ABSTRACT
The rise of grass-dominated ecosystems is one of the most profound paleoecological changes in the Cenozoic1 (Crepet and Feldman, 1991). According to Stromberg (2011) the evolution and subsequent ecological expansion of grasses (Poaceae) since the Late Cretaceous2 have resulted in the establishment of one of Earth’s dominant vegetation formations, the temperate and tropical grasslands, at the expense of forests. The development of grassland ecosystems on most continents was a multistage process involving the Paleogene3 appearance of (C3 and C4) open-habitat grasses, the mid-late Cenozoic spread of C3 grass-dominated habitats, and, finally, the Late Neogene
4 expansion of C4 grasses at tropical-subtropical latitudes. Macrofossil records reveal that grasses were established in northern South America, northern Africa, and India in the latest Cretaceous and Paleocene (Anderson, 2006; Stromberg, 2011). According to Mix et al. (2013) changes in vegetation in western North America, for example, played a critical role in establishing the modern hydrologic regime. They attribute this to three primary reasons: (1) increased evaporation and transpiration fluxes in grassland regions affected the water balance, (2) shallower rooting depths of grasses led to the transpiration of soil water enriched in δ18O5 due to evaporation, and (3) grasslands transpire δ18O rich waters during a shorter, more punctuated growing season. Mix et al. (2013) argue that the observed isotope signals are indicative of a feedback mechanism wherein grasslands not only respond to regional
a Institute of Desertification Studies, Beijing, China. b Qinzhou University, Guangxi, China. * Corresponding author: dryland1812@internode.on.net 1 Cenozoic was about 70 MaBP [Ma is short for mega annum (million) and BP is before the present]. 2 Late Cretaceous about 60 MaBP. 3 Paleogene was about 43 MaBP. 4 Late Neogene lasted about 43 million years end about 23 MaBP. 5 Oxygen isotope δ180 is widely used to help determine sequence stratigraphy.
