Fire and the Miocene expansion of C4 grasslands

C-4 photosynthesis had a mid-Tertiary origin that was tied to declining atmospheric CO2, but C-4-dominated grasslands did not appear until late Tertiary. According to the 'CO2-threshold' model, these C-4 grasslands owe their origin to a further late Miocene decline in CO2 that gave C-4 grasses a photosynthetic advantage. This model is most appropriate for explaining replacement of C-3 grasslands by C-4 grasslands, however, fossil evidence shows C-4 grasslands replaced woodlands. An additional weakness in the threshold model is that recent estimates do not support a late Miocene drop in pCO(2). We hypothesize that late Miocene climate changes created a fire climate capable of replacing woodlands with C-4 grasslands. Critical elements were seasonality that sustained high biomass production part of year, followed by a dry season that greatly reduced fuel moisture, coupled with a monsoon climate that generated abundant lightning-igniting fires. As woodlands became more open from burning, the high light conditions favoured C-4 grasses over C-3 grasses, and in a feedback process, the elevated productivity of C-4 grasses increased highly combustible fuel loads that further increased fire activity. This hypothesis is supported by paleosol data that indicate the late Miocene expansion of C-4 grasslands was the result of grassland expansion into more mesic environments and by charcoal sediment profiles that parallel the late Miocene expansion of C-4 grasslands. Many contemporary C-4 grasslands are fire dependent and are invaded by woodlands upon cessation of burning. Thus, we maintain that the factors driving the late Miocene expansion of C-4 were the same as those responsible for maintenance of C-4 grasslands today.