A brief history of Eucalyptus, Angophora and Corymbia
Although eucalypts must have been seen by the very early European explorers and collectors, no botanical collections of them are known to have been made until 1770 when Joseph Banks and Daniel Solander arrived at Botany Bay with James Cook. There they collected specimens of C. gummifera and later, near the Endeavour River in northern Queensland, they collected E. platyphylla; neither of these species was named as such at the time.
In 1777, on Cook’s third expedition, the botanist David Nelson collected a eucalypt on Bruny Island, southern Tasmania. This specimen was taken to the British Museum in London, where it was named Eucalyptus obliqua by the French botanist, Charles-Louis L’Héritier de Brutelle, who was working in London at the time. He coined the generic name from the Greek roots eu and calyptos, meaning ‘well’ and ‘covered’, in reference to the operculum of the flower bud. This organ protects the reproductive structures during their development and sheds under pressure from the emerging stamens at flowering. The name obliqua was derived from the Latin, obliquus, meaning ‘oblique’, describing a leaf base where the two sides of the leaf blade are of unequal length and do not meet the petiole at the same point.
In the publication of Eucalyptus obliqua, L’Héritier recognized in the generic name a feature common to all eucalypts – the operculum. In his choice of specific name, he recognized not only a characteristic feature of E. obliqua but one that occurs in most other eucalypts as well. E. obliqua was published in 1788 and coincides with the date of the first official settlement of Australia.
Between 1788 and the beginning of the nineteenth century several more species of Eucalyptus were named and published. Most of these were by the English botanist James Edward Smith and most were, as might be expected, trees of the Sydney region. They include the economically valuable E. pilularis, E. saligna and E. tereticornis, each of which also occurs in Queensland, with the distribution of E. tereticornis extending to the island of New Guinea.
Also in this period the genus Angophora was published, in 1797, by the Spanish botanist Antonio Jose Cavanilles, based on specimens collected at Port Jackson by Frenchman Luis Née in 1793. Née was botanist with the Alejandro Malaspina expedition. Various authors have considered Angophora to be sufficiently distinctive that it should be maintained as a separate genus. Others believe it is a ‘eucalypt’. We recognize both Eucalyptus and Angophora in EUCLID, reflecting results of recent research and usage by the general community.
The nineteenth century was a period of extensive land exploration. This resulted in the discovery of many new eucalypts and their subsequent naming by several of the great botanists in Australian history, particularly Ferdinand von Mueller, whose work on eucalypts contributed greatly to the first comprehensive account of the genus in George Bentham’s Flora Australiensis (1867). Bentham never visited Australia, but his account is the most important early systematic treatment of the genus Eucalyptus.
Some earlier authors had constructed classifications, but the distinctions they used – for example, shape of the operculum and the juvenile leaf arrangement – were only applicable to far fewer species than were known to Bentham; they were of little use when applied to a much larger number of species. One useful study before that of Bentham, however, was Mueller’s description of different bark types (Mueller, 1858). These still have relevance in distinguishing between, for example, groups that shed or retain dead bark and, in the latter case, between ironbark and other types of rough bark.
Bentham divided the genus into five series whose distinctions were based on characteristics of the stamens, particularly the anthers. Categories within each series were based largely on the leaves, and on bud and fruit shape. He was obviously working with limited botanical specimens, and field characters were not available to him unless communicated by others from Australia.
Mueller, working in Australia, devised another classification based on the anthers (Mueller, 1879-84), while Joseph Henry Maiden (1924) elaborated on the anther system, which was taken even further by William Faris Blakely (1934). By this time, classification based on the anther system had become too complex to be workable.
Other more consistent characters have been sought in recent years to aid in the construction of classifications. Of these, leaf venation, the nature of bristle glands, the morphology of the seeds, nature of the operculum and the structure of the inflorescence are fundamental. More sophisticated equipment has usually enabled the examination of these leaf and floral structures early in and during their development. Similarities thus recognised usually provide the evidence of natural affinity between species and groups of species. In other words, botanists became better equipped to decide whether these similarities noticed in different species and groups were the results of inheritance from a common ancestor or if they had independently evolved, in many cases as an adaptive necessity such as lignotuber formation or salt tolerance.
A comprehensive but informal classification of all known eucalypt species was published in 1971 by the late L.D. Pryor and L.A.S. Johnson. It comprised seven major groups based on the association of many morphological characters and suggested by the breeding incompatibility between them. Their system has been subjected to close scrutiny in the past 30 years. Many improvements to this classification were proposed by Johnson himself and by others, although he never formally published a system of classification.
Briggs and Johnson (1979) contributed a major advance in the botany of the whole family Myrtaceae, in which they outlined for the first time a comprehensive analysis of inflorescence structure in all genera and its indication of evolutionary trend.
In Volume 19 of the ‘Flora of Australia’, all eucalypts published to 1988, were comprehensively treated (Chippendale, 1988). This work includes 513 species of Eucalyptus arranged in 92 series, many of which were published formally in this volume. This is not a structured classification as there are no subgenera or sections. The work is of particular value for its typology and erection of many new taxonomic series.
The decade after 1988 saw the application of advanced methodology in the study of the genus Eucalyptus, especially in phylogenetic analyses of taxonomic series (e.g. Ladiges et al., 1987; Hill and Johnson, 1995) and in the use of molecular techniques in the estimation of infra-generic relationships within the genus and between cognate genera (Ladiges et al., 1995; Ladiges and Udovicic, 2000).
Most notably in 1995 K.D. Hill & L.A.S. Johnson published a monograph on the bloodwoods and ghost gums in which they described the genus Corymbia, with species grouped in seven sections which intentionally follow from the earlier work of Pryor & Johnson (1971). This grouping at section, and at series and subseries rank is, according to the authors, intentionally informal, i.e. outside the International Code of Botanical Nomenclature (Hill & Johnson, 1995, p. 186)
In 2000, M.I.H. Brooker published a formal classification of the genus, which is a synthesis in the form of an updated taxonomy to accommodate the numerous taxa published since Chippendale’s 1988 treatment. While based conceptually on the work of Pryor & Johnson, it recognizes one genus, Eucalyptus and includes Angophora and Corymbia as 2 of a total of 13 subgenera, and assigns all species known to the year 2000 to a heirarchical system of subgenera, sections, subsections, series, subseries and supraspecies (Brooker 2000).
The 5 years following Brooker’s 2000 classification has seen further systematic and phylogenetic investigations of the eucalypts. Increasingly, molecular data are incorporated into studies of the relationships between the subgenera recognised by Brooker, and this is reflected in ongoing refinements to eucalypt systematics. The various concepts proposed from those studies are discussed in the accompanying section “Evolutionary Relationships in Eucalyptus sens. lat.”.
Whilst hybridism has been frequently reported between species in Corymbia, and also between species from within the same subgenus of Eucalyptus, rarely are hybrids between species from different subgenera of Eucalyptus seen in the field. An example of the latter is hybrids between E. cloeziana (subgenus Idiogenes) and E. acmenoides (subgenus Eucalyptus) (see Stokoe et al., 2001). Manipulated hybrids between species from different subgenera have never been successful.
Hybridism requires genetic compatibility and synchronous flowering times (unless manipulated). Hybrids between species within a subgenus are rare in natural forests or scrubs. They are more likely to be seen as regrowth trees or mallees in disturbed areas where changed environmental conditions may be more amenable to the success of the hybrid combination. Hybrids in stands appear to be selected against in favour of the parent species.
Many reported hybrids are better interpreted as natural intergrades between closely related species. e.g. E. burgessiana and E. stricta in eastern New South Wales, E. dalrympleana subsp. dalrympleana and E. viminalis subsp. viminalis in Tasmania, E. angulosa and E. incrassata in coastal South Australia and Western Australia, E. brownii and E. populnea in Queensland.
Hybrids can complicate the identification of specimens and there are a couple of ways of investigating more closely whether the problem specimen is a hybrid. Firstly, close observation of the features of eucalypts in the stand where you collect the specimen may indicate that some trees appear intermediate in some features such as bark type or crown colour. Secondly if there is seed on your specimen you may be able to raise a moderately large number of seedlings (say 10-20) and look for gross variation in seedling leaf shape and other characteristics, and compare these with similarly grown seedlings raised from individuals of the purported parents. This is of course a lengthy business but can give good information.
There are a few well-known formally named hybrids where both parents have been identified and are seen in the field, e.g.
E. brachyphylla (E. kruseana × E. loxophleba subsp. lissophloia)
E. erythrandra (E. incrassata or E. angulosa × E. tetraptera)
E. missilis (E. cornuta × E. angulosa)
and possibly E. balanites (E. lanepoolei × E. decipiens).
In another example, E. annuliformis, only one parent is evident, E. drummondii, the other being a mystery.
Many presumed hybrids have been formally named by botanists of the eucalypts over the last 200 years. The most recent annotated list of many of these names can be found in Chippendale (1988, pp. 428–442). Hill & Johnson (1995) list, in an appendix, hybrids and intergrades they have observed in Corymbia.