No 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.

Savannas first began to spread across Africa during the Miocene. A major hypothesis for explaining this vegetation change is the increase in C 4 grasses, promoting fire. We investigated whether mammals could also have contributed to savanna expansion by using spinescence as a marker of mammal herbivory. Looking at the present distribution of 1,852 tree species, we established that spinescence is mainly associated with two functional types of mammals: large browsers and medium-sized mixed feeders. Using a dated phylogeny for the same tree species, we found that spinescence evolved at least 55 times. The diversification of spiny plants occurred long after the evolution of Afrotherian proboscideans and hyracoids. However, it is remarkably congruent with diversification of bovids, the lineage including the antelope that predominantly browse these plants today. Our findings suggest that herbivore-adapted savannas evolved several million years before fire-maintained savannas and probably, in different environmental conditions. Spiny savannas with abundant mammal herbivores occur in drier climates and on nutrient-rich soils, whereas fire-maintained savannas occur in wetter climates on nutrient-poor soils.Africa | Bovidae | coevolution | mammalian herbivory | savanna T he origin and spread of savannas have been topics of intensive research, but many questions remain. The C 4 grasses that dominate savannas emerged in the late Oligocene (∼30 Ma), but savannas only began to emerge as one of the world's major biomes in the late Miocene more than 20 My later (1). What changed to roll back the forests, allowing the rapid spread of grasslands? Ehleringer et al. (2) first linked the rise of savannas to a drop in atmospheric CO 2 , which would favor C 4 grasses over their C 3 grass predecessors. Low CO 2 can also reduce woody cover by increasing the risk of recruitment failure in woody plants whether from drought, fire, or browsing (3). However, the timing of the onset of low CO 2 is much earlier than the spread of savannas; therefore, although low CO 2 may have contributed to savanna expansion, it cannot explain the long time lag between C 4 origins and savanna spread. Climate change is the usual explanation for changing vegetation over time. Increased aridity in the late Miocene has been shown to cause the retreat of forests in North America and Eurasia, allowing grasslands to spread in their place (4, 5). However, large areas of extant savannas occur in climates that are wet enough to support forests and other closed woody types (6-8). Fires are frequent in high-rainfall savannas and have been considered the major agents accounting for open ecosystems in climates that can support forests. Fossil charcoal, mostly from marine cores, shows a surge in fire activity from the late Miocene correlated with the spread of savannas (9, 10). Phylogenetic studies have shown the emergence of fire-adapted woody plants from the late Miocene through to the Pleistocene in both Brazil and Africa, consistent with fossil evidence for increasing f...

Previous phylogenetic studies have indicated that Acacia Miller s.l. is polyphyletic and in need of reclassification. A proposal to conserve the name Acacia for the larger Australian contingent of the genus (formerly subgenus Phyllodineae) resulted in the retypification of the genus with the Australian A. penninervis. However, Acacia s.l. comprises at least four additional distinct clades or genera, some still requiring formal taxonomic transfer of species. These include Vachellia (formerly subgenus Acacia), Senegalia (formerly subgenus Aculeiferum), Acaciella (formerly subgenus Aculeiferum section Filicinae) and Mariosousa (formerly the A. coulteri group). In light of this fragmentation of Acacia s.l., there is a need to assess relationships of the non‐Australian taxa. A molecular phylogenetic study of Acacia s.l and close relatives occurring in Africa was conducted using sequence data from matK/trnK, trnL‐trnF and psbA‐trnH with the aim of determining the placement of the African species in the new generic system. The results reinforce the inevitability of recognizing segregate genera for Acacia s.l. and new combinations for the African species in Senegalia and Vachellia are formalized. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 172, 500–523.