Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Tuesday, 27 October 2015

The rise of grasses and grasslands in Australia

It's been a while since I wrote. The perils of too many things on, a loooonnnngggg list, and not enough time.
Recently I've been thinking about my favourite biome - the grasslands - and when they came to prominence in Australia. Was it because of megafauna? Was it because of climate? Was it because of Aboriginal burning?  So, I started reviewing the literature and thought I'd share my findings here. It's a fascinating topic and one that I'm pretty sure most biologists / grassland aficionados  /ecologists aren't entirely familiar with.
Mitchell Grass grasslands - widespread in
north-eastern Australia, but when did they
come to prominence?
While tussock grasslands are a widespread vegetation type in Australia, grassland ecosystems per se were largely absent from Australia until fairly recently. It is also likely that the C4 contingent of grasses that currently occur in Australia are recent arrivals to the continent. Jacobs et al. (1999) provide a comprehensive review of the topic.

Grasses originated in Gondwana during the Cretaceous (>65 million years ago (Ma)) with some speculation that graminoids possibly originated in East Gondwana, notably the area that later became Australia. In Australia, however, there are almost no early preserved grass macro-fossils, possibly due to the bias towards their preservation in wet environments. The earliest record of Australian grass pollen is from the mid Eocene (~45 Ma), but it has always been relatively sparse in the Australian fossil record, only becoming most abundant within the last 2 million years.

Grass pollen first occurs in north-western Australia, possibly forming savannah by the mid-Miocene (~15 Ma). The expansion of open vegetation types accelerated in the late Miocene due to increased climate seasonality. There were rapid evolutionary radiations in many large Australian groups such as the sclerophyll taxa Eucalyptus, Banksia and Allocasuarina at this time, as well as grasses. Austrostipa, for example, originated and began to diversify between 25 and 10 Ma, and a rapid radiation occurred, indicated by a high diversification rate at that time. Increasing taxonomic diversity may have resulted from adaptation to newly derived arid niches caused by climatic changes.

From north-west Australia, grasses expanded south-east through central Australia as aridity intensified through the Late Tertiary. Grasses were present in northern and central Australia, extending into the Murray Basin, in the early to mid Miocene. The entry of grasses into more southern and eastern areas of Australia occurred in the mid to late Miocene and early Pliocene (~5 Ma). Fires were part of the landscape in the Murray Darling throughout the Miocene, increasing with climate seasonality, potentially facilitating the spread of grasses and grasslands, particularly those dominated by C4 grasses.

Themeda triandra - one of the most widespread
C4 grasses in Australia

Many grasses that are now common in Australia appear to have migrated from Asia during the Miocene when Sundaland (the Indonesian archipelago) collided with the Australian plate, including the C4 genera Themeda, Dichantheum and Bothriochloa. A number of tropical south-east Asian genera have strong representation in Australia due to this early migration. All are tropical grasses which extend into the temperate zone of southern Australia. The Andropogoneae (which includes all the above genera) have major centres of distribution in south-eastern Indonesia and India, with a lesser centre in central eastern Africa. This infers that the taxa have entered Australia from the north since its collision with Asia (although Andropogoneae may have existed in Australia before this time). It is likely that Themeda migrated from south-east Asia into both South Africa and Australia in the late Tertiary; it is now widespread in both continents.

C4 grasses extended into the temperate regions of the south of Australia, most notably, Themeda, the genera that subsequently became the dominant mesic grassland type in the south-east of the continent. In southern Australia, however, grasses did not achieve their current prominence until the Late Pleistocene. Explanations for the C4 expansion across the landscape in the Miocene have invoked changes in the seasonality of climate, particularly climate drying, given the C4 pathway appears to give grasses a competitive advantage in arid environments, and to changes in fire regimes. Increases in the abundance of the major C4 clades Paniceae and Andropogoneae were thought most favoured by these changed conditions, although the reasons for the rise of C4 grasslands per se are still debated. New research, for instance, suggests that the C3 Pooideae (which include the Stipeae) expanded into cooler climates rather than being outcompeted by C4 grasses, an event that is possibly as important as the global C4 expansion.

Further Reading
Jacobs, B.F., Kingston, J.D. & Jacobs, L.L. (1999) The origin of grass-dominated ecosystems. Annals of the Missouri Botanical Garden 86, 590-643.