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

Abstract

SummaryThe 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' (=geoxy-les) of southern Africa, one of the most distinctive growth forms characteristic of fire-prone savannas.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.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 million yr. When contrasted with their tree relatives, geoxyles occur in regions characterized by higher rainfall and greater fire frequency.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.

Tropical grassy biomes: linking ecology, human use and conservation

“…While true in some locations [40], in the majority of areas this perspective is misplaced [34]. A wealth of new information including dated phylogenetic analyses demonstrates the antiquity of both tree and grass species (and lineages) specialized to these biomes [25,27,28]. The presence of endemic plant lineages and species, as well as species with unique life histories, functional traits and architectures, including forbs with large underground storage organs, are strong indicators of the antiquity of TGBs [35,41].…”

The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes

“…Increased fire activity within the last approximately 10 Myr, is a worldwide phenomenon with evidence for a surge in charcoal fluxes in the North Pacific, the North and South Atlantic off Africa, and terrestrial records in Australia [36]. In Africa, dated molecular phylogenies indicate that growth forms characteristic of fire-maintained savannas developed at least 6-8 Ma in the tropics spreading to South Africa by 1-2 Ma [37]. Both fossil and phylogenetic evidences of the radiation of the bovids, the lineage of antelopes characteristic of contemporary African savannas, show a rapid proliferation of grazing mammals from the Pliocene [38][39][40].…”

“…Underground trees are thought to be slow growing so that their presence indicates an ancient grassland system. Their life history and ecology is poorly studied in Africa but they do provide tracers of the ancient origins of fire-maintained African savannas [37].…”

“…'Underground' trees (geoxylic suffrutices) are common in frequently burnt savannas, growing in environments with slow woody growth rates [37,84]. Underground trees are taxonomically diverse but, by definition, have tall-tree relatives in forests or savannas [84].…”

Reforestation or conservation? The attributes of old growth grasslands in South Africa

“…They support a rich fauna and flora endemic to the grassland habitat. Growth forms adapted to savannah fire regimes are also millions of years old [18][19][20].…”