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African Acacia tree against an orange sunset

African Acacia Thorn Trees Reclassified

Africa’s thorny trees and shrubs are no longer “Acacias.” The taxonomic status of these trees proposed in 2003 has been adopted. The genus Acacia is reserved for the Australian native species while Africa will now have to call its acacias by the name Senegalia with the African species subdivided into two genera, Vachellia and Senegalia.

According to Colin Dyer, Director at the Institute for Commercial Forestry Research (ICFR) in Pietermaritzburg, South Africa, the debate around the taxonomic status of the genus Acacia goes back a long way, with many taxonomists attempting to bring taxonomic order to this extremely diverse and species-rich group of mainly trees and shrubs.

With biochemical and molecular tools becoming available, taxonomists soon realised that this diverse group contained large subgroups of related species and that the groups had little in relation to each other (Haddad 2011).

“The genus Acacia in the broad sense has been synonymous with Africa since it was first described by Philip Miller in 1754. The genus Acacia contains a large number of species (approximately 1 500), making it the largest genus within the pea family (Fabaceae) and is widespread, occurring in Australia, Asia, Africa and the Americas,” says Dyer.

Illustrations of Acacia genera Vachellia and Senegalia

A reclassification of the genus Acacia was proposed by an Australian group of botanists in 2003, including typification of the genus. After a long, complex and emotional debate, the revision of Acacia was approved at the 17th International Botanical Congress, held in Melbourne in 2011. These changes have been accepted by the international botanical community and are already in use in the botanical literature (Boatwright et al. 2014).

What does this mean for Africa?

The first consequence is that the genus Acacia is reserved for those species native to Australia, about 900 of them. Many of these have become naturalised in other parts of the world, and familiar species names such as Acacia mearnsii remain valid. One of the arguments used by the botanists for reclassification was that 72% of the species would remain unchanged.

For the African species of Acacia in the broad sense, these have been grouped into two distinct genera, Vachellia and Senegalia, which are clearly separated based on a number of morphological, anatomical and biochemical attributes.

The main differences are that Vachellia has capitate inflorescences (round, head-like flowers) and spinescent stipules (thorns). Senegalia has spicate inflorescences (flowers in spikes) and the stipules are non-spinescent.

The well-known Acacia karroo, A. nilotica, A. xanthophloea and A. sieberiana now become Vachellia karroo, V. nilotica, V. xanthophloea and V. sieberiana, respectively. Similarly, Acacia nigrescens, A. caffra and A. senegal become Senegalia nigrescens, S. caffra and S. senegal, respectively. The revised classification for the African species is presented in Kyalangaliwa et al. (2013).

“Although the name Acacia has become entrenched amongst botanists, foresters and conservationists in Africa and the International Code of Nomenclature for algae, fungi, and plants are not prescriptive, it is imperative that we embrace the new classification and use the new names when we refer to these iconic trees and shrubs from our continent,” concludes Dyer.

References: Boatwright JS, van der Bank M, Maurin O. 2014. Name changes in African Acacia species. Veld and Flora 100: 33. Haddad WA. 2011. Classification and nomenclature of the genus Acacia (Leguminosae), with emphasis on Africa. Dendron 43: 34–43. Kyalangalilwa B, Boatwright JS, Daru BH, Maurin O, van der Bank M. 2013. Phylogenetic position and revised classification of Acacia s.l. (Fabaceae; Mimosoideae) in Africa, including new combinations in Vachellia and Senegalia. Botanical Journal of the Linnean Society 172: 500–523.

About the ICFR: The Institute for Commercial Forestry Research is an independent provider of project-based research solutions and other related services in support of the industrial wood plantation sector in southern Africa. To achieve desired research project goals the ICFR works closely with other research institutes and universities. For more information visit www.icfr.ukzn.ac.za

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