Climate controls on evolution of grassland ecosystems since late Cenozoic: A phytolith perspective

The rise of grasses using C4 photosynthesis and dominating grasslands during the latest Cenozoic is one of the most dramatic events in Earth's history. A growing body of geological records adds more details to this process, such as the study of fossil phytoliths. Phytolith data make it possible to track major grass lineages over time, and distinguish which kind of C4 grasses rise to dominate in a particular region. If the environmental niches of this kind of C4 grass are well studied, the corresponding environmental controls and driving mechanisms could be identified. Here, we combine geospatial data and climate data from two public archives to produce a database of-12 million collections spanning-6000 species of grasses with corresponding climate data to explore the quantitative relationship between the major grass lineages (subfamilies) and climate at global and regional scales. Our results show that the 10 C isotherm of mean annual temperature is the transitional boundary of Pooideae and Panicoideae/Chloridoideae distribution in North America and East Asia; the 400 mm isohyet of summer precipitation is the transitional boundary of Panicoideae and Chloridoideae distribution in North America and South Africa. However, the detailed environmental niches of certain grass lineages vary from region to region and are controlled by regional climate, topography and local species. Based on these data, the reanalysis of phytolith data since the Neogene reveals a distinct evolutionary history of grassland ecosystems. The onset of modern like grassland happened in the early Miocene in both North America and East Asia, those original grasslands were most likely dominated by C3 Pooideae with minor Panicoideae (C4) and few Chloridoideae (C4); C4 expansion on both continents was based on substantial heterogeneity in vegetation, with similar aridity in climate as a possible driver, but with different timings and results: the rise of Chloridoideae in North America at 8-5 Ma and the rise of Panicoideae in East Asia at-11 Ma. We suggest that synchronous changes in Asian monsoon precipitation and temperature are responsible for the rise of Panicoidaeae in East Asia, whereas the combination of warm and semiarid climates is responsible for the rise of Chloridoideae in North America. We emphasize the importance of regional environmental control on the evolution and distribution of the major grass lineages and offer a database that could be used to reconstruct palaeoclimate by phytoliths in different regions.

Automated summary

This study establishes a database using geospatial and climate data to explore the quantitative relationship between major grass lineages and climate at global and regional scales. The results show that environmental factors play a significant role in the distribution of grass lineages, and there are variations in environmental niches among different regions.