New perspectives on tropical grassy biomes and the trophic consequences of the evolution of grass.

BACKGROUND: Biomes are physiognomically distinct large vegetation units traditionally defined by climate. Yet, grassy biomes differ by growing in diverse climates and on diverse soils. Consequently they were long assumed to be secondary vegetation produced by human deforestation. However, since the new millennium, studies have shown grassy biomes have endemic species-rich biotas contradicting their supposed recent origin. Here I review novel perspectives on the nature of tropical grassy biomes, how they are maintained where forests can also grow and why the evolution of grasses displaced ancient forests. SCOPE: A significant problem is defining grassy biomes as distinct from forests. Controversy over definitions has contributed to the lack of a widely accepted global grassland map. Nevertheless, generalizations from studies from diverse regions are that ancient grasslands have diverse floras, especially rich in herbaceous plants. Plants are adapted to high sunlight and frequent disturbance contrasting with forest taxa. Plant adaptations to grassland disturbances depend on whether mammalian herbivory or fire is the dominant disturbance. There is considerable interest in which grassy biomes could potentially switch to forests for sequestering carbon. The new perspective is that neither site nor disturbance predicts tree densities but rather their interaction. Predicting potential tree biomass remains controversial with little consensus among methods and many failing to include fire or herbivory. Alternative stable state theory has been applied as an explanation for forest/grassy biome mosaics driven by frequent fires. CONCLUSIONS: The evolution of grasses has been transformative in creating grassy biomes by promoting novel trophic interactions with grazing mammals and grass-fuelled fire regimes. The extirpation of large mammals by humans and their replacement by livestock, and human-driven changes in grassland fire regimes are altering the domain of grassy biomes. An understanding of the unique features of grassy biomes can contribute to wiser management in the future.