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The Vegetation of New Zealand

The problematical history of the flora

page 422

The problematical history of the flora.

It seems fairly certain that since early Mesozoic times New Zealand has never been completely submerged. From the fossils which have been collected Arber (The earlier Mesozoic floras of New Zealand. Pal. Bull. No. 6. N. Z. Geolog. Surv., 1917) has shown that there was a Triassic-Jurassic flora (more than one flora really) consisting of Equisetales, Fern-like plants (some probably seed-bearing), Cycadophyta, Podozamiteae, Ginkgoales and Coniferales. But of more interest is the record of two Dicotyledons of the Lower Cretaceous which were associated with Cladophlebis australis — a fern-like Mesozoic plant of wide distribution the world over and extremely common in Triassic New Zealand.

In the Cretaceous the Mesozoic rocks were folded and by degrees the land rose and extending north and south and south-east became virtually a continent. In the north, basing the statement on the present ocean depths and on the magnitude of the palaeotropic element, Great New Zealand included Norfolk Island, the Kermadecs, Lord Howe Island, the New Hebrides and New Caledonia (i. e. their present sites formed a small part of the area) and extended to New Guinea and northern Queensland. In the south, mainly for zoological reasons as already explained and partly through disbelief in long-distance transit over the ocean of seeds &c., my rather unwilling opinion is that the land extended to the Antarctic Continent; and, in the east what are now the Chatham Islands would undoubtedly be included.

During the Great New Zealand period the palaeotropic element would people the north while from the fairly warm Antarctica would come the subantarctic element. Synchronously with the invasion of northern and southern plants the palaeozelandic element, which in part had its beginnings in the Jurassic, would advance north and south. On the wide area of Great New Zealand, genera and perhaps families would come into being — these also palaeozelandic.

The main difficulty which now comes to the front is the question of the subantarctic and New Zealand high-mountain elements in the flora of Tasmania. There can have been no direct "land-bridge" from New Zealand to Tasmania, for had there been one the true Australian element would have been fully represented. It is also nearly impossible to believe that seeds of high-mountain species quite unfitted for wind or bird carriage can have passed from New Zealand to the mountains of Tasmania. The only alternative is to postulate a great extension of Tasmania southwards. With Skottsberg1 in contradistinction to W. R. B. Oliver2 I consider Nothofagus

1 1) Skottsberg (195: 138 — 139) defines his "Old Antarctic element" which consists of "genera or even orders which are virtually bicentric" and, to cite a few, includes Oreobolus, Rostkovia, Libertia, Nothofagus, Laurelia and Donatia — there are 22 in all. Nor does that eminent, much-travelled botanist favour long-distance carriage over oceans.

2 2) For Oliver's arguments regarding Nothofagus, see 1925: 120. He considers that "It is probable that Fagus and Nothofagus originated in North America and spread thence east south and west. The western moiety passed, via Japan, round the Pacific, reaching Australia and New Zealand".

page 423a critical genus from the biogeographical standpoint — originated in the far south.

Remnants of the rich upper Cretaceous and early Tertiary Flora occur in many localities, especially in Otago and Southland. The fossils have been studied by Ettingshausen, who refers some to various Northern genera which one would not expect in the Southern Hemisphere, especially Australasia, e. g. Myrica, Alnus, Quercus, Ulmus and Acer. He also records the Australian Casuarina and Eucalyptus. Mixed with these were plants referred to existing New Zealand genera or their representatives. Judging from the figures accompanying the descriptions, Ettingshausen's identifications, if accepted at all, must be received with the greatest doubt, indeed I do not think they are of any value. But were these genera present, then there must have been a universal temperate flora, a matter hardly conceivable in the fact of the tropical climate as a barrier. This tertiary fossil flora, however, is a fact, and it teaches us that many species and genera have passed away, just as the present species would have gone, in the future, or changed by natural means, had New Zealand remained a virgin land.

The great elevation and extension of the land was succeeded by an equally great depression which is the most critical occurrence in regard to any conclusions concerning the origin of the flora, or its distribution in New Zealand itself, for if the great area was reduced to a few small, flat islands what would happen to the high-mountain species? With this question in my wind I wrote to Professor R. Speight regarding the area &c. of New Zealand at the maximum period of depression. His reply, which gives an excellent concise account of much of the geological history of New Zealand is here quoted almost in its entirety.

"New Zealand was raised into a mountain region at the close of the Jurassic or the beginning of the Cretaceous, that is, the post-Hokonuian revolution.

By the close of the Cretaceous it was reduced to a peneplain, with probabale elevations of no great height standing above its surface and then this peneplain was slowly depressed beneath sea-level so that it was fairly completely covered with a layer of Sediments. There were no doubt islands of relatively small extent and comparatively low elevation which were not covered with this veneer. It is not postulated that sinking took place all over the land at the same time and it is certain that the sagging of the crust in some areas was posterior to the sagging in adjoining areas. For example the sagging in the North Canterbury-Kaikoura area reached a maximum in the late Eocene, whereas it commenced later in the South Canterbury-North Otago area and reached a maximum in the Miocene. page 424Probably the depression went on later in the North Island. This has, however little to do with your problem. We may take it therefore that by the Early-Middle Tertiary the reduction of the land area of New Zealand had reached its limiting value, but there may have been adjacent high land of whose precise location we can say nothing.

From the period of maximum submergence onward, the land rose and there must have been land of decided relief in the near neighbourhood in order to furnish the thick deposits of ancient gravels which occur over wide areas in the South Island. You get such gravels in North Canterbury. In the Castle Hill area, in South Canterbury, in the Mackenzie Country, &c. These date from the late Pliocene. This elevation continued till there was the onset of the Pleistocene Glaciation, when the land probably stood higher than at present. There were probably two periods of glacier advance, but it is not thought that the interglacial period was one of very mild climate or of reduced height of the land. I look on the two glacial periods as merely marked fluctuations in the ice front.

From then on there has been a reduction in level, probably amounting to 1500 or 2000 feet, but these figures are mere conjectures. The evidence from our artesians certainly show a lowering in level of 700 feet and if bores are put down further this figure will no doubt be substantially increased.

I do not think that the land area during the Oligocene-Miocene times could have been very large and we have little evidence as to height. I think Morgan considered the presence of glaciers on mountains over the present site of Western Nelson though I fancy that he came to the conclusion that the beds were not glacial. In any case they were coarse in texture and were probably shed from an area of pronounced relief, but of course one cannot say how high it was in so many feet. Coarse material may come from low hills if the slopes are abrupt. Some of the coarse Tertiary conglomerates of Central Otago, if not morainic as postulated by Park and others, imply the action of rivers with considerable transporting power and this may indicate high land. A consideration of these cases certainly does suggest that there was land with considerable relief but no indication of its height can be arrived at

The only point which I can see which presents any difficulty is the date of arrival of the high-mountain flora. If it came late in the Tertiary there is no difficulty as far as I can see but if it is a survival of a Cretaceous flora then there are difficulties in providing a refuge for it or a location for it to be established on. There is always the possibility that the refuge may have been lands which have sunk out of sight wether beneath the Tasman Sea or to the east of the present New Zealand. My own investigation of Cretaceous Coal conglomerates leads me to think that the present land features are in no way related to those of that period, and that page 425mountain ranges then existent, which may have persisted fairly far on int the Tertiary, may have entirely disappeared from adjacent areas."

The great Oligocene-Miocene reduction of the land must have brought about the extinction of many species especially among the high-mountain plants and plastic species, perhaps the sole survivors of large genera, would find a haven of refuge on rock-faces and other habitats, perhaps fairly high, unsuited for forest-plants. It follows then that the primitive Subantarctic element would be decimated and that the present species or genera are a mere fraction of the original extensive company. But had there been no high land and the climate warmer than now, which was most likely the case, hardly a true high-mountain species could have survived and it seems impossible that a new subantarctic flora could have arrived during the Pliocene-Pleistocene without another great extension of land southwards, but this appears most unlikely. As for the lowland forest species they would not suffer nearly so much, for it is surprising how great a plant-population can thrive on a small island, such as the Little Barrier, where there are, at the present time most of the forest-plants of Northern Auckland.

During the Pliocene, elevation again took place and extended far into the Pleistocene. The alpine plants could return to the mountains, and under the new stimuli, ancient genera would be revivified and new forms appear. Again the land extended to the north, south and east and to some extent to the west. Exchanges would be possible with Australia by transoceanic methods and with northern palaeotropic floras. Towards the end of the period of elevation in the Pleistocene, when the mountains were at their highest, came the great extension of the glaciers. Then would the palaeotropic element be driven northwards and perhaps eastwards, especially if as is more than probable, the glacial advance was, in part, the result of a colder climate. East of the Southern Alps there would be a steppe-climate on the plateau. Then by hybridism and perhaps by epharmonic change would arise the intense xerophytes, descendants of mesophytes it may be, such as Carmichaelia Petriei from a leafy forest-species, or Edwardsia prostrata from E. chathamica. Then, too, would appear that semi-stable xeromorphy seen in the palaeotropic Hoheria and Pennantia.

Towards the end of the Pleistocene, depression of the land set in once more, the glaciers retreated far into the mountains and the re-peopling of the glaciated land, as already described, began. Once more the descendants of the ancient species, some perhaps themselves of high antiquity, commenced to make new forms, acted on by the novel and diverse environments and probably by secular changes of climate first wet, then drier. Then would Celmisia, Hebe, Epilobium and many other genera, no longer held in check by uniform conditions, burst forth into their multiplicity of forms, many of them hybrids (species usually isolated from one another page 426coming together) or epharmones and others true species. Even yet, species-making is in progress.

The land having receded beyond its present limits, elevation once more took place, and the New Zealand of to-day came into being, peopled by its heterogeneous gathering of plants, children of north and south and east and of the New Zealand soil itself, moulded by great earth-movements and climates of extreme variety.1 In one thing they differed from the plants of other regions; no grazing mammals had ever been present to molest them, they possessed no structures that could claim to be defensive.

Finally came man; first the Maori, or it may be his predecessor, but their influence on the vegetation was but slight. Then arrived the European. It is more than 100 years since he began to occupy the land, but how great the change his operations have wrought, has been already told. We, who now live in this wonderful country, and love its marvellous vegetation, have set aside sanctuary after sanctuary where the palaeotropic, Australian and palaeozelandic plants, the survivors of that bitter strife with Nature, that commenced millions of years ago, can still pursue their destinies if unmolested by their human enemies and the horde of foreign plants and animals he has let loose.

Will our descendants prize this unique heritage from the dim past and preserve these sanctuaries intact?

1 1) Nothing shows more clearly how greatly climate must have changed in the temperate Southern Hemisphere than do the angiospermous fossils of Seymour Island and the Antarctic coal discovered by Shackelton's expedition. A considerable rise in temperature, accompanied by depression of the land, would undoubtedly be detrimental to the well-being of alpine plants, but, as seen from their present distribution in New Zealand, many thrive under warm lowland conditions, so that a fair number could probably tolerate a considerable rise in temperature. The fossil plants mentioned above clearly show that Antarctica possessed a Tertiary flora distinguished by a small but most characteristic New Zealand element, as evidenced by the following species: Laurelia insularis, Knightia Andreae, Drimys antarctica and two species of Nothofagus. With these species in one's mind it is easy to agree with Skottsberg that "the Antarctic Continent may have been a centre of evolution from which animals and plants wandered north".