Savanna fire and the origins of the ‘underground forests’ of Africa

What is it about?

Geoxyles are one of the most distinctive plant forms in fire-prone savannas. These small shrubs produce large woody underground structures that allow rapid resprouting after a fire. The authors investigated the evolution of these plants to try to answer a fundamental question: how did grassland savannas replace ancestral forests? RESULTS The authors sampled the DNA of 1,400 woody plant species from southern Africa, including 53 geoxyles. They found that the geoxyle form had evolved independently in many plant lineages, often around 2 million years ago. This correlated well with the timing of the appearance of similar geoxyle lifeforms in the South American cerrado savannas. All the geoxyles in the study grew in fire-maintained savanna ecosystems characterised by a higher average rainfall and greater fire frequency. They were ecologically similar to their tree relatives except for their underground growth, which is believed to help them survive and regenerate after a fire. WHY IT MATTERS The authors suggest that geoxyles may be considered markers of fire-maintained African savannas occurring in climates suitable for forests, and as such, can provide insights into the origin of savannas and their replacement of ancestral forests. They hypothesise that fires pushed back the boundary of the ancestral forest and enabled the formation of a stable savanna ecosystem. The oldest known geoxyles species were found near the equator approximately 5 million years ago, suggesting a gradual expansion of savannas from the tropics to higher latitudes, remarkably concurrent to that of the South American cerrado savannas.

Featured Image