The mud in the lake floor of this lagoon on Truwana/Cape Barren Island may hold evidence of the environment 20,000 years ago and longer when the Bass Strait was land. Photograph by Simon Haberle. CC BY-NC-ND 4.0.

Pollen is the fine powdery substance produced by most flowering plants and is essential in the reproduction of those plants. Each individual pollen grain is small, generally 0.01 to 0.06 millimetres across – 10,000 pollen grains can fit on the head of a pin. Some pollen travels easily in the wind and is blown across the surface of a landscape. When it lands on the surface of a body of water, like a lake, it will slowly sink to the bottom and become preserved within the soft sediment. Over time, layers form and slowly build up a record with older pollen being buried deeper in the sediment, and modern pollen at the top.

By taking a core of these undisturbed sediments, researchers can extract pollen from regular intervals of the core, correlate them to age through radio-carbon dating, and start building up a picture of what past environments looked like.

Critically, different plants produce different types of pollen, meaning the pollen from a wattle shrub, for example, will look different to the pollen from a gum tree. Researchers can analyse and identify these pollen grains, to determine what specific plants were growing in a particular area, at a particular time.

Madeleine Bessell-Koprek collecting core samples. Photograph by Simon Haberle. CC BY-NC-ND 4.0.

Today the Bass Strait contains numerous small islands. These would have formed high plateaus during the Bassian Plain period. The islands contain many lakes and lagoons that provide the perfect conditions to collect cores and sample pollen. These cores show us that around 30,000 years ago the environments of the Bassian plain changed from open woodland (dominated by gum trees with a shrub and herb understory) to grassland. We know this because there is a dramatic increase in grass pollen (Poaceae) in the record at this time. So, those lakes and lagoons of the plateaus became surrounded by grassy plains. While the Bassian Plain was predominantly grassland, we can also tell that the landscape had some scattered trees. We know this because there is some pollen from she-oaks (Casuarina) and gum trees (Eucalyptus) in the pollen record after about 30,000 years ago. Overall, in this period we see more grass pollen and less tree pollen in the paleoenvironmental record than in the period prior. This environmental shift from woodland to grassland is associated with the onset of the Last Glacial Maximum. The world had reached a full glacial climate – the peak coldest point of the ice age. Overall colder and drier conditions resulted in multiple environmental changes. Not only did grassland expand at the expense of woodland, but lakes across the Bassian Plain experienced drops in water levels and turned into wetlands and swamps.

The second key to understanding past environments, and their management, is charcoal. When a fire occurs in a landscape, charcoal is produced. Charcoal particles, like pollen grains, are blown across a landscape, land on water bodies, settle to the bottom, preserve in the sediment, and can be extracted in the form of a core by researchers. By counting charcoal particles present at regular intervals in the cores, we can begin to construct a fire history of the area. Sudden peaks in the amount of charcoal in the record correlate to a large fire event, or a rapid series of fire events.

What we find is that at the same time as the grassland expanded on the ancient Bassian Plain, we see a decline in fire activity. We know this because we see a reduction in the abundance of charcoal present in the sediment cores collected from the Bass Strait islands. First Peoples started living in the Bass Strait region some 40,000 years ago. Between 40,000 and 30,000 years ago we see a high amount of charcoal in the palaeoecological record, indicating that the Old People used managed burning practices on the wooded land bridge as a land management strategy. As the environment shifted from woodland to grassland 30,000 years ago, and the climate became much colder, we see a decline in charcoal present in the record. This is in part because there is a reduction in woody biomass to burn, however it also likely reflects a change in First Nations fire management strategies as an adaptation to ongoing climate change. The increasing abundance of grasslands across the Bassian Plain would need less burning than that of an open woodland, requiring a land management regime of infrequent, low-intensity burning. This indicates that the Old People were adapting their land management strategies to fit the changes in environment.

Collecting mud from the lakes of the Bass Strait islands reveals a wealth of information about life on the ancient Bassian Plain. From analysing the charcoal particles and pollen grains preserved within this mud, we can start to picture and reconstruct what vegetation and landscapes would have looked like tens of thousands of years ago. We can also uncover compelling evidence that First Nations People have effectively used fire to manage, shape and take care of their environments for thousands of generations.