Journal Article

Atmosphere, ecology and evolution: what drove the Miocene expansion of C₄ grasslands

Osborne, Colin P.

2008

Overview

1. Grasses using the C₄ photosynthetic pathway dominate today's savanna ecosystems and account for ~20% of terrestrial carbon fixation. However, this dominant status was reached only recently, during a period of C₄ grassland expansion in the Late Miocene and Early Pliocene (4-8 Myr ago). Declining atmospheric CO₂ has long been considered the key driver of this event, but new geological evidence casts doubt on the idea, forcing a reconsideration of the environmental cues for C₄ plant success. 2. Here, I evaluate the current hypotheses and debate in this field, beginning with a discussion of the role of CO₂ in the evolutionary origins, rather than expansion, of C₄ grasses. Atmospheric CO₂ starvation is a plausible selection agent for the C₄ pathway, but a time gap of around 10 Myr remains between major decreases in CO₂ during the Oligocene, and the earliest current evidence of C₄ plants. 3. An emerging ecological perspective explains the Miocene expansion of C₄ grasslands via changes in climatic seasonality and the occurrence of fire. However, the climatic drivers of this event are debated and may vary among geographical regions. 4. Uncertainty in these areas could be reduced significantly by new directions in ecological research, especially the discovery that grass species richness along rainfall gradients shows contrasting patterns in different C₄ clades. By re-evaluating a published data set, I show that increasing seasonality of rainfall is linked to changes in the relative abundance of the major C₄ grass clades Paniceae and Andropogoneae. I propose that the explicit inclusion of these ecological patterns would significantly strengthen climate change hypotheses of Miocene C₄ grassland expansion. Critically, they allow a new series of testable predictions to be made about the fossil record. 5. Synthesis. This paper offers a novel framework for integrating modern ecological patterns into theories about the geological history of C₄ plants.

• Publisher: Oxford, UK : Blackwell Publishing Ltd,Blackwell Publishing,Blackwell Publishing Ltd,Blackwell Science
• Subject: Animal and plant ecology / Animal, plant and microbial ecology / atmospheric CO / atmospheric CO2 / Biological and medical sciences / C4 photosynthesis / C4 plants / carbon / carbon dioxide / Climate change / Climate models / Climatology. Bioclimatology. Climate change / C₄ plants / data collection / Earth, ocean, space / ecosystems / Essay Review / evolution / Exact sciences and technology / External geophysics / fire / fires / fossils / Fundamental and applied biological sciences. Psychology / General aspects / Geology / Grasses / grassland / Grassland fires / grasslands / grazing / Meteorology / Paleoclimatology / Plant ecology / Plants / Poaceae / prediction / Rain / rainfall / Review / savanna / savannas / seasonality / species diversity / uncertainty