Forest Vines to Snow Tussocks: The Story of New Zealand Plants
The New Zealand Pattern
The New Zealand Pattern
Such then is the 'isolated island syndrome'. The question now arises: 'does the New Zealand flora also conform to this pattern?' The answer this time is 'yes and no'.
The strongest difference from the isolated island pattern lies in the presence in New Zealand, mainly in forests, of plants belonging to groups of poor dispersal ability.
|(a)||Conifers are a prominent feature of the forests with the giant kauri (Agathis australis) in the far north and the more widespread species of the family Podocarpaceae ('Podocarps') including rimu (Dacrydium cupressinum), kahikatea (Dacrycarpus dacrydioides) and totara (Podocarpus totara).|
|(b)||The largely northern temperate group of wind-pollinated trees is represented in New Zealand by species of southern hemisphere beech (Nothofagus) which form extensive forests at mostly higher altitudes and latitudes.|
|(c)||The woody families of the order Ranales are represented in New Zealand by several species, including horopito (Pseudowintera species), tawa (Beilschmiedia tawa) and pukatea (Laurelia novae-zelandiae).|
The presence of representatives of these and other ancient and mostly southern groups not represented on islands which have always been isolated, indicates that New Zealand has not always been as isolated as it is now and must once have had continental connections.
Figure 2 The New Zealand crustal complex. Dark grey - dry land. Light grey-submerged continental to subcontinental crust. Dashed lines - volcanic ridges.
According to the land-bridge theory, then, if Tasmantis were once largely above the sea then dry land may have extended to New Guinea and perhaps even to south-east Asia. Such a land extension might explain some of our floristic links with the tropics, but not so readily those with the former flora of Antarctica and the present flora of South America, as the 2000 km gap between the Campbell Plateau and Antarctica makes it difficult to envisage a former dry land connection between them.
Another theory provides a more likely explanation. Recent geophysical evidence of sea floor spreading has led to a fairly general acceptance of the old theory of continental drift, with some modification.10 As far as the southern hemisphere is concerned it is believed that all its land areas, as well as India, were once united in a single large continent known as Gondwana. India and Africa/South America first separated off and later became separated from each other. Through its southern extremity South America probably retained tenuous links with the Antarctica /Australia/Tasmantis remnant of Gondwana (Fig. 3). It is suggested that Tasmantis separated and moved into isolation about 80 million years ago and that Australia and Antarctica fully separated about 50 million years ago. If the timing of these separations is correct then the ancient southern floral element could have reached New Zealand and New Caledonia before Tasmantis drifted into isolation. The presence of this element in the New Zealand flora is the most notable contrast with the floras of isolated islands, but in other respects there are several points of agreement:
|(a)||A number of genera in New Zealand, like many genera of isolated islands, exhibit a wide morphological and ecological range. To cite Coprosma again, some species are small trees with large leaves in the lowland forests, others are densely branched small-leaved shrubs mostly of open habitats from the sea coast to above tree-line in the mountains, and a few are mat-forming, near-herbs of the high mountains and montane riverbeds. Similar wide ranges are evident in a number of other genera including Pittosporum and Myrsine. Even in genera with only a few species in New Zealand there may be an
Figure 3 Proposed reconstruction of the Australian/Antarctic/Tasmantis portion of Gondwana during the mid-Cretaceous about 90 million years ago. After Tasmantis separated lateral movements along the alpine fault gradually rearranged the crust of present New Zealand.
Figure 4 Kaikomako (Pennantia corymbose): A New Zealand example of dioecism. The female flower on the left has a well developed ovary at the centre, but the small stamens do not produce viable pollen. The male flower on the right has only a rudimentary nonfunctional ovary, not discernible in this photo, but has large stamens with long filaments. Kaikomako is probably wind-pollinated. Photo: B. V. Sneddon.
|(b)||The level of dioecism in the New Zealand flora (Fig. 4) is, at 12 per cent,11 much lower than that of Hawaiʻi, but still much higher than that of Europe. In particular some genera shared with Europe, such as Clematis and Rubus, are entirely dioecious in New Zealand and predominantly hermaphrodite in Europe. In the Umbelliferae (carrot family) of New Zealand about 80 per cent of the species are dioecious or gynodioecious (female and hermaphrodite plants). Elsewhere in this large family such sexual patterns are very rare. Recently it has been shown that some tropical forests can have a level of dioecism between 20 and 30 per cent.12 However, affinities with tropical dioecious plants would offer an explanation for only a minority of New Zealand's dioecious species.|
|(c)||Natural hybridism has also long been recognised as a feature of page 23 the New Zealand flora,13 in many cases involving species of widely different form and ecology.|
|(d)||With the exceptions of the brightly coloured, bird-pollinated flowers such as the ratas (Metrosideros) and kowhais (Sophora), New Zealand plants on the main islands, like those of Hawaiʻi, are not notable for size or colour of flowers. Even among alpine plants, New Zealand species of genera which may be colourful elsewhere (Ranunculus, Myosotis, Gentiana) are often white. Here too the suggested explanation is a lack of specialised insect pollinators. 14 Perhaps in compensation many New Zealand plants with small sometimes wind-pollinated flowers (Fig. 5) produce an abundance of brightly coloured berries which are bird dispersed. Paradoxically, a number of the plants of the New Zealand subantarctic islands have flowers more brightly coloured than their New Zealand relatives even though these islands too lack specialised pollinators.11|
Figure 5 Kanono (Coprosma grandifolia): A New Zealand example of both wind pollination and dioecism. The flowers are male. As is generally the case with wind-pollinated flowers they are small and inconspicuous, but have disproportionately large dangling stamens with large anthers. The hanging stamens move readily with the wind, which shakes out the large quantities of pollen necessary for this rather wasteful method of pollination. The family to which Coprosma belongs, the Rubiaceae, is mostly insect-pollinated with often showy flowers. The species of Coprosma are unusual in the family in being both dioecious and wind-pollinated.
Photo: M. D. King.
Probably the isolated island characteristics in the New Zealand flora developed after isolation had been attained, and, more particularly, since high mountains and glacial/interglacial fluctuations have developed in recent geological times. When mountains first became significant and temperatures cooled sufficiently the climate would have become too cold on the mountain tops for tree growth and alpine habitats would have come into existence like new islands, although in this case not page 24surrounded by a sea of water, but by a sea of mostly unsuitable plants. Because of New Zealand's isolation, plants suited to cold conditions elsewhere could arrive only slowly by long distance dispersal so there would have been plenty of opportunity for the establishment of hardy variants of forest genera in progressively colder habitats. It was probably in these circumstances that genera such as Coprosma attained their present, unusually wide, morphological and ecological ranges by radiating into non-forest habitats.
As each glacial phase progressed the area of alpine vegetation would extend, reaching sea level in many places. The forests would become greatly restricted with some species becoming extinct and others barely surviving. With the warmer conditions of the interglacials the situation would be reversed and the alpine flora would come under pressure. Such climatic fluctuations, combined with an isolation that would prevent both the ready recruitment by long distance dispersal of species into new habitats, and the ready survival by migration of species already present, would put variable populations, including dioecious species and hybrid swarms, at a distinct advantage. In times favourable to their vegetation type they would be best able to take advantage of new habitats and in unfavourable times they would be more likely to survive.