Woodland community origins - how has our woodland evolved?
If you leave a garden to itself, derelict, some plants will persist but others, generally the weeds, will grow vigorously in the spaces previously cultivated. At first some plants will do well, the quick-growing ones, then others will gradually take over, the taller-growing ones or longer-lived ones. The quick growing ones may persist in open spaces or along path edges, with slightly different conditions. Some new seeds may blow in on the wind or be brought in by birds. Meanwhile some of the original garden plants may persist, while others may produce seed and increase in abundance. Those that lack the ability to spread vegetatively, or are unable to produce seed (because it has been bred out of them), will disappear. Some which can seed, but require special conditions for being pollinated or germinating, will also die out. After the next 10-20 years probably about a dozen species will end up surviving, each with individual patterns of distribution, and responding to the local climatic and soil factors in particular ways.
And so it is with the plants of the Cumberland Plain woodland.
How has our woodland community, with its 100 or more individual plant species, developed?
The plants themselves have evolved, many from the original ancient Gondwanan stock, with others being added by chance invasions. Over many millions of years climates have changed gradually from wet to dry to wet, and back again; soil landscapes have formed on bedrocks and eroded again. Plant species have been sifted into communities and then re arranged as conditions have changed. Even over the last 100,000 years, as geological and landscape features have become stable, and our plant species have probably become recognisable as the ones we know today, climate and sea levels have continued to fluctuate resulting in waves of ice ages and warmer interglacials. The last cold period was as recent as 20 000 years ago. The factors driving plant species distributions today would have been operating during these times, that is, rainforests would have occurred on the wet sheltered sites on good soils, eucalypt forests on the poorer soils, with sclerophyll heaths on sandy soils with very low nutrients and in regions of low rainfall, grassy woodlands, shrublands or grasslands on the better soils.
Mount Annan 20,000 years ago …
Mount Annan 20 000 years ago would have had its deep clay soils and gently undulating landscapes, but instead of being 30 km from the sea it would have been nearer to 50. Instead of the sandstone foreshores along Sydney there is likely to have been a sandy coastal plain with sclerophyll woodland on its dunes stretching out 10-20 km. The alluvial deposits of the Georges River would have meandered across this plain, while the deep gorges that are now filled by the present river may have provided habitats for rainforest species suffering under the drier and colder conditions, or perhaps rainforest species survived as littoral rainforest behind the dunes closer to the seashore.
At Mount Annan 20 000 years ago rainfall would have been much lower than the current 700-800 mm per year, perhaps down to about half, say 400–500 mm per annum. What plants could have been here?
Many of the species present in the woodland today (about 65% of the ground cover species), also still grow in parts of NSW with a low rainfall like this, at Condobolin and even Menindee. These include 9 out of 13 daisy species and 4 out of 7 chenopods. These species are very likely to have been part of the Ice Age grasslands and herblands growing at Mount Annan, and indeed probably spreading across most of western Sydney's Cumberland Plain. The current trees, particularly the eucalypts, and shrubs would not have tolerated these dry conditions however and would have been absent.
8000 years ago
Rainfall and temperatures increased over the next 12,000 years and by about 8000 years ago had reached more or less today’s conditions. Sea levels rose gradually, destroying sand barriers previously pushed up across Bate Bay, flooding Botany Bay, the lower Georges River and Port Hacking. Gradually Eucalypts would have invaded the grasslands of Mount Annan. They may have survived the previous dry conditions in woodlands along the gorges of the Georges, or on clay soils in places closer to the coast. Eucalyptus tereticornis still occurs on coastal headlands around Sydney harbour and Port Hacking and may have invaded the woodlands from there.
Other species from grassy forests may also have invaded the grassland, some of the daisy species that are wind dispersed could have blown in while other bird and animal dispersed species with fleshy fruits such as Solanum and Einadia could have gradually arrived.
Fires would have burnt through the woodlands as a result of lightning strikes or human activities. Humans have been in eastern Australia since long before the last Ice age and are likely to have increased natural fire frequencies. The grassland herb species, with their adaptions to survive the dry conditions, their underground storage rhizomes and tubers, would also have been able to tolerate fires. The fire regimes may have assisted the eucalypts to invade the grasslands by providing open conditions necessary for their seedlings to establish, though such conditions could also have been provided by grazing macropods. However the particular species that invaded, Eucalyptus moluccana and Eucalyptus tereticornis, would not have been able to invade without the increased rainfall.
Whether Aboriginal people facilitated the introduction of particular species into the woodland is not known, but many species, including many grasses, have hooks, barbs or appendages on their seeds that would have enabled inadvertent transport by humans, or other animals such as dingoes or kangaroos.
Since 1800 - the exotic weed species
The movement of Europeans and their domestic animals into the Campbelltown area in the 19th century, added a major group of species to the woodland flora – the exotic weeds. Some were introduced very early with grazing and agriculture and were reported to be naturalising very soon after settlement e.g. *Lolium perenne and *Gomphocarpus fruticosus by 1804; some were introduced deliberately for pastures e.g. *Paspalum dilatatum; some came accidentally e.g. with stock moved in from drier parts of western NSW such as Einadia species
Later came species that were first grown in gardens and then escaped into the surrounding paddocks and bushland: *Heliotropium amplexicaule, *Verbena rigida, *Araujia sericifera *Hypericum perforatum. *Olea europaea subsp. cuspidata was used as hedges and then got away. In the 20th century escapes continued *Senecio madagascariensis spread in the 1960s. *Nasella neesiana has begun to spread in the last decade.
Find out more about the history of vegetation of the Cumberland Plain and across the Western Sydney Woodland.
Asterisk * indicates exotic species naturalised at the Australian Botanic Garden.