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Forest Vines to Snow Tussocks: The Story of New Zealand Plants

Botanical History

page 242

Botanical History


The fossil floras of New Guinea, New Caledonia and other smaller islands are too fragmentary and/or poorly known to provide any coherent picture. Australia, New Zealand, Antarctica and southern South America will be considered together for the Cretaceous period,15,195,200 when they were still joined and shared a largely common flora. For the periods of the Tertiary era, when they had separated and their floras had undergone different trends, they will be treated individually.

In the middle Cretaceous when Australasia, Antarctica and southern South America were joined in high southern latitudes their climates appear to have been much warmer than those of similar latitudes today. The linked lands also seem to have been generally moist, even in parts now arid, and they shared a type of forest dominated by conifers and ferns with an increasing flowering plant component. The tree fern family Cyatheaceae was prominent and among conifers the Araucariaceae201 were represented and the Podocarpaceae were becoming dominant. The earliest flowering plants are known mostly from pollen that cannot be matched with any present day types.

In the latest Cretaceous, when this part of Gondwana began to break up, flowering plants increased in the southern lands and a number of the micro-fossils and macro-fossils can be assigned to modern families, or in some cases genera. Nothofagus is a notable ease with brassii group pollen appearing at about the same time in Australia, New Zealand and Antarctica (from fossil pollen near McMurdo Sound) and somewhat later in South America where it was associated with the fusca and menziesii groups. Pollen from these two groups appears in New Zealand at the end of the Cretaceous and in Australia in the early Tertiary. Pollen assignable to the Proteaceae is also found in the four regions in the Late Cretaceous. In South America, New Zealand, and Australia in particular, a number of other angiosperm fossils of late Cretaceous age have been found but these have not yet been identified. Some of the families that have been recognised, however, include the Winteraceae, Epacridaceae, Chloranthaceae (Ascarina), Loranthaceae (mistletoes), (?)Scrophulariaceae, Lauraceae, and, rather surprisingly, several families which are largely herbaceous at present, although they may not have been so then page 243— Ranunculaceae, Haloragaceae (Gunnera), Cruciferae and Caryophyl-laceae.15

Tertiary Australia198, 202, 203

For most of the Paleocene Australia was still joined to Antarctica although a rift valley was developing between them. Southern Australia was at 65°S but there is evidence that sea temperatures were of subtropical warmth so temperatures on land were probably warm also. Microfossils indicate widespread forests dominated by Podocarpaceae; Nothofagus was rare. Araucariaceae, Proteaceae, and Myrtaceae were also represented and in addition Anacolosa and possibly Cupania, genera now restricted to the tropics. Forests of this type were also present in Central Australia so the general climate was moist as well as warm. High rainfall, warm temperatures and low relief at this and later times resulted in many places in intensely leached infertile soils which have persisted to the present day.

At the beginning of the Eocene, final separation of Australia and Antarctica began. At first conditions continued to be warm and moist and further tropical genera were added to the forests such as Bombax (the kapoc genus), and the tropical mangrove palm Nipa. Conifers and Nothofagus were still poorly represented. From the middle Eocene the climate cooled, a number of the tropical genera disappeared from southern Australia, and pollen of N. brassii group increased in importance.

During the Oligocene the climate cooled further and the sparse fossil record indicates similar floras to the late Eocene although with some reduction in diversity. It is suggested that with the lower temperatures the climates would also have been drier and perhaps relatively arid in northern parts. An ice cap probably began to develop in Antarctica at this time.198

In the early Miocene the climate became distinctly warmer and moister than it was in the Oligocene. The fossils suggest extensive moist forests over southern Australia with Nothofagus of the brassii group, conifers, Myrtaceae and Lauraceae prominent. In central Australia, localised forests of the same type occurred but so did open habitats as indicated by the record of Acacia, Casuarina and grasses. Temperatures dropped again by the late Miocene when Australia had reached its present latitudes and the Antarctic ice sheet had reached its present page 244dimensions. The rather limited evidence198 suggests a retreat of rain forest with the cooler and drier climates.

The Pliocene also began with a warmer phase resulting in some rain forest expansion, but eventually the temperature declined leading to the severe glacial/interglacial oscillations of the Pleistocene. This finally led to the widespread disappearance of rain forest and expansion of plants adapted to arid conditions. These included Eucalyptus, Casuarina, Acacia and various Proteaceae. Most of the arid and semi-arid floral components in Australia are thought to be derived from families of early rain forest origin adapted to heavily leached infertile soils during the Tertiary.

Tertiary Antarctica195

A site near McMurdo Sound (presently at 78°S) yielded a Cretaceous flora of Podocarpaceae, Nothofagus and Proteaceae. Palm pollen is recorded in early Tertiary strata from the same locality, indicating a relatively mild climate, even though the latitude was much the same as the present day. The richest fossil floras of Antarctica are from Seymour Island, off the Antarctic Peninsula at 64°S, where leaves and pollen have been recovered. The date of these deposits is not certain, but it seems likely to be lower Tertiary. The flora is reasonably diverse and of rain forest character, although 23 species cannot be identified and of the remainder 27 are ferns including Cyatheaceae and Schizaeaceae. There is a strong conifer component including the genera Araucaria, Agathis, Dacrydium, Phyllocladus and Podocarpus. Angiosperms include Nothofagus of the fusca and brassii groups and the following families: Cunoniaceae, Lauraceae, Monimiaceae (Laurelia), Myrtaceae, Proteaceae (Knightia), Winteraceae, Loranthaceae, Leguminosae, Aquifoliaceae (includes Ilex, the holly genus), Cruciferae and Cyperaceae (sedges). This assemblage is similar to that produced by the rain forests of Australia in the early Tertiary with the difference that there are no 'tropical' taxa, such as Anacolosa and Cupania.

The most recent record of vegetation in Antarctica comes from pollen in Ross Sea deposits of the late Oligocene age. At this stage an ice sheet was beginning to form. The assemblages were dominated by Nothofagus pollen (with the fusca group most common) plus lesser amounts of Proteaceae, Myrtaceae and Podocarpaceae.

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Tertiary South America195,198

The general progression in South America seems to have been similar to that of Australia although arid climates, while strongly developed in places, did not become so widespread as in Australia.

In the Eocene, genera and families now largely restricted to tropical northern South America appeared in the fossil records of Chile and Argentina. This is comparable to the appearance of a tropical element in the southern Australian floras at about the same time. Similarly, with cooling from the Oligocene on, the tropical element disappeared from southern South America and southern rain forest groups — Podocar-paceae, Nothofagus, Proteaceae, Cunoniaceae — became increasingly important.

Tertiary New Zealand15, 204, 205, 206

Mountains were raised in the New Zealand crustal complex (Rangitata orogeny) in the late Jurassic and early Cretaceous when it was still part of Gondwana. By the late Cretaceous, when separation of New Zealand from Gondwana began, it is believed the mountains had been largely eroded, so the physical background to most of the Tertiary fossil record we are about to consider is one of a low lying peneplain with strongly oceanic climates.

During the Paleocene, conifers of the family Podocarpaceae were dominant and Nothofagus of all three groups was fairly rare. Proteaceae were quite common and, as in Australia, Anacolosa appeared as well as a number of palms including probable Nipa. Casuarina also appeared and remained a significant element for most of the Tertiary. Casuarina no longer occurs in New Zealand as a native genus, but fossil pollen considered to be Casuarina had been known for some time. The recent discovery of impressions of the distinctive cone-like fruits of Casuarina and the related genus Gymnostoma has confirmed the presence of the family in New Zealand.207 The Myrtaceae also appear for the first time in the Paleocene with pollen referable to both Leptospermum and Metrosideros having been identified.

In the Eocene the podocarps declined in importance, although Phyllocladus is recorded for the first time. Nothofagus became dominant, firstly the fusca group and latterly the brassii group. Cupania and Bombax appeared, rather later than in Australia, along with a number of new page 246proteaceous genera and Freycinetia, Astelia, Quintinia, possibly Phormium, and the family Araliaceae (includes Pseudopanax). A genus now extinct in New Zealand, Ilex (which includes the holly), is also first recorded in the Eocene. Cooling at the end of the Eocene may have been the reason for the extinction of Anacolosa, Nipa and a number of Proteaceae.

During the Oligocene, New Zealand reached its present latitudes and also suffered its greatest reduction in area as a result of sea encroachment. The cooler climate of the late Eocene seems to have continued into the Oligocene, perhaps partly due in New Zealand's case to the development of cooler westerly winds and currents following the separation of Australia and Antarctica. The forests were dominated by the Nothofagus brassi group, although the N. fusca group, Casuarina, Myrtaceae, Palmae and Podocarpaceae were also prominent. Among genera to first appear in this period are Weinmannia, Elaeocarpus, Myrsine, Fuchsia, Coprosma, Laurelia and Epilobium.

The Compositae (daisy family) made its first appearance in the late Oligocene.

Warmer climates prevailed in the early Miocene and the land began to rise. Swampy areas were common and surrounding them were dense floristically rich forests. Many of the species cannot be identified with modern plants, but several Nothofagus brassii group species were common together with Myrtaceae (possibly including Eucalyptus), Casuarina, Podocarpaceae and tree ferns. Palms also were common and included a small fruited species of coconut in the northern North Island — Cocos zeylanica (Fig. 124). Among genera recorded for the first time are: Cordyline, Ripogonum, Dysoxylum, Alectryon, Macropiper, Pittosporum, Muehlenbeckia and Melicytus. An interesting incomer is Acacia, no longer present in New Zealand.

Temperatures declined towards the end of the Miocene and this trend continued into the Pliocene. Uplift continued during the Pliocene and the Southern Alps and other ranges were formed. This phase of uplift is known as the Kaikoura orogeny and continues today. Extensive development of alpine vegetation, as we have discussed earlier, would have commenced from this time. In the lowlands the Nothofagus brassii group was prominent in the early Pliocene but only in the northern North Island. South of there Myrtaceae, rimu (Dacrydium cupressinum), Nothofagus fusca group and ferns were common. By the end of the Pliocene a number of forest genera and species had become extinct in- page 247
Figure 124 Two fossil coconuts of the extinct Cocos zeylanica. The larger of the two nuts is only about 4 cm across. The fossils are associated with a coal seam of Miocene Age at Cooper's Beach in the far north of the North Island. The coal seam dips below the sea and the small coconuts are washed onto the beach from time to time. Photo: J. E. Casey.

Figure 124 Two fossil coconuts of the extinct Cocos zeylanica. The larger of the two nuts is only about 4 cm across. The fossils are associated with a coal seam of Miocene Age at Cooper's Beach in the far north of the North Island. The coal seam dips below the sea and the small coconuts are washed onto the beach from time to time. Photo: J. E. Casey.

most of the Nothofagus brassii group, Bombax, Cupania and many palms.

During the Pleistocene the climate deteriorated greatly with a succession of long glacials and shorter interglacials. The last species of the Nothofagus brassii group disappeared together with Microstrobus and Microcachrys (now restricted to Australia) of the Podocarpaceae, Casuarina, (?) Eucalyptus, Acacia and all Proteaceae except two species, namely Knightia excelsa and Toronia toru. With each glacial the alpine vegetation expanded and diversified; with each interglacial the forests expanded from refugia (albeit reduced in diversity) to largely reclothe the landscape.