What Limits Trees in C4 Grasslands and Savannas?

Though the distribution of global vegetation can generally be predicted from climate, grasslands are an exception. C4 grassy biomes cover vast areas that are warm enough and wet enough to support closed forests. The extent of this climate mismatch has been revealed by physiologically based global vegetation simulations and by large empirical data sets. Reasons for the existence of grassy biomes have long been debated, polarized into bottom-up (resources) or top-down (fire, herbivory) arguments. Recent studies indicate that both are important, especially in suppressing woody recruits. Grasses are formidable competitors belowground, create highly flammable fuels, and can support large herbivore densities. The net effect on trees is rare and episodic recruitment of adults in tree-fall gaps. The implication is that ecosystem structure and function depend on demographic transitions. Tree cover is increasing and grass/forest boundaries are changing. These changes can have large feedbacks to the earth-atmosphere system. Though progress has been made, there is still great uncertainty in predicting the future of C4 grassy biomes.