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Abstract
The two species from St Helena usually included in Trochetia differ sufficiently from the Mascarene species to constitute a distinct genus. Trochetiopsis is described and the combinations T. erythroxylon (Forst. f.) Marais and T. melanoxylon (R. Brown ex Ait. f.) Marais are made.
... Marais, and ebony T. melanoxylon (Ait.f.) Marais). It is closely related to the Mascarene genus Trochetia, in which it was formerly included (Bentham, 1862;Marais, 1981). Interestingly no closely related species are found in the intervening African continent. ...
... 410 m]; but altogether not more than seventeen or eighteen plants are now to be found in the island--viz., two at Arnos Vale; one at Oakbank; three or four at Bowers's in Sandy Bay; two at Sam. Alexander's; one at Southens; six or eight young trees at the Hermitage; one at Diana's Peak and one at High Peak'. DISAPPEARANCE FROM THE WILD (1936-1981 In 1936 In 1970 N. R. Kerr visited the island to assess the possibilities of conserving the remaining endemic plants. He reported one wild tree and 'several' in gardens. ...
Redwood Trochetiopsis erythroxylon was still a common tree when the English East India Company colonised St Helena in 1659. It was, however, the most important indigenous plant to the settlers as its bark is suitable for tanning leather, and it provided the best timber. Within 50 years it was very scarce but was saved from extinction by being planted in island gardens from the early 18th century onwards. By the end of the 19th century only two wild trees remained even though the pressure of exploitation had by then been removed by the introduction of new timber trees and the importation of timber. It still fails to regenerate in St Helena and only through an exact knowledge of its requirements can vigorous trees be re-established there.
... The generic name Trochetiopsis was coined by the Kew botanist Marais (1981) to encompass the two St Helenan species formerly united with the Mascarene genus Trochetia DC. It was not until 1995 that it was realized that there had been three species of Trochetiopsis, not just the two that survive (albeit precariously) today. ...
The rediscovery, history, flower maturation and cultivation requirements of the St Helenan endemic Trochetiopsis ebenus Cronk (Sterculiaceae), which is known on the island as ‘EBONY’, are discussed; a full description and illustrations are provided.
Molecular phylogenetic analyses of Malvaceae subfamily Dombeyoideae based on a 6‐marker dataset sampling all 21 currently accepted genera (including extinct Astyria) yielded improved resolution and evidence for multiple generic circumscription problems. The taxonomy of the subfamily is adjusted with the synonymization of 10 genera (eight in current use) and the description of one new one, for a revised Dombeyoideae comprised of 14 genera and supported by morphological, anatomical, and/or palynological evidence. Eriolaena is expanded from a narrow Indo‐Asian group to include the Malagasy endemic genera Helmiopsis and Helmiopsiella, and a recently described species from continental Africa. A conspectus is presented of the new broadly circumscribed Eriolaena with 27 species (28 taxa, including 3 new Malagasy species and 16 new combinations) and defined by a unique winged‐seed morphology. A new monotypic Malagasy genus (Hafotra), resolved as sister to Eriolaena, is described. A conspectus is presented of a broadly circumscribed Mascarene‐Malagasy Ruizia (+ Astyria and Trochetia, with 13 new combinations). Two other genera are expanded and recircumscribed: Melhania (+ Trochetiopsis and Paramelhania, with one new name and two new combinations) and Corchoropsis (+ Paradombeya, with two new combinations). Additionally, two genera are typified, and 21 names are lectotypified.
In this study, we used an integrative approach combining morphological and molecular data with ecology, in order to clarify the identity of a Dombeya newly recorded as occurring in Mayotte. Morphological and molecular evidence are in agreement and convincingly show that specimens of this newly recorded species can be confidently assigned to D. rosacea, previously known only from four herbarium sheets from Madagascar. The new populations of this species in the Comoros significantly expand the extant area of occupancy of D. rosacea. Ecological preferences are also discussed, as populations of D. rosacea were found in greatly contrasting sub-humid habitats. This newly reported geographical disjunction between the Comoros and Madagascar strengthens the evidence for a biogeographic connection between the floras of these two regions.
The discovery in historic herbaria of an overlooked extinct endemic from the island of St Helena is reported. The first descriptions of St Helena Ebony, Trochetiopsis melanoxylon (Sterculiaceae), and the specimens associated with them in the herbaria of Oxford University (OXF) and the Natural History Museum, London (BM), do not match living and later-collected material, and instead represent an extinct plant. A new name is therefore needed for living St Helena Ebony: Trochetiopsis ebenus Cronk sp. nov. The hybrid between this species and the related T. erythroxylon is also described here: Trochetiopsis × benjamini Cronk hybr. nov. (Sterculiaceae), and chromosome counts of 2n = 40 are reported for the hybrid and both parents for the first time. The re-assessment of the extinct ebony emphasizes the importance of historic herbarium collections for the study of species extinction.
S ummary
When the endemic plant species of St Helena are ordered according to their increasing taxonomic isolation, their corresponding ‘sister groups’ show a ‘relictual series’ of increasing geographical disjunction. This is best explained by progressive extinction and evolution in continental areas rather than by particularly high rates of evolutionary change on the island (i.e. the taxonomically isolated endemics are ancient relicts). Few of the supposed trends of oceanic island evolution are convincingly evident in the flora of St Helena. St Helena was formed in the Miocene, and it is likely that the relict genera colonized St Helena from Southern Africa before the wet forest flora in this area largely disappeared as the climate deteriorated from the late Miocene onwards.
Re-examination of a fossiliferous sediment from St Helena has revealed the presence in the late Miocene of pollen grains very similar to modern Trochetiopsis pollen. Trochetiopsis is found only in St Helena and is assumed to be a palaeoendemic genus. The late Miocene material provides striking confirmation of this view.
The two extant Trochetiopsis species are compared with other species in the Sterculiaceae: Dombeyeae, using multivariate methods. It is suggested that the two extant Trochetiopsis species diverged on St Helena comparatively recently, probably as a result of disruptive selection followed by ecological isolation caused by environmental change at the beginning of the Quaternary.
It is also suggested that the ancestor of the extant Trochetiopsis species arrived in St Helena by long distance dispersal from African or Madagascar Dombeyeae stock at least 9 million years ago. However, since this time the pollen morphology of the Trochetiopsis lineage has changed little. The taxonomic isolation of Trochetiopsis can thus be explained partly by evolution on St Helena, but mainly by evolution and extinction in the Dombeyeae elsewhere (reliction).
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