Phylogeography of Two Widespread C4 Grass Species Suggest That Tableland and Valley Grassy Biome in Southwestern China Pre-Date Human Modification

The Yunnan-Guizhou Plateau in southwest China lies at the intersection between East and South Asia, and is characterised by highly complex vegetation ranging from subtropical forest to open grassland. There is a long history of human modification to the landscape, but we know surprisingly little of the biogeography of open habitats such as grassy biomes in the region. To investigate the historical continuity of grassy biomes in southwest China, we examined the biogeographies and evolutionary histories of two widespread and dominant C4 grasses, Themeda triandra and Heteropogon contortus, in Yunnan and southern Sichuan provinces, using chloroplast DNA markers in combination with climate data. We discovered that cool- and warm-adapted lineages have been widely distributed across the Yunnan-Guizhou Plateau for at least the last 2 million years, pre-dating any possible anthropogenic impact. Moreover, the high genetic diversity and strong spatial structure of both species suggests the continuous presence of multiple large populations, rather than a recent expansion from the dry valleys in response to anthropogenic deforestation. This is the first evidence for the long-term existence of extensive populations of C4 grasses in this region outside the dry valleys, and calls for a reappraisal of the conservation value of these grass-dominated landscapes.