Chapter 17 The Geologic History of C4 Plants

Abstract

Our understanding of C4 plant history has been revolutionized by the use of carbon isotopes to construct geologic records of photosynthetic pathway. Through isotopic analyses of fossil teeth and soils, geochemists have discovered that the dominance of low latitude ecosystems by C4 species is a relatively recent phenomenon. A major expansion of C4 grasslands occurred across four continents only during the Late Miocene and Pliocene (2–8 Myr ago, Ma), with intriguing evidence suggesting a presence of C4 plants at low abundance for at least 10 Myr before this event. Analysis of calibrated molecular phylogenies for the grasses indicates that declining atmospheric CO2 began to select for C4 photosynthesis during the Oligocene (25–30 Ma), but there remains an important gap in the geochemical data between this event and Miocene evidence of the pathway. A similar atmospheric selection pressure may have operated during the Permo-Carboniferous (270–330 Ma), but isotope surveys have so-far failed to detect any direct evidence of C4 species. Understanding when C4 plants first originated, and why they remained sub-dominant components of ecosystems for so long, therefore remain important unresolved problems in this field. However, the worldwide expansion of C4 grasslands is better understood. A range of complementary geologic data now indicate that increasing climatic seasonality or aridity caused a retraction of woodland vegetation and allowed the incursion of C4 grasses. Abrupt increases in charcoal abundance in the Late Miocene and analogies with modern fire-maintained mesic grasslands indicate an important additional role for fire in this vegetation change. However, significant uncertainties remain, especially in explaining why earlier seasonal climates did not promote C4 grassland expansion, and what drove this event in North America, where there is no evidence of abrupt climate change. I propose that the evolution of grazing resistance in C4 grasses could have promoted fires, providing a mechanism for vegetation change without the need to evoke paleoclimate change.