Forest Vines to Snow Tussocks: The Story of New Zealand Plants
Derivation from Plants Already Here
Derivation from Plants Already Here
Cockayne suggested the possibility that a few alpine species from the much earlier mountain building period of about 100 million years ago (Rangitata Orogeny) may have survived through the warmer, mountainless times in suitable sites such as rocky bluffs.63 He described these species as 'plastic', that is, that although requiring open habitats, they would have had a wide temperature tolerance. With the coming of the recent Ice Age and high mountains these surviving 'alpines' or their derivatives would have diversified into the new cold habitats.
Present rocky coast species of otherwise alpine genera could appear to support Cockayne's hypothesis. Some of these are: the spaniard, Aciphylla squarrosa, of the Cook Strait coasts; Gingidia montana on the Marlborough coast; Celmisia mackaui of Akaroa, Banks Peninsula and C. lindsayi of the south-east Otago coast; Anisotome lyallii of the south-east Otago coast, Fiordland and Stewart Island; and Phormium cookianum, perhaps equally common on coastal rocks as in the high mountains. The problem here is that these are all herbaceous flowering plants and 100 million years ago fossil evidence indicates that flowering plants, then in the early stages of their evolution, were predominantly woody. If the fossil record can be taken as a reliable guide (and some question page 195this) it was not until long after the time of Rangitata Orogeny in higher northern (and perhaps also southern) latitudes that herbaceous flowering plants began to expand and diversify.
However, a modification of Cockayne's theory might be tenable. After herbaceous angiosperms became common some of them may have reached New Zealand by long distance dispersal from the northern hemisphere, and perhaps from a then warmer Antarctica, and, if tolerant of warmer temperatures, they may have established in various open lowland habitats. With the coming of alpine habitats these species would have been able to move into them and diversify.
This is not to say that the lowland species of herbaceous, predominately alpine genera in New Zealand are necessarily primitive and ancestral. During glacials, alpine vegetation probably extended at times right to the coasts, particularly in southern New Zealand. With each interglacial, forests would replace alpine plants in the lowlands, except on some exposed rocky coastal sites. Here some alpine species could persist and even give rise to new forms. In these circumstances we would have to regard the coastal species as derived from alpine species rather than as being their ancestral stock.
Wardle108 suggested that in warmer pre-Ice Age times New Zealand may have extended much further south to the limits of what is now the submerged Campbell Plateau. Even in those warmer times he felt that such a southern extension would have had a cool temperate climate able to support a flora 'which would later give rise to the present mountain flora'. He based this idea on the present, fairly diverse, mostly herbaceous floras of the subantarctic islands, which share a number of genera with the New Zealand mountains, but which also have a number of endemic genera which, Wardle suggests, did not reach the main islands.
Unfortunately geologists do not support the idea of such a southern extension at that time, although even scattered islands to the south in warmer times may have provided adequate sites for the evolution of a stock of cool temperate plants.
In a later paper Wardle146 proposed that a small element of the alpine flora may have been present in New Zealand throughout the warmer climates of the Tertiary, a view partly supported by fossil evidence. The generic and subgeneric groups concerned 'are small and taxonomically isolated, restricted to cool habitats, and either endemic to New Zealand page 196or occurring elsewhere only in the Australian or American sectors of the South Temperate Zone'. Wardle thinks it unlikely that the species of this element have been derived from warm-climate Tertiary ancestors, or that they reached New Zealand by long distance dispersal. He lists 29 flowering species in 15 genera which are mostly confined to cool, wet, infertile soils. Of these, 'Nothofagus (N. menziesii — silver beech) is a forest dominant, Stilbocarpa is a tall herb of far southern coasts, while the remainder are low growing plants ascending to subalpine or alpine levels, eight of them being more or less cushion or mat-forming'. Genera with species of the latter form are Hectorella, Phyllachne, Donatio, Tetrachondra and Oreobolus. He also suggests that certain shrubby, cold-tolerant, higher altitude conifers such as Podocarpus nivalis, Lepidothamnus laxifolius and Phyllocladus aspleniifolius var. alpinus may also have been present through the Tertiary rather than having been derived more recently from taller, lower altitude relatives. This view is based on a study which indicates that the shrubby forms are more primitive cytologically than the trees, and, if the cytological interpretation is valid, it follows that the former are unlikely to have been derived from the latter.
It had been earlier suggested147 that this alpine element may have reached the New Zealand mountains from Antarctica, as presumably would also have been the case for the comparable elements in Tasmania and South America. For New Zealand this would imply a migration via the subantarctic islands, but on the whole Wardle does not favour this view as only 6 of the 18 genera concerned occur on these islands, and in the case of Nothofagus poor dispersal ability would suggest that crossing such wide ocean gaps would not have been possible. So if species of this element were already present in New Zealand before colder climates and mountains developed, where did they grow? Wardle points out that even today a number of the species of the group occur in the wet Westland climate within 300 m of sea level on infertile soils, and in Hawaiʻi, whose climate may be similar to that of New Zealand during the Tertiary, otherwise alpine species can be found in a bog at only 600 m altitude. 'If, during the period of Tertiary warmth (in New Zealand), there were infertile soils on peneplained uplands drenched by persistent mist and rain, some cool climate plants would undoubtedly exist … On the other hand, considerably colder conditions would seem necessary for the Tertiary existence of the high altitude genera Hecto-page 197rella, Haastia, Phyllachne, Donatia and Rostkovia, and it is perhaps in respect to these that the suggestion of immigration from Antarctica at the end of the Tertiary is most attractive.' Finally, Wardle suggests, the steep slopes with young soils of most of the present mountains would not have been suitable for this element, so it has remained as a minor component of the alpine flora as a whole.
The possibility of the derivation of alpine species from warm-climate forest ancestors in New Zealand has already been mentioned. Most authors would accept this as one source, but most also consider its contribution as relatively minor — involving mostly subalpine and alpine shrubs. For example, some botanists consider that divaricate shrubs are basically adaptations to Ice Age cold and drought and some of these, which belong to such genera as Coprosma, Pittosporum, Melicytus, Myrsine, Aristotelia, are to be found in subalpine and alpine habitats. In Coprosma and Myrsine there are even a few completely prostrate alpine species. Among non-divaricate derivatives, Dracophyllum has a number of needle-leaved and some broader-leaved shrubs in lower alpine and near cushion forms in higher alpine situations and Hebe is well represented by alpine shrubs, some of which are of whipcord form. Astelia is an herbaceous genus with a number of forest floor species and one epiphytic species, which is also quite well represented in the alpine zone.