ECR feature: Susanna R. Bryceson on the distribution of grasses in Australia

Susanna Bryceson has recently completed her PhD at La Trobe University, Melbourne. She is an ecologist primarily researching the migration of C4 grasses to Australia and their effect on the resident flora and fauna with which they had not co-evolved. Here, Susanna shares her experience and exciting findings.

Susanna Bryceson at savanna fire research site near Darwin, Northern Territory Australia.

Institute. La Trobe University, Australia.

Academic life stage. Recent PhD completion.

Major research themes. I’m interested in the interface of Australian ancient plants and animals, with taxa immigrating into the country (particularly through Southeast Asia) over the past few million years, and how interactions between them—and potentially with humans—shaped what we see in Australia today.

Current study system. My study system is the Australian continent and my focal species are the grasses, particularly those using C4-type photosynthesis. In a global context, the Australian flora and fauna stand alone, having evolved in isolation over tens of millions of years. In Gondwana, the Australian landmass was at the ‘end of the road’ in the east, and today it has a similar position in relation to the main source of immigrant taxa, Southeast Asia.

Recent JBI paper. Bryceson, S. R., Hemming, K. T., Duncan, R. P., & Morgan, J. W. (2023). The contemporary distribution of grasses in Australia: A process of immigration, dispersal and shifting dominance. Journal of Biogeography, 50(9): 1639-1652.

Motivation behind this paper. Today, grass-driven fire is a prominent feature in many parts of Australia. My recent research found that Australian ecosystems evolved largely free of grasses until the past few million years, unlike on other continents which had massing herds of grazing animals co-evolving with grasses over tens of millions of years, forming complex savanna ecosystems of plants and animals. With Peter Linder’s words about Africa ringing in my ears, that ‘C4 grasses likely created an orgy of extinction’, my mission was to work out how C4 grasses that originated on other continents could become so prominent in Australia. I started thinking about the consequences of immigrant grasses entering a woodland, where they would be bulkier and taller than the existing ground cover. For example, a fire in a tall, bulky grass would potentially affect more of the neighbouring vegetation than a flaming small plant with fine leaves. I wanted to find out where these grasses speciated and spread, particularly the tall species.

Key methodologies. Earlier phylogenetic research showed that not all grasses were the same and that C4 grasses inherited the morphology of their C3 ancestors (Erika Edwards et al, 2010). To get a picture of the progress of grasses in Australia, we combined morphology (height), phylogenetic lineage, photosynthetic type, endemism and migration period. The grass species were categorised as endemic or shared (with other continents) and using environmental models and the combined analysis enabled us to track their distribution across time. We gathered data on the heights of all Australian grass species and linked this to the distribution of C3 and C4 lineages over about 30 million years.

Susanna Bryceson in tall C4 grass covering a previously clear track through savanna country, Northern Australia.

Unexpected challenges. One of our aims was to create a map showing grass heights across Australia. We thought that by combining location data for each species with their height data, that would be straightforward, but the maps we produced were baffling and didn’t represent what we had observed in the field. We realised that the problem arose with the height data: in the various floras we consulted, the height of a species was often expressed as a range, e.g. 50-80 cm. We applied the tallest height to all occurrences of that species across the country but what this could not account for was that generally, the height of C4 grasses in the cooler south was at the very low end of the catalogued range. Without more detailed adjustments to represent local grass heights, producing a map that made sense was impossible.

We were also stunned by the way the endemic and shared species of the different phylogenetic types were distributed and how, when informed by the environmental modelling, they could describe a picture of dispersal (or not!) through the dry interior of the country, when most of the ancient plants and animals were retreating to more mesic regions. Matched up with time of arrival in the country, we could also see patterns in which some grass types had spread before the aridification in the inland but had retreated into a disjunct and relictual pattern, another type had been favoured by aridification and was widespread inland, and a further group showing that the recent arrivals had barely spread from the north coast.

Major results. This paper lowers its focus to the ground level of vegetation. Grasses are such a ubiquitous element of many ecotypes that they are easily overlooked in favour of larger or more showy plant forms—trees, shrubs and flowering curios—when describing ecosystem patterns. From an ecological perspective, the tendency of grasses to create dry fuel brings the potential of fire into the ecosystems they infiltrate, making them potent factors in landscape function and, potentially, transformation. This paper shows the transformative power of grass as an evolutionary force, a migrator, an invader, and an opportunist. Over 20 million years, grasses gradually moved into Australian ecosystems, with that migration becoming a surge as the continent drew closer to Southeast Asia, culminating in the arrival of Andropogoneae grasses in the far north where their presence or absence dictates the fire regimes of the northern savannas today.

Tall grasses of the Asian-origin Androgooneae tribe have transformed landscapes across Australia. This species, Themeda triandra Forssk, is now distributed across the country to far southeast and southwestern regions.

Next steps for this research. Some of the world’s most flammable grasses have their origins in Asia, dominating the savannas which ranged through to Sunda (the former Southeast Asian landmass). But it wasn’t just the grasses that migrated, it was a system: many trees and shrubs from ancient Sundanese savannas are also present in northern Australia and as such, they are adapted to more frequent fire. I’ll be looking in more detail at Australia’s savannas and analysing the distributions of plants of different origins (either ancient Australian or Sundanese) in relation to fire frequency.

If you could study any organism on Earth, what would it be? I am intrigued by the Eriachne genus, the only Australian-origin C4 grass. This speciose, finely-statured group seems to have a low-key role in ecosystems, being able to grow in harsh sun as well as in shady and long unburned places. Where it is the dominant grass, it probably means that fire is infrequent.

Is there anything else you would like to tell us about yourself or your featured research? This paper was five years in the making. I met my collaborator, Kyle Hemming, at a conference, surprised that he was also looking at grass distribution and our very different approaches turned out to be a great match. We kept adding new dimensions to the analysis but as this expanded, I realised a ‘prequel’ was needed, spawning my paper that described Australasia’s grasses in a global context (Bryceson & Morgan, 2022). Once the global picture was clarified, work on the Australian distributions paper resumed with many further iterations, thanks to Kyle’s endless patience. It was intriguing to see the patterns being revealed.

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