Climate, not Aboriginal landscape burning, controlled the historical demography and distribution of fire-sensitive conifer populations across Australia.
Climate and fire are the key environmental factors that shape the distribution and age of plant populations in Australia. However, because of limited fossil records in this arid continent, it is unclear as to which of these factors have impacted vegetation more strongly, and what were the roles of human fire activity and the extinction of the megafauna about 45,000 years ago.
To answer these questions, this study analysed historic changes in the genetics of Cypress Pine (Callitris species) and its distribution. Cypress Pine is a widespread native Australian tree that grows in fire-prone regions but is paradoxically very sensitive to fire. By looking at the population genetics of these trees it was possible to calculate when they became genetically isolated from each other. If the trees became isolated around 45,000 years, about the time that Aboriginal Australians first arrived, it would suggest that human activity may have played a role.
The study examined DNA markers in samples from pine trees collected across the continent for signs of population ‘bottlenecks’. When populations shrink and stop exchanging genes with each other they lose genetic diversity and risk becoming inbred. Looking at changes in genetic diversity tells us if and when a population of animals or plants goes through a period of decline. The results varied from northern to southern Australia. In the monsoon tropics, where fire risk is highest, the pines showed no evidence of having passed through a population bottleneck. Pine trees in the arid zone however showed bottlenecks around 18,000 years while those in the temperate south rapidly increased and expanded around 10,000 years ago.
These dates don’t match human arrival in Australia, but they do broadly match Australia’s climate history. Around 18,000 years ago was the height of the last ice age when central Australia was dry, windy, and colder than today. Around 10,000 years ago was the end of the last ice age where warmer conditions favoured tree growth in previously treeless areas. The monsoon tropics have always been seasonally dry and hot, so no changes were expected in these areas.
Article accepted: 07 October 2013, Published 22 December 2013 - doi.org/10.1098/rspb.2013.2182
|Type of Publication||Journal Article|
|Year of Publication||2013|
|Authors||Sakaguchi S, Bowman DMJS, Prior LD, Crisp MD, Linde CC, Tsumura Y, Isagi Y|
|Pagination||20132182 - 20132182|
|Journal||Proceedings of the Royal Society B: Biological Sciences|