The effects of fire, climate and pCO2 on C4 grass abunadance in equatorial East African grassland communities over the past 25,000 years: An evaluation of the C4-fire hypothesis

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 East Africa. Fire enacts a strong influence on the maintenance of modern tropical grasslands where C₄-grasses are most abundant and fire has been suggested as a main driver for the Miocene expansion of C₄-grasslands. 

Results/Conclusions

Preliminary SPIRAL data from Lake Challa in equatorial East Africa indicate that C₄-grass abundance in the Mt. Kilimanjaro area averaged 45 + 25% during the last 25,000 yr BP. Analyses show that charcoal accumulation rates (i.e., fire), had no significant relationship with C₄-grass abundance throughout the record. Temperature/pCO₂ showed the strongest relationships with C₄ abundance with relatively more C₄ grass when pCO₂ temperatures were lower during the last glaciation generally agreeing with bulk-sediment and leaf-wax d¹³C records from the lake. However the SPIRAL data is systemically below leaf-wax by ~30-40% during the late Pleistocene while converging during much of the Holocene The disparities are probably a result of differences in the carbon isotope source (i.e., pollen vs. leaf wax) and dispersal area of the substrates for these proxies. Moisture seasonality also appears to be a strong positive control suggesting that C₄ grasses favored long seasonal droughts. Strikingly, C₄-grass pollen abundance has reached its highest levels (>80%) of the past 25,000 years only within the last millennium while remaining relatively low during much of the Holocene. The modern day rise in C4-grasses cannot be easily attributed to any of the climate proxies despite that we can uniquely control for them.