Article: S. Hoetzel, L. Dupont, E. Schefuß, F. Rommerskirchen, G. Wefer. (2013) The role of fire in Miocene to Pliocene C4 grassland and ecosystem evolution. Nature Geoscience. Vol. 6, 1027-1030. DOI:10.1038/NGEO1984
The African Savanna is known for its picturesque yellow grasses dotted with exotic shrub-like trees such as Acacia, teeming with impressive mammals including prides of lions, herds of giraffes, zebras, and elephants, leaping antelope, and gigantic rhinoceros. Anyone who has seen Disney’s The Lion King will know this grassland biome. However, these Savanna grasslands are geologically young and only evolved around 8 million years ago towards the end of the Miocene.
The Miocene is an epoch during the Neogene period that occurred roughly between 23 and 5.3 million years ago. Towards the end of the Miocene, the Earth started to cool down due to the formation of the Himalayan Mountains. Mountain building causes cooling because it increases the amount of exposed rock available for erosion, which ultimately leads to a reduction of the amount of carbon dioxide in the atmosphere.
This type of climate, dry with low CO2, led to the evolution and expansion of C4 plants in subtropical regions. C4 plants, such as most grasses, use a special type of carbon fixation (part of photosynthesis) that excels in warm regions prone to drought and low carbon dioxide. C4 plants are much more successful in these conditions than C3 plants, such as most trees.
Paleo-scientists have hypothesized that C4 plants, and grassland biomes, evolved during the end of the Miocene due to these arid and low CO2 conditions. However, the optimal decrease in CO2 happened much before the large expansion of C4 grasslands. This new study hypothesizes that wildfires were the initial trigger for the expansion of the African Savanna because they burned down forests and made room for grasses to grow.
Hoetzel et al. (2013) used a sediment core collected off the coast of Namibia and analyzed it for the grass pollen Poaceae, pollen from desert plants, fire-charred particles, and biomarkers called n-alkanes (Figures 1 and 2). They used a section of the sediment core from the end of the Miocene to the beginning of the Pliocene (9- 2.5 years ago) in order to determine the role wildfires played in expanding grasslands. Marine sediments are excellent at preserving these biomarkers and pollen, allowing paleoceanographers to reconstruct what the conditions on earth were like millions of years ago.
The amount of C4 grass pollen increased by 30% starting about 8.4 million years ago, and then increased steeply up to 70% around 6.8 million years ago at the end of the Miocene. The n-alkane biomarkers also indicate a large rise in C4 grass plants over C3 plants. N-alkanes are derived from leaf waxes and C4 grasses typically have 31 or 33 carbons and an enriched δ13C. These n-alkanes can be used to determine the relative inputs of C4 grasses versus C3 plants. This rise of C4 grasses coincides with a spike in the amount of fire-charred particles as well as a prolonged dry period that would be optimal for wildfires.
These researchers noted that upwelling from the Benguela current (located off the coast of Southwest Africa) greatly increased during the end of the Miocene, which led to cooler temperatures, stronger winds, and an increasing aridification (dryness) of Southern African. This oceanographic event led to strong wildfire breakouts that burned down many of the woodlands in the area, allowing C4 grasses to grow in place of the trees. Grasses grow faster than trees, especially after wildfires. Thus, wildfires are hypothesized to have caused the initial expansion of the C4 grasslands by clearing space, and since the atmosphere was dry and had low CO2, C4 plants easily out-competed C3 trees are the region.
But the influence of fire did not just initiate the expansion of C4 grasslands in Africa; it created a positive feedback mechanism to continue its expansion. As grasslands out-competed tree biomes, the local region became even drier. All plants release some water back into the atmosphere in a process called evapotranspiration. Grasses put less water in the atmosphere than trees, which caused Southern Africa to become even drier, which caused more wildfires to occur, which burned down more C3 biomes, and led to a greater expansion of the grassy Savanna. This feedback continued until the local climate actually got too dry even for grasses, resulting in the expansion of deserts.
The expansion of deserts occurred around 5.8 million years ago. The sediment record shows that the amount of fire-charred material also decreased around this time, most likely since deserts do not have a lot of plants for fires to burn. However, C4 grasslands were already established and the subtropical climate even today favors their dominance over woodlands. The expansion of these C4 grassland biomes changed the ecology of Africa, allowing the animal diversity we know today to evolve.
Atmospheric conditions of low water vapor and CO2 during the late Miocene favored the evolution and expansion of the modern day African Savanna. However, wildfires caused by drought are now hypothesized to have triggered this expansion by clearing woodlands and allowing C4 grasses to quickly dominate this region. The increasing arid conditions also led to the growth of deserts, which is ultimately what stopped C4 grassland expansion. Thus, lions, cheetahs, and elephants can thank wildfires for giving them their home.
Figure 1: Present day vegetation map of the study area. The Benguela Current, and associated upwelling, is in blue. The dashed black line represents the 4,000 m ocean depth line. Black arrows indicate the modern prevailing wind direction. The fire symbols show the areas were large wildfires have occurred from October-Novemebe 2011.
Figure 2: The sediment record used to reconstruct Africa’s climate and vegetation between 9 and 2.5 million years ago. A) The grass pollen Poacaea shows an increase between 8 and 6.8 million years ago, demonstrating the expansion of grasslands. B) Stable carbon isotope ratios of grass waxes called n-alkanes also showed an increase in grasses. C) The concentration of fire charred particles occurred during the same time as grass expansion. D) Pollen from desert plants began to increase after the peak of wildfire frequencies, showing the expansion of deserts stopped the expansion of grasslands. E) Alkenones, a biomarker used to predict seas surface temperature, showed that the ocean (thus atmosphere) got cooler during this expansion due to an increase in upwelling from the Benguela Current.
I received a Ph.D. in oceanography in 2014 from the Graduate School of Oceanography (URI) and am finishing up a post-doc at the University of Maryland Center for Environmental Science (Horn Point Laboratory). I am now the Research Coordinator for the Delaware National Estuarine Research Reserve.
Carbon is my favorite element and my past times include cooking new vegetarian foods, running, and dressing up my cat!