A southern hemisphere view of nature

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Don Adamson

School of Biological Sciences, Macquarie University, Sydney

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A relic population of Wollemi pine (Wollemia nobilis) in a steep-sided canyon northwest of Sydney. Discovery of these large trees (35 m high, 1 m diameter) in August 1994 caused great excitement because this species had been considered extinct for 150 million years. Their continued existence was attributed to a habitat which offered protection from fires common on platueax in that region. "Pine" is something of a misnomer; they are taxonomically closer to species of Aracaria than to true pines (genus Pinus).

Original photograph by Jaine Plaza, an supplied courtesy Royal Botanical Gardens, Sydney)

Geology

Australia and New Zealand once formed part of a huge southern land mass now referred to as Gondwanaland, whereas northern hemisphere continents were once aggregated into Laurasia. Gondwanaland and Laurasia began to disaggregate about 160 million years ago. Prior to this time, the southern hemisphere land masses and India were connected into Gondwanaland, while North America, Europe and much of Asia formed Laurasia. South Africa, Madagascar, India, South America, New Caledonia, New Zealand, Australia and various other fragments broke away and drifted northwards, leaving Antarctica behind. Australia and South America were the last major land masses to separate from Antarctica, Australia beginning slowly about 90 to 100 million years ago and establishing a deep ocean passage some 30 to 40 million years ago. Opening of the Drake Passage between South America and Antarctica completed an ocean connection around Antarctica, allowing development of circumglobal cold currents in the Southern Ocean and the thermal isolation of Antarctica from warm tropical water and air.

Plant life

Before this cooling there was a continuity of vegetation which only became fragmented as Gondwanaland broke up into separate land masses. Angiosperms were already established worldwide as a major successful group of plants. The climate was warm, moist and conducive to plant growth even at high latitudes where daylength in winter would have been the same as that in the Arctic today. Forests were widespread, diverse and dominated by gymnosperms and angiosperms (presumably deciduous). As a result of the break up of Gondwanaland, entire plant communities containing representatives of the major types of plants now alive were carried northwards. A legacy of these events can be seen today in many plants of southern hemisphere lands and is well illustrated by the distribution of major plant families such as the Proteaceae (banksias, grevilleas, waratahs) and Myrtaceae (eucalypts, bottlebrushes, titrees, lillipillis). Despite subsequent colonisation by long-distance dispersal of propagules and/or merger with northern lands (South America with North America, Africa with Europe, India and Australia with Asia), the floras of the continents and islands of the southern hemisphere remain more closely related to each other than they are to northern hemisphere floras derived from fragments of Laurasia.

As Australia drifted northwards from Antarctica about 40 million years ago it moved into warmer climatic zones. Cool temperate forests, abundant over most of Australia, became confined to wetter areas. The vegetation as a whole became more open and in places heathlike.

Until about 15 million years ago one of the most abundant and widely distributed angiosperms in Australia was Nothofagus (southern beech). Evidence from a site in the Lachlan region of central New South Wales indicates that the decline in abundance of Nothofagus was accompanied by a corresponding increase in pollen from eucalypts and other mainly rainforest species from the same family (Myrtaceae). Somewhere between 15 and 8 million years ago the amount of charcoal relative to pollen at the same site also increased. Most likely, communities of eucalypts and related genera were subject to more frequent burning than the Nothofagus-dominated forests that preceded them. Similar evidence from pollen profiles from Lake George near Canberra reveals an abrupt change from a mixed forest of Nothofagus and gymnosperms to an open shrubland with grasses and daisies about three million years ago.

From the Gondwanan stock, and by evolution of new life forms, plants adapted to match emerging conditions of low and unpredictable water supply, strong sunlight, higher temperatures and frequent burning. The new combinations of environmental conditions produced the vegetation types which now dominate the Australian continent. Genes for many of the adaptive attributes were presumably already present, scattered through the Gondwanan flora as a consequence of prior exposure of ancestral stock to environmental stress associated with life on land.

During its drift northwards Australia was subject to only relatively minor geological disturbance although some volcanic activity occurred on the eastern side of Australia and large parts were inundated by shallow seas which for long times isolated southwestern Australia from eastern Australia. Such isolation aided evolution of a remarkably rich endemic flora in southwestern Western Australia. The sea retreated, and limestone soils may have sustained the isolation; developing aridity certainly did so.

By contrast, New Zealand after its separation from Antarctica and Australia some 60 million years ago has had a tumultuous history of geological disruption involving partial or possibly complete inundation by the sea, severe volcanic activity and glaciation. New Zealand vegetation, like that of Australia and other southern continents, also changed in response to climatic change and environmental stress. As New Zealand broke away and moved north conifers became less abundant and a mixed evergreen angiosperm and gymnosperm flora developed. A change from Nothofagus-dominated rainforest to drier vegetation occurred in the Miocene around about the same time as there was clear evidence of drying in Australia. Today, conifers are again abundant in rainforest communities and the climate is wetter due to New Zealand’s present location across the path of moist oceanic winds.

Biogeography

Because of global position and stable geological history, Australia is dry and intensely weathered. In addition, large areas are covered by sedimentary rocks whose minerals have already been through at least one cycle of weathering. There has also been negligible glacial activity to grind rock and expose unweathered minerals. As a result, most of Australia offers plants with a poor supply of both water and nutrients. Moreover, plant communities are also subject to burning by fires ignited by electrical storms.

In exploring the question of adaptation of plants to aridity, fire and mineral deficiency, note that only two genera (Eucalyptus and Acacia) dominate the top stratum of vegetation over three-quarters of Australia. Eucalypts occur in open forests and woodland. Acacias occur in nearly all plant formations but are particularly prominent in semi-arid and arid regions. Both genera are obviously very well adapted to present-day conditions; both are of ancient Australian origin; both are sclerophyllous (i.e. have leathery/rigid leaves); both could be described as scleromorphic (extremely woody). The same could be said of many other typically Australian plants.

Water shortage is such an obvious limitation to plant growth in Australia that it is easy to conclude that specialised plant structures and behaviours are adaptations to conserving water or surviving under water stress. Scleromorphy in the Australian flora can be interpreted as an adaptive response to an increase in aridity and/or low nutrient supply.

Scleromorphy is also important for fire ecology. Scleromorphic vegetation provides a mass of dry woody fuel that is very slow to decay. Decay of lignin by microorganisms occurs much more slowly than decay of most other plant products. Consequently lignin-rich plant litter (dead leaves, branches, fruit and bark) accumulates faster than it decays. Fire therefore becomes inevitable and increasingly severe as litter accumulates. Many plants, such as eucalypts, paperbacks, titrees and boronias, also accumulate large amounts of volatile flammable oils in their leaves. During hot fires the canopy produces an explosive mixture of vapourised oil. Scleromorphic vegetation promotes its own burning. Consequently only plant species that have developed characteristics that allow their individuals or their seeds to survive fire will survive. Not only does scleromorphic vegetation promote fire, but some species are so coupled to a fire-prone environment that they would probably become extinct without periodic burning.

Charcoal in ancient sediments implies that fire has been a feature of the Australian landscape for a very long time. However, the fire regime of the whole continent must have begun to change when humans first occupied Australia at least 50 to 60 thousand years ago. Fire was used for purposes of hunting, safety and access to the countryside. Fire is a frequent event across all woodlands, shrublands and grasslands of Australia and all plants beyond rainforests are adapted to cope. Replacement of the leafy canopy after fire by sprouting of dormant buds is one of the main adaptations which allow well-insulated plants to survive in a fire-prone environment.

The history of Australian vegetation since the break up of Gondwanaland provides some insight into the magnitude of the changes that have occurred in relative isolation from the rest of the world and the nature of the selection pressures that have operated. Over millions of years, cool temperate forests that covered much of the continent when it was 60°S were replaced by open forest, woodland, shrubland and grassland. Those communities were better suited to the warmer and drier conditions of its present location.

Human impact

In stark contrast to geological events, and on a time scale of only decades since European settlement, 70% of Australia has been turned over to agriculture and forestry. Almost all potentially productive and marginal farming and grazing land has been cleared. Soil degradation, including wind and water erosion, acidification and salinisation have become severe national problems. Large numbers of pest species, both plant and animal, have emerged from the huge numbers of newly introduced organisms while many indigenous plants and animals are extinct or endangered. Much of the Australian landscape is not sustainably productive after almost 200 years of exploitation. Such ‘mining’ of vegetation and soil continues unabated despite wide recognition of precious ‘capital’ that is irretrievably lost in the process.

New Zealand faces its own set of severe problems including plant and animal extinctions, rapid spread and dominance of pest species ranging from European gorse to Australian brush-tailed possums which, by selective browsing, are threatening extinction of some native plant species. New Zealand has been severely over-cleared of native vegetation even on marginally productive land. Steep slopes have been destabilised and some types of vegetation have been lost.

Southern hemisphere plants are worth preserving as a heritage from the distant past, as vegetation for the future, and for numerous practical present-day purposes ranging from erosion control to sources of useful chemicals. In Australia and New Zealand substantial areas have been set aside for the protection of vegetation but in both countries some types have disappeared entirely and others are unrepresented in the reserves. However, compared to many countries which have much less of their original vegetation to conserve, Australia and New Zealand are well off.

The basic tenet of Plants in Action is that every facet of a plant’s existence is an outcome of genotype × environment interactions. In present times, as distinct from geologic past, humans must now be included as a significant environmental factor for that interaction.

Further reading

Adamson, D.A. and Fox, M.D. (1982). ‘Change in Australasian vegetation since European settlement’, in History of Australasian Vegetation, ed. J.M.B. Smith, 109–146, McGraw-Hill: Sydney.

Flannery, T.F. (1994). The Future Eaters: An Ecological History of the Australasian Lands and People, Reed Books: Melbourne.

Groves, R.H. (ed.) (1981). Australian Vegetation, Cambridge University Press: Cambridge.

Ladiges, P.Y., Humphries, C.J. and Martinelli, L.W. (eds) (1991). Austral Biogeography, Australian Systematic Botany, 4(1), CSIRO: Melbourne.

Pole, M. (1993). ‘Keeping in touch: vegetation prehistory on both sides of the Tasman’, Australian Systematic Botany, 6, 387–397.

Truswell, E.M. (1993). ‘Vegetation changes in the Australian Tertiary in response to climatic and phytogeographic forcing factors’, Australian Systematic Botany, 6, 533–557.

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