivalis
, and leatherwood in places.
The forests and shrublands were divided into 17 associations by using a numerical procedure. The composition, structure and habitat of each is described. The influence of ungulates (particularly red deer) and of opossums on each association and on sub-units of the survey area was determined by considering the susceptibility of the vegetation to browsing, the extent of modification in the vegetation resulting from past use, and the pattern of use at the time of the survey. It was found that in most cases the seral lacebark-broadleaf and kamahi-broadleaf associations were the most susceptible to the ungulates, were the most modified from the original condition and were also receiving the greatest ungulate use at the time of the survey. The sub-alpine shrub associations were generally the least susceptible, the least modified and were receiving the least ungulate use. The kamahi-dominated associations were the most susceptible to opossum damage and were receiving the greatest opossum use at the time of the survey, while the Nothofagus forests were the least susceptible and were receiving the least use.
The areas most susceptible to ungulate damage were in the south where the forest was predominantly mixed podocarp and large-leaved hardwood and the least susceptible were in the north where Nothofagus species dominate. The areas where the vegetation was most modified by ungulates were the Crooked and Haupiri Catchments and the Elliot Range while the least modified were in the vicinity of Mt Te Kinga, around Bald Hill and in the Upper Grey. Ungulate use at the time of the survey was greatest in the Crooked, Haupiri and Upper Grey Catchments and least around Mt Te Kinga and Bald Hill. The most susceptible areas to damage by opossums were in the south and at the time of the survey opossum sign was most evident in the Crooked and Haupiri Catchments and in the vicinity of Mt Te Kinga.