Long term controls of fire, climate and pCO2 on C4 grass abundance in southeastern Australian grasslands

Background/Question/Methods

Grass-dominated ecosystems cover one-third of Earth’s land surface, influence key biogeochemical processes, and serve as major food sources. A challenge in studying the response of grasslands to environmental change in paleorecords is that grass pollen generally has low taxonomic specificity, which means that pollen assemblages are uninformative about C₃ and C₄ grass variations. To overcome this difficulty, we have developed a novel technique for analyzing the stable carbon isotopic composition of individual grass-pollen grains (SPIRAL). Here we utilize SPIRAL to assess how fire and climate affected C₄-grass abundance during the late Quaternary in SE Australia. Fire enacts a strong influence on the maintenance of modern grasslands even in both tropical and temperatre regions of the globe. 

Results/Conclusions

Preliminary SPIRAL data from Victoria, Australia indicate that C₄-grass abundance varied both spatially and temporally during the last 25,000 yr BP. Caledonia Fen, a high elevation site in the Snowy Mountains, ranged from ~80% C₄ grass in the late glacial to ~0% near the present. Tower Hill, a low elevation site, had highly variable C₄ abundance over the last 25,000 yr BP with values ranging from 20-60%. Overall analyses show that charcoal accumulation rates (i.e., fire), had no significant relationship with C₄-grass abundance throughout either record. However, a relatively large increase in C₄ grasses in the mid-Holocene of Tower Hill is coincident with a shift to a higher fire regime suggesting that fire may be important in maintaining high C₄ grass abundance during certain periods. Temperature/pCO₂ showed the strongest relationship with C₄ abundance at Caledonia Fen with relatively more C₄ grass when pCO₂ temperatures were lower during the last glaciation. Periods of relatively higher aridity in the Tower Hill record coincide with peaks in C₄ grass abundance in the late glacial and early Holocene indicating that moisture may be controlling C₄ grass abundance during these period. Similar patterns in C₄ grass abundance are observed at Lake Rutundu (high elevation) and Lake Challa (low elevation) in east Africa.