The role of fire in Miocene to Pliocene C4 grassland and ecosystem evolution

Abstract

Modern grasslands are dominated by grasses that use the C4 photosynthetic pathway, and were established about 8 million years ago. A sediment record suggests that in southwestern Africa, the expansion of grasslands was associated with increasing aridity and fire activity, both of which favour grasses that use the C4 pathway. Modern savannah grasslands were established during the late Miocene and Pliocene (8–3 million years ago)1,2. In the tropics, grasslands are dominated by grasses that use the C4 photosynthetic pathway, rather than the C3 pathway. The C4 pathway is better adapted to warm, dry and low-CO2 conditions, leading to suggestions that declining atmospheric CO2 levels3, increasing aridity and enhanced rainfall seasonality allowed grasses using this pathway to expand1,2,4 during this interval. The role of fire in C4 expansion may have been underestimated1,5,6,7. Here we use analyses of pollen, microscopic charcoal and the stable isotopic composition of plant waxes from a marine sediment core off the coast of Namibia to reconstruct the relative timing of changes in plant composition and fire activity for the late Miocene and Pliocene. We find that in southwestern Africa, the expansion of C4 grasses occurred alongside increasing aridity and enhanced fire activity. During further aridification in the Pliocene, the proportion of C4 grasses in the grasslands increased, while the grassland contracted and deserts and semi-deserts expanded. Our results are consistent with the hypothesis1,5,6,7 that ecological disturbance by fire was an essential feedback mechanism leading to the establishment of C4 grasslands in the Miocene and Pliocene.