Gwilym Lewis, Research Leader in Kew’s Comparative Plant and Fungal Biology department, explains how and why some plants are reclassified, and reveals the complications of renaming such an iconic species.
Taxonomists have long wondered about the correct classification of Brazil’s national tree. A new DNA study which analyses the evolutionary relationships of some 200 related species (known as the Caesalpinia group) confirms that Brazil`s iconic tree represents an isolated evolutionary lineage, and merits recognition as a new genus.
Resolving the taxonomy of morphologically complex genera containing many species takes time. For the last twenty years an international group of legume researchers, including myself, has been studying the taxonomy of the pantropical genus Caesalpinia and its allied genera in the pea and beans family (Leguminosae). This research has involved extensive field, herbarium, laboratory and analytical work.
Since Caesalpinia was first described by Linnaeus in 1753 many species have been added to the genus. Conversely, many separate genera have been described to accommodate groups of species considered to be distinct. Historically, however, there has been a lack of agreement on which of those separate genera should be retained as ‘good genera’ and which, if any, should be added back into a broader concept of Caesalpinia.
By the beginning of this century the genus Caesalpinia was considered to include as many as 25 published genus names in synonymy, even though some of these were readily recognisable by a number of morphological characters, geographical distributions and habitat preferences (Lewis, 1998; Lewis 2005). Once species and genera are ‘lumped’ together it can be difficult to know when to stop, just as the opposite process called ‘splitting’ can be equally problematic when deciding where to draw the boundaries amongst genera and species.
Advances in molecular analysis, especially during the past 20 years, have given systematists the data needed to test previous hypotheses of relationships and groups based largely on morphological and anatomical characters.
As DNA sequences were generated for a growing number of species, inter- and intra-generic relationships could be explored in more depth using more robust data sets. The main aim driving the research on Caesalpinia was to establish solid foundations for deciding how many good genera to recognise, and to resolve which species belonged to which genus, providing a reliable classification for users of taxonomy (such as ecologists, foresters, horticulturalists, and conservationists).
A collaboration between legume researchers in Brazil, Canada, Mexico, Switzerland, the US and Kew has resulted in a series of publications refining the classification of Caesalpinia and related genera (e.g., Lewis & Schrire, 1995; Lewis, 1998; Simpson et al., 2003, 2004; Gagnon et al., 2013). The most recent of these was published online in the open access journal PhytoKeys in October 2016 (Gagnon et al., 2016). In this publication we present a new classification for all the taxa considered to belong to the pantropical Caesalpinia group, which now comprises 26 genera and 205 species. The molecular analyses which underpin our 2016 publication clearly revealed three species which do not belong to any of the main groups highlighted in the study. These have been recognised as the new monospecific genera: Hererolandia, Hultholia and Paubrasilia. We also redefined the genus Caesalpinia itself to include only nine species, and the PhytoKeys paper includes many other nomenclatural changes and novelties, including new names for 75 taxa (species, subspecies and varieties) which were given new generic placements.
One species in particular, has attracted much interest because it is the national tree of Brazil. First scientifically described as Caesalpinia echinata by Lamark in 1785, pau-brasil, as it is commonly known (Lewis, 1998; Bueno et al., 2002), is the tree which gave its name to the country Brasil (spelt Brazil in English). In our molecular analyses, however, this iconic tree does not group with any other species analysed. In fact, in all molecular analyses it sits alone on a long branch of the phylogenetic tree, which is an indication of early evolutionary divergence and isolation from other members of the Caesalpinia group. The species is also morphologically unique: possessing a combination of features not seen in any other species of the 26 genera in the Caesalpinia group.
Pau-brasil, is a medium to large tree (5– over 15 metres in height) armed with prominent up-turned prickles which usually arise from woody protrusions on the trunk and main branches. It has bright yellow, bee-pollinated flowers with the uppermost (standard) petal with a central, blood-red blotch on its inner surface, and spiny, woody, finely pubescent, sub-lunate, 1–2-seeded pods (the species epithet echinata alludes to these spiny fruits). Pau-brasil is native just in eastern Brazil and is highly endangered in its native habitat, but it is widely cultivated as an ornamental street and park tree and sometimes in plantations.
To correctly classify pau-brasil based on the accumulated scientific evidence, we needed to change its name. However, changing the name of such a well-known and nationally loved species carries with it wider political and cultural implications and ramifications beyond the confines of plant taxonomy. After much deliberation, we decided that the best new genus name would be Paubrasilia, a Latinization of the common name, and thus a name that we hoped would be acceptable and favourably received by Brazilians.
Professor Luciano Paganucci de Queiroz, a Brazilian legume specialist, is a co-author of the PhytoKeys publication; and Doctor Haroldo Cavalcante de Lima, who has studied pau-brasil for many years, is a co-author of the new genus name. Both Brazilian researchers sanctioned the choice of name and are promoting the name change in the Brazilian media.
Red sap exudes from cut or damaged bark and is extracted from Paubrasilia wood. In the past the sap was used for dying luxury textiles. In addition, the dense heartwood of Paubrasilia is much prized for manufacturing high quality violin bows.
Cutting trees for these two economic uses over several centuries has led to decimation of the species across most of its native geographical range. Furthermore, less than eight percent of Brazil’s Atlantic forest, in which the richest populations of pau-brasil once grew, remains intact today. Despite Paubrasilia echinata being listed on CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora), prohibiting trading of the wood, the authors have witnessed, first-hand, continued illegal logging of pau-brasil trees during recent fieldwork.
We hope that the emblematic new genus name for Brazil`s national tree will draw added attention to its endangered status and highlight the fragile state of the fragmented forests of coastal Brazil.
- Gwilym -
Bueno, E., Roquero, A., Fernandes, F.L., Lewis, G.P., Lima, H.C. de, Montaigne, J.-M., Guedes, M.J. & Manzano, N. (2002). Pau-brasil. São Paulo, Axis Mundi.
Gagnon, E., Lewis, G.P., Sotuyo, J.S., Hughes, C.E. & Bruneau, A. (2013). A molecular phylogeny of Caesalpinia sensu lato: Increased sampling reveals new insights and more genera than expected. South African Journal of Botany 89: 111–127. Available online.
Gagnon, E., Bruneau, A., Hughes, C.E., Queiroz, L.P.de & Lewis, G.P. (2016). A new generic system for the pantropical Caesalpinia group (Leguminosae). PhytoKeys 71: 1–160. Available online.
Lewis, G.P. (1998). Caesalpinia: A revision of the Poincianella-Erythrostemon Group. Royal Botanic Gardens, Kew.
Lewis, G.P. (2005). Tribe Caesalpinieae. In: Legumes of the World, eds. G. Lewis, B. Schrire, B. Mackinder & M. Lock, pp. 127–161. Royal Botanic Gardens, Kew.
Lewis, G.P. & Schrire, B. D. (1995). A reappraisal of the Caesalpinia group (Caesalpinioideae: Caesalpinieae) using phylogenetic analysis. In: Advances in Legume Systematics, part 7: Phylogeny, eds. M.D. Crisp & J.J. Doyle, pp. 41–52. Royal Botanic Gardens, Kew.
Simpson, B.B., Larkin, L. & Weeks, A. (2003). Progress towards resolving the relationships of the Caesalpinia group (Caesalpinieae: Caesalpinioideae: Leguminosae). In: Advances in Legume Systematics, part 10: Higher Level Systematics, eds. B.B. Klitgaard & A. Bruneau, pp. 124–148. Royal Botanic Gardens, Kew.
Simpson, B.B., Tate, J.A. & Weeks, A. (2004). Phylogeny and character evolution of Hoffmannseggia (Caesalpinieae: Caesalpinioideae: Leguminosae). Systematic Botany 29: 933–946. Available online.