Changes in vegetation and seasonal climate since the last full glacial at Lake Frome, South Australia
Abstract Results are presented from stratigraphy, radiocarbon, and pollen analysis from Lake Frome, South Australia, close to the summer/winter rainfall boundary at 30° south latitude. The pollen sequence shows that tree vegetation was minor around 18,000 yr B.P. The landscape was dominated by chenopod low shrublands and ephemeral (Tubuliflorae) vegetation. By ∼17,000 yr B.P., Callitris moved in and rose to 25% of the pollen sum. Values of Casuarina, myrtaceous shrubs, Acacia, Dodonaea and Cyperaceae also rose at about the same time. Between ∼17,000 and ∼ 14,500 yr B.P., Callitris and Eucalyptus dominated the tree vegetation with Callitris reaching its maximum values. Eucalpt woodlands were associated with undershrubs belonging to Myrtaceae, Acacia, Dodonaea and Gyrostemonaceae, whereas Callitris woodlands had probably supported only a sparse understorey. Chenopodiaceae, today distributed mainly in the winter rainfall zone, continued to show high values (>; 25%). Gramineae, today associated with high summer rainfall, generally remained below 20%. The sporadic presence of a number of taxa, which now grow in temperate areas further south, suggest that average annual temperatures were a little lower than at present. From ∼ 14,500 to ∼ 13,000 yr B.P. there was a sudden decline in the values of Callitris and a more gradual decline in Chenopodiaceae and Dodonaea. The rise of ephemeral vegetation together with a fall in Chenopodiaceae would indicate that winter precipitation had started to decline. Between ∼ 13,000 and ∼ 4500 yr B.P., Chenopodiaceae and Tubuliflorae (ephemerals) declined, and Gramineae rose to its highest values. Tribulus pollen, a subtropical/tropical taxon, first appeared ∼ 13,000 yr B.P. Overall, plants from the summer rainfall zone became more prominent, and taxa from the temperate south declined. Summer monsoon incursions apparently strengthened, and temperature increased during this phase. Since ∼ 4500 yr B.P. both Chenopodiaceae and Tubuliflorae returned to high values whereas Gramineae declined. Values of Acacia and myrtaceous shrubs rose but those of Casuarina declined and Callitris remained extremely low. These changes were accompanied by the onset of playa environments, which have since persisted. Vegetation during this phase suggests that summer precipitation declined and summer monsoon incursions became rare during the late Holocene.