On the Young Savannas in the Land of Ancient Forests

Abstract

Covering ancient geomorphological landscapes, and surrounded by some of the most diverse forests on Earth, the Neotropical savannas were once perceived by naturalists as ancient environments. However, current evidence suggests that tropical forests have existed in the Neotropics since the Paleocene, whereas most plant lineages present in South American savannas are recently derived from clades from the surrounding forested biomes. This chapter provides a multidisciplinary overview on the origin, assembly and expansion of Neotropical savannas, with focus on South America. For this, we consider available evidence from the fossil record, paleoenvironmental proxies (phytoliths), and phylogenetic information for both plants and animals. Paleoenvironmental reconstructions indicate suitable climates for central South American savannas since the middle Miocene, which is also when molecular phylogenies indicate the origin of some vertebrate groups typical of savannas. Fossil data indicate the ecological expansion of both C₃ and C₄ grasses in southern South America by the late Miocene. Fossil information also indicates the onset of savannas in northern South America during the Pliocene, a period in which most woody plants of the largest extension of Neotropical savannas (the Cerrado) are thought to have diversified, as inferred by dated phylogenies. Although the combined lines of evidence indicate that Neotropical savannas in South America are indeed younger than their surrounding forests, the precise timing and factors that influenced the origin, assembly and expansion of Neotropical savannas remain contentious. Future research should aim at (1) increasing and integrating knowledge about the diversification of important taxa characteristic to Neotropical savannas, (2) establishing continuous sequences of fossils, and (3) building accurate paleoenvironmental reconstructions for the entire Neogene.