Forest Vines to Snow Tussocks: The Story of New Zealand Plants
The fate of the original vegetation cover of the various regions of the Australasian/Antarctic portion of Gondwana has depended on crustal movements and climatic change.
In the case of Antarctica, deterioration of the climate in the later Tertiary eventually led to the extinction of vascular plants.
The northward movement of Australia during the Tertiary counter-balanced the temperature decline to some degree, but the pronounced cold spells of the Pleistocene played some part in the disappearance of closed forest from most of the continent. However the major cause of the drastic vegetational changes from the late Tertiary would have been increasing aridity associated with declining temperatures. In the southern hemisphere, aridity is most marked along the Tropic of Capricorn which Australia now straddles.page 249
The extent to which the rich rain forests of New Guinea correspond to those which were formerly widespread in Australia is uncertain. Until recently it was thought that much of New Guinea's rain forest flora was derived from south-east Asia after contact was made with that region. This was also thought to be the case with the rain forests of north Queensland, but this has recently been questioned as many of the genera concerned have a fossil record in Australia extending back to times when Australia and its northern portion which became New Guinea was still far south of Asia.209 Certainly the rain forests of New Guinea in near equatorial latitudes would not have been greatly affected by cooler, drier climates during the Pleistocene. The alpine flora of New Guinea, as already mentioned, although interesting is not particularly rich and includes Asian and Australasian components.
Botanically speaking, New Caledonia is one of the most interesting fragments of Gondwana. Being tropical like New Guinea, although not equatorial, with a strongly oceanic climate due to its narrowness, it too has probably not been greatly affected by coolness or drought. As a consequence its inheritance of Gondwana conifer and angiosperm families would have survived better than those of Australia and New Zealand. This makes it of special importance to botanists in the latter countries attempting to reconstruct the history of their own floras, as now extinct genera in Australia or New Zealand may still have extant species in New Caledonia. A special development in New Caledonia has been the evolution of many species, mostly of Gondwana genera, tolerant of the soils of the extensive areas of ultramafic rock.
Finally we come to New Zealand. Like Australia, New Zealand's northward movement during the early Tertiary would have moderated the effects of any temperature declines and its oceanic climate would have countered any tendency to aridity. However, New Zealand had attained its present position some time before the more drastic climate deterioration of the Pleistocene and the cold climates deriving from the combined effects of high mountains and glaciations, although ameliorated to some extent by an oceanic climate, greatly affected the vegetation. Forests retreated and some of their genera and species became extinct, while new species evolved in colder open habitats contributing to the present relatively rich alpine flora.
The native forests include genera with a long history in the region; the plants of the mountains are a more recent burgeoning of specialised page 250forms in response to the uplift of the mountains and episodes of climatic refrigeration.
The recent advent of human habitation has greatly reduced New Zealand's natural cover — Polynesians achieved it with fire and, to more effect in a shorter time, Europeans with fire, the plough and the introduction of alien plants and animals.
The time has surely come to call a halt, to preserve those areas of largely intact vegetation we still have and to actively encourage the regeneration of native plants in areas abandoned from farming where fire and browsing have taken their toll.