INVITED: POSITIVE FEEDBACKS PROMOTING THE RAPID EXPANSION OF C4 GRASSLANDS

C₄ plants dominate today's tropical savannahs and grasslands. Their success is due to a physiological CO₂-concentrating pump, which raises photosynthetic efficiency in warm climates and low atmospheric CO₂ concentrations. Remarkably, C₄ plant dominance of tropical ecosystems was achieved abruptly and near-synchronously across four continents in the late Miocene. We critically review the leading explanation for this major ecological shift, which posits that declining atmospheric CO₂ promoted the dominance of C₄ plants by progressively limiting photosynthesis in their C₃ competitors. Recent atmospheric CO₂ determinations from fossil leaves and marine phytoplankton challenge this view, indicating stable CO₂ concentrations during the period of rising C₄ plant abundance. We propose an alternative mechanism based on a synthesis of geological evidence from South Asia. In this region, the decline of humid tropical forest and its replacement by C₄ grassland were likely initiated by seasonal drought that accompanied intensification of the Indian monsoon. A concurrent increase in charcoal abundance in marine sediments records a major shift in the fire regime and points to critical positive feedbacks between fire frequency, regional climate and grassland evolution. Frequent fires suppress the recruitment of trees and permit the encroachment of C₄ grasslands. Grasses in turn promote fires by providing fuel in the dry season, and complete a strong positive feedback loop. Further positive feedbacks involve the interaction of aerosols from smoke with the hydrological cycle. Aerosols inhibit cloud formation and precipitation, extending the dry season, and increasing tree mortality and the incidence of fire. In a final feedback, deep convective clouds form over wildfires and cause intense storms. High lightning activity raises the likelihood of igniting more fires. Together, these coupled vegetation-fire and fire-climate feedbacks would have rapidly accelerated the loss of forest, causing a rapid and irreversible transition to C₄ grasslands. While declining CO₂ was a necessary precondition for a strong vegetation-fire feedback, it was not the direct trigger for C₄ plant success.