Abstract:
1. The origin of fire-adapted lineages is a long-standing question in ecology.
Although phylogeny can provide a significant contribution to the ongoing debate,
its use has been precluded by the lack of comprehensive DNA data. Here we focus
on the ‘underground trees’ (= geoxyles) of southern Africa, one of the most
distinctive growth forms characteristic of fire-prone savannas.
2. We placed geoxyles within the most comprehensive dated phylogeny for the
regional flora comprising over 1400 woody species. Using this phylogeny, we
tested whether African geoxyles evolved concomitantly with those of the South
American cerrado and used their phylogenetic position to date the appearance of
humid savannas.
3. We found multiple independent origins of the geoxyle life-form mostly from the
Pliocene, a period consistent with the origin of cerrado, with the majority of
divergences occurring within the last 2 Ma. When contrasted with their tree
relatives, geoxyles occur in regions characterized by higher rainfall and greater fire
frequency.
4. Our results indicate that the geoxylic growth form may have evolved in response to
the interactive effects of frequent fires and high precipitation. As such, geoxyles
may be regarded as markers of fire-maintained savannas occurring in climates
suitable for forests.
