For more than 30 years, scientists have suggested that the first human immigrants into Australia dramatically changed the continent's vegetation with the use of fire. However, few vegetation records from the vast Australian interior exist, and none extend beyond the last 18,000 years. The traditional tools for reconstructing vegetation - such as imprints and remnants of plants and the accumulation of pollen grains - are not preserved in the harsh outback environment.
A report in the May 14 issue of Science, describing a novel approach to reconstructing paleovegetation, presents the first continuous vegetation record from the Australian interior extending back 65,000 years. This, in turn, serves as a proxy for the predominent season of rainfall. The evidence is consistent with a human over-print on environmental change, the authors say.
The paper was authored by Beverly Johnson, a post-doctoral fellow with the University of Washington's School of Oceanography; Gifford Miller of the University of Colorado at Boulder; Marilyn Fogel of the Carnegie Institution of Washington (D.C.); John Magee and Michael Gagan of the Australian National University; and Allan Chivas of the University of Wollongong in Australia. To reconstruct changes in vegetation, and the season of predominant rainfall, the research team analyzed the stable carbon isotope composition of fossilized eggshells laid by emus, large flightless birds that are still found today in Australia.
The eggshells reflect the emu's diet, including the relative amounts of vegetation consumed with a C3 signal (mostly trees, shrubs, and winter grasses) or a C4 signal (tropical grasses). Modern emu eggshells have a stronger C3 signal if the animals live in regions where the plants predominately receive winter precipitation. The shells of modern emus have a stronger C4 signal if they live in areas dominated, instead, by the summer monsoon and the resulting weather systems carrying rain into the interior.
The paper reveals a vegetation record from fossilized eggshells from the southern portion of the Lake Eyre Basin, which comprises 1/6 of the Australian continent. The record indicates that summer rains (December to February), which occur across the northern part of the continent and produce weather systems that carry rain into the interior, dominated the region between 45,000 and 65,000 years ago supporting trees, shrubs and C4 grasses. Between 15,000 and 30,000 years ago as the world entered the Last Glacial Maximum, Lake Eyre became almost devoid of C4 grasses, and the vegetation became dominated by desert scrub. Even when the last glaciation ended about 10,000 years ago, and the planetary monsoon systems were re-invigorated, C4 grasses reached only moderate levels at Lake Eyre. Today, the desert scrub found there survives on less than 10 inches of precipitation that falls throughout the year.
Just three months ago, in the Jan. 8 issue of Science, researchers from these same institutions presented evidence that the ways humans changed the vegetation could have caused 85 percent of Australia's animals weighing more than 100 pounds to go extinct. The research team speculated that many browsers, such as the large flightless bird Genyornis newtoni that was the focus of their research, became extinct after human-set fires across the continent's interior changed the ecosystem's flora. The new paper strengthens the earlier speculation by demonstrating for the first time that arid-zone vegetation did indeed change dramatically, consistent with the proposed burning hypothesis.
"The summer monsoon remained only moderately effective during the last 10,000 years and has never been as effective as it was 65,000 to 45,000 years ago" Johnson says. The researchers speculate that the failure of monsoon precipitation to reach the interior may be due to systematic burning by humans, which led to large-scale modification of the vegetation across the semi-arid zone with trees and shrubs being replaced by desert scrub.
Such a change would have decreased the transfer of moisture from the biosphere to the atmosphere, an important feedback mechanism for the effective penetration of the monsoon, Johnson says. For instance, plants with less leaf area would have reduced evapotranspiration, in which plants give off moisture to the atmosphere. This is moisture that would have contributed to additional rainfall into the continental interior. Soils would have had less organic matter and less ability to hold moisture.
Fire is presumed to have been used by the first human colonizers for hunting purposes, path-clearing, and communication, among other things. The soils in the Australian interior are low in nutrients thus rendering the ecosystems more sensitive to disturbances and less likely to recover from the impact of changes in burning regime.
Additional vegetation and fire history records from northern and central Australia are needed to prove these hypotheses, the researchers say.
The work reported in Science was funded by the National Science Foundation and the Carnegie Institution of Washington's Geophysical Laboratory.
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