Late-Quaternary variation in C₃ and C₄ grass abundance in southeastern Australia as inferred from δ¹³C analysis: Assessing the roles of climate, pCO₂, and fire

Climate, atmospheric pCO₂, and fire all may exert major influences on the relative abundance of C₃ and C₄ grasses in the present-day vegetation. However, the relative role of these factors in driving variation in C₃ and C₄ grass abundances in the paleorecord is uncertain, and C₄ abundance is often interpreted narrowly as a proxy indicator of aridity or pCO₂. We measured δ¹³C values of individual grains of grass (Poaceae) pollen in the sediments of two sites in southeastern Australia to assess changes in the proportions of C₃ and C₄ grasses during the past 25,000 years. These data were compared with shifts in pCO₂, temperature, moisture balance, and fire to assess how these factors were related to long-term variation of C₄ grass abundance during the late Quaternary. At Caledonia Fen, a high-elevation site in the Snowy Mountains, C₄ grass abundance decreased from an average of 66% during the glacial period to 11% during the Holocene, primarily in response to increased pCO₂ and temperature. In contrast, this pattern did not exist in low-elevation savannah woodlands around Tower Hill Northwest Crater, where C₄ grass abundance instead varied in response to shifts in regional aridity. Fire did not appear to have strongly influenced the proportions of C₃ and C₄ grasses on the landscape at millennial timescales at either site. These patterns are similar to those of a recent study in East Africa, suggesting that elevation-related climatic differences influence how the abundance of C₃ and C₄ grasses responds to shifts in climate and pCO₂. These results caution against using C₄ plant abundance as a proxy indicator of either climate or pCO₂ without an adequate understanding of key controlling factors.