The [British South Africa] Company...recognising
the desirability of tree-planting, established a
nursery on the eastern bank of the Chirapagura
stream (at present-day Newlands) and, in due
course, the [Sanitary] Board was provided with
gum-trees which were planted on the edge of the
vlei and along the drain, but the cows, which
roamed at will, ate them.
GH TANSER - A SCANTLING OF TIME
The events recorded here seem to have occurred early in 1892, and give us the first known account of eucalypt planting in Harare - or anywhere else in Zimbabwe. Since then eucalypts have become so much a part of the Zimbabwean landscape that Mashonaland, in particular, would now seem strange without them. From practically every vantage point on the highveld eucalypts can be seen as plantations, farm woodlots, or solitary trees standing high above the indigenous vegetation. To the Shona people they have become mapuranga or magamutirii, and to the English-speaking people they are gums, blue gums (often said as one word), or bluegumtrees.
But back to Harare. Mid-1896
The [Sanitary] Board decided that tree planting
was an essential matter...The contractor who had
undertaken to plant 3,000 trees per month had
failed to do so, and now a full time employee of
the Board was appointed. A large plantation of
blue gums was made at the south-east corner of
the town, and a new tree planting scheme started
in the streets.
GH TANSER - A SCANTLING OF TIME
[Earl] Grey wrote to [Sir Alfred] Milner, "We are
to the Queen...
Grey's appeal to the residents to show their
loyalty by planting a tree in a hole which each
of them had dug fell on deaf ears; but a short
time before Jubilee Day, he, discarding his coat
and waistcoat and watched by a number of Natives
who were greatly interested in this strange
proceeding, set to work to dig a hole in the
Hospital grounds. On Jubilee Day no one shared
his labours: Grey planted his gum-tree alone.
GH TANSER - A SCANTLING OF TIME
As soon as the Commonage had passed into the hands
of the [Town] Council, a number of plots...bordered
by the Marimba...had been surveyed. There was keen
demand by would-be leaseholders who put up grass
shelters for their cattle, planted gum trees for
shade, and sank wells...
GH TANSER - A SEQUENCE OF TIME
And so the planting of eucalypts got under way in what was to become Harare. We don't know how many different species were planted in those early days - to the settlers they would all simply have been gum trees - but we do know that among the earliest eucalypts in the present-day city were Eucalyptus citriodora (lemon-scented gum), E. grandis (flooded gum),
E. longifolia (woolybutt), E. tereticornis (forest red gum),
E. camaldulensis (river red gum), and E. paniculata (grey ironbark). Other species might have been tried without success, such as E. viminalis (manna gum), which was planted in the village of Melsetter (now Chimanimani) in 1897, and still survives there. Over the years about 200 species of eucalypts have been planted experimentally, commercially, or ornamentally in Zimbabwe, but the purpose of these notes is to describe the species that occur within Harare, hopefully to enable interested members of the Tree Society to identify them and to get to know them. But if we want to know the eucalypts we commonly see, we really need to understand something of their background and taxonomy. This should provide an apprec- iation of the nature of the eucalypts and, perhaps, help to dispel some of the myths surrounding their behaviour as exotics.
The story of the eucalypts in modern times began in the year 1642 AD, when they were noted by Abel Janszoon Tasman (1603-1659), a Dutch navigator, during his 1642-43 voyage of dis- covery of Tasmania, New Zealand, and the Tonga and Fiji Islands. In 1688 an English freebooter, William Dampier, landed on the northwest coast of Australia, and noted the occurrence of trees that exuded a gummy substance, but he confused them with the dragon tree, Dracaena draco, which produced the resin of commerce known as "dragon's blood". Then, in 1770, Captain James Cook made his first voyage to the Pacific Ocean. Attached to Cook's staff were botanist Joseph Banks and his assistant Daniel Carl Solander, who collected the first eucalypt known to science, E. gummifera (red blood- wood), from the shores of Botany Bay on the east coast of Australia, where the city of Sydney now stands. Later, they collected E. platyphylla (white gum) in north Queensland, but neither species was named at the time. On Cook's third expedition in 1777 David Nelson collected E. obliqua (messmate stringybark) on Bruny Island, off southern Tasmania, and it was this species that became the first eucalypt to be named.
Those species were probably not the first eucalypts known to western civilization, for the Portuguese, who had established a settlement on the island of Timor in 1519, would have been familiar with E. alba (white gum) growing on the coast, and
E. urophylla (Timor mountain gum) in the mountains. From those early times to the present day the eucalypts have become widely known and planted in practically every country that has a subtropical or mediterranean-type climate, and even in some regions that experience severe winters, and they have been the subjects of intensive study and massive volumes of scientific literature. New species are still being discovered and described, and today the number of validly named species stands at about 725.
The generic name Eucalyptus was coined from two Greek words, eu (well) and kalyptos (covered), in allusion to the operc- ulum, or cap, covering the flower bud. When the flower bud is fully developed this operculum is shed to expose the stigma, and the stamens with their anthers.
NATURAL OCCURRENCE OF EUCALYPTUS
The eucalypts have been described as Australia's gift to man- kind, but, while it is true that the overwhelming majority of the 725 or so species are Australian endemics, perhaps 9-10 extend northwards into the neighbouring islands, and 5-6 specific and infraspecific taxa do not occur in Australia at all. Among the latter is E. deglupta (kamerere), which occurs from New Guinea northwards to Mindanao in the Philippines, the only eucalypt to cross the equator into the northern hemisphere.
Essentially, though, the eucalypts are Australian. They dominate the Australian continent to an extent that few other trees have done in any other country, and they give the Australian landscape its distinctive appearance. But there are parts of the continent where eucalypts are notably absent: they are excluded from the tropical and subtropical rain-forests of Queensland and New South Wales, and from the temperate rainforests of Victoria and Tasmania; they cannot survive the intense cold and the strong winds of the highest summits of southeastern Australia; and they do not occur commonly in the extensive arid zones that cover nearly 50% of the Australian mainland, although a few species may be locally abundant along watercourses in those regions.
Eucalypts do not extend westwards to the Asian side of the Wallace Line, nor eastwards to New Caledonia or New Zealand. Within Australia the most southerly occurrence of the eucalypts is at latitude 43 degrees South in Tasmania, and although most of the Tasmanian species also occur on the mainland, there are about a dozen that are confined to the island, probably by climatic factors resulting from the southerly latitudes and the fairly high altitudes.
Most eucalypts grow in localities where there is a marked shortage of soil moisture for much of the year, and they are well adapted to drought. At the more arid side of their environment the species that do occur there have adopted specialized life-forms, in particular the form known as mallee, in which the plant is no longer a single-stemmed tree, but one with numerous slender stems arising from a large underground rootstock that is well protected from fire. Scattered trees of larger size may be found in the mallee areas, and the mallee gives way to better-quality eucalypt woodland as rainfall improves. The woodlands contain a wide variety of species, and they, in turn, give way to open forests of taller trees, but fewer species, as conditions improve further still. Finally, the best soils in the wetter regions of southeastern ansd southwestern Australia carry magnificent closed forests consisting of a few, sometimes only one, eucalypt species, with tree heights reaching 60 metres or more.
MATURE SIZE OF EUCALYPTS
The eucalypts show great variation in mature size, from one-metre shrubs like E. cunninghamii(cliff mallee ash) to the giant E. regnans (mountain ash) towering more than 100 metres, and the tallest hardwood species in the world. The present-day champion mountain ash is a specimen in Tasmania with a height of 99 metres, but there is a record of a tree felled in Victoria in 1872 that measured 132 metres to the point where the top had broken off in the fall. That measurement was never properly authenticated and is very rarely quoted, but there is a reliable record, also from Victoria, of a tree felled in 1918 that measured 114 metres. Other measurements of mountain ash of 107, 104, 99, and 98 metres have been recorded. From Western Australia there has been a fairly reliable record for a specimen of E. diversicolor (karri) that measured 104 metres when felled in 1901, while the tallest known standing karri today has a height of 88 metres.
The tallest eucalypt in Zimbabwe is a specimen of E. grandis (flooded gum) planted in 1905 on the farm Inodzi, near Penhalonga in the Eastern Highlands. In 1984 this tree was measured with instruments and found to have a height of 73 metres. Another specimen of E. grandis, at Tilbury Estate in the Chimanimani District, was found to have a height of 74 metres in 1988, but that measurement has not been verified.
EVOLUTION OF EUCALYPTUS
Fossil evidence indicates that a eucalypt type made its appearance in Australia about 25 million years ago. That was long after the break-up of Gondwanaland (135 million years ago) and the final separation of the Australian landmass from Antarctica (55 million years ago), which would explain why the eucalypts are almost exclusively Australian. The northwards migration of the eucalypts into New Guinea, the Lesser Sunda Islands, and the Philippines became possible after the Austr-alian plate collided with the Asian plate about 15 million years ago.
At the break-up of Gondwanaland Australia was largely covered by cool-temperate rainforest, but as the continent drifted northwards and eastwards to its present position, there were changes in its climate, and massive erosion of its surface, which left Australia as one of the driest and flattest land surfaces in the world. The impact on the Australian vegetation was immense, and it would seem likely that the great profusion of Eucalyptusspecies evolved as a consequence of those climatic changes following rapidly on one another. The eucalypts evolved because Australia had become dry. And it is against this background of the remorseless deterioration of the Australian environment that we can begin to appreciate what outstanding evolutionary capacity the progenitors of the eucalypts must have had in order to keep pace with the environmental changes of their habitat.
There are many links between the flora of Australia, South America, and southern Africa that show the Gondwanaland connection, but eucalypts, or anything like them, are entirely absent from South America and southern Africa, although the botanical family to which they belong, Myrtaceae, is present in both; for example, our well-known waterberries, Syzygium spp, belong to that family.
Three powerful factors have been at work in shaping the eucalypts. Firstly, they evolved as a response to declining soil fertility associated with the erosion, weathering, and laterization of the Australian soils; secondly, they were subjected to a rapidly drying climate; and, finally, they had to cope with fire.
Up to this point we have been looking at Eucalyptus as a single genus, but there is evidence to show that the eucalypts may have evolved from more than one progenitor or prototype. Scientific opinion on the evolution and development of Eucalyptus has swung from the early single-genus approach to a concept of up to nine genetically independent subgenera (or even genera), then to a belief that only two genera were involved, which led to the revolutionary step by some botanists to excise, as a separate genus Corymbia, two of the nine subgenera that had earlier been recognized. Currently, a new phylogenetic approach to the eucalypts recognizes 13 sub- genera, including the previously independent genus Symphyomyrtus_,_Minutifructa_,_Alveolata_,__Cuboidea'>Angophora'>Angophora. With the passage of time and further study the complete picture will undoubtedly emerge. If it is finally concluded that Eucalyptus comprises more than one genus, the very close similarities between the species of the different genera will have to be acknowledged as the result of evolutionary converg-ence: this is seen particularly in the development of the operculum in all species of all groups, of bark shedding of some species of each group, and in the absence in some species of some groups of the woody swellings (lignotubers) that occur at the base of the tree, and provide a reserve of buds and food to assist survival under adverse conditions.
CLASSIFICATION OF EUCALYPTUS
Since the first discovery of the eucalypts there have been many attempts at formal (and informal) classifications that grouped together species having similar characteristics of bark, buds, fruit, oils, cotyledons, wood properties, and so on. The earliest attempts at formal classification made use of such obvious features as operculum shape or bark type, but it was soon found that those characteristics were not necess- arily reliable guides to the various groups, particularly when it was realized that the genus was far larger than the dozen or so species originally collected and described. Bark types are still relevant in distinguishing between groups that shed or retain dead bark, and, in the latter case, between iron- barks and other types of rough bark, but they are not of major importance.
The first major classification of the eucalypts was by George Bentham in 1867, and he based his system on the morphology of the anthers because he found that other eucalypt characters varied so widely as to make it impossible to base a satisfac- tory system on any one of them. Subsequent workers, notably Joseph Henry Maiden and William Farris Blakely, elaborated on Bentham's classification, and, in 1934, Blakely's "A Key to the Eucalypts" was published, a work that remained the stand- ard classification into the 1970s. Present-day taxonomists no longer attach as much weight, as did their predecessors, to the morphology of the eucalypt anther, because the anther system has become too complex to be workable. But present-day taxonomists do not, by any means, ignore the anther.
The next major classification of the eucalypts, published by LD Pryor and LAS Johnson in 1971 (and subsequently modified), was developed over a period of 20 years, and proved to be a milestone in Eucalyptus biology and the benchmark for all eucalypt scholars. In this classification (as subsequently modified) the authors separated the traditional genus Eucalyptus into eight subgenera, to which they added a ninth, Angophora, previously regarded as distinct from Eucalyptus. These subgenera were
Angophora, Blakella, Corymbia, Eudesmia, Gaubaea,
Idiogenes, Monocalyptus, Symphyomyrtus, and Telocalyptus,
and were based on the association of many morphological char- acters, backed up by the apparent breeding incompatibility between them. There was a view that these subgenera should be elevated to the rank of independent genera, but more conserv- ative opinion prevailed.
The Pryor and Johnson classification is a heirachy comprising subgenera, sections, series, subseries, superspecies, species, and subspecies, but there are no diagnostic data for any taxa above species. This classification, therefore, remains extra-codical, or informal, no matter how valuable it has proved to be.
In 1995 KD Hill and LAS Johnson formally erected a new genus, Corymbia, that included the subgenera Blakella and Corymbia of Pryor and Johnson, but this does not seem to have gained wide acceptance at the present time.
A new classification of the eucalypts has been submitted for publication by MIH (Ian) Brooker, who is probably the leading eucalypt taxonomist of the present day. This classification is based on that of Pryor and Johnson, but it recognizes 13 subgenera (7 polytypic and 6 monotypic), and the heirarchy comprises subgenera, sections, subsections, series, subseries, supraspecies, and species. Brooker has deliberately used the term supraspecies instead of superspecies because of the ambiguous connotations of the latter. He has also omitted all taxa at infraspecific level, a decision that is bound to be criticised. Brooker's classification is unquestionably an advance on anything that has gone before, and includes Latin diagnoses for all taxa above species. It covers 725 species, compared with 461 species and 91 subspecies in Pryor and Johnson, and 522 species and 150 varieties in Blakely. An earlier version of Brooker's classification covered 748 species and 234 subspecies, but it is clear that a number of his original species have been reduced to synonymy. Of the 725 species that occupy the base of Brooker's system there are many that are relatively easy to identify from their distinct and constant characters. Where species do not lend themselves to such easy recognition it is usually due to their more recent evolution, and their small divergence from the charac- ters of their congeners. Such species share many characters with their close relatives, and may still be exchanging genes with them, and they require a cluster of characters to determine their identity. They cannot be recognized by one feature alone. Brookers subgenera are (in taxonomic order)
Angophora, Corymbia, Blakella, Eudesmia, Acerosa,
Cuboidea, Idiogenes, Primitiva, and Eucalyptus,
and will be discussed below.
EUCALYPTUS SUBGENERA IN ZIMBABWE
Angophora Etymology: from the Greek angos (a goblet, vessel), and phorus (carrier), alluding to the shape of the fruit resembling a goblet. Angophora is distinguished from other subgenera in having opposite leaves, and flowers with persistent calyx teeth (sepals) and separate petals. The
subgenus comprises a heirarchy of 4 series, 1 subseries, and 14 species, of which two have definitely been introduced into
Zimbabwe in the past, but none is known in the country today.
Corymbia Etymology: from the Greek korymbos, a cluster of flowers, referring to the large, terminal, corymbose inflorescences that characterize this subgenus. This results in the flowers being conspicuous on the outside of the tree's crown, as is so noticeable in the handsome red-flowering gum, E. ficifolia. The flower buds of all species have double opercula, and in most species both opercula are held until flowering. Corymbia has a heirarchy of 2 sections, 2 subsections, 10 series, 3 subseries, 2 supraspecies, and 70 species. Most of the species have woody, urn-shaped fruits, and collectively they are known as the woody-fruited bloodwoods. Thirteen species have been introduced into Zimbabwe, of which nine are known to have been planted in Harare (see Appendix 1).
Blakella This subgenus was named after the notable Queensland botanist, Stanley T Blake, and comprises a heirarchy of 2 sections, 2 series, and 17 species, mainly from tropical habitats. All have thin-walled fruits, which give them their collective common name of paper-fruited bloodwoods. All species have flower buds with double opercula, but the outer operculum is shed early in bud development. Only three species have been
introduced into Zimbabwe, but none has been planted in Harare.
Eudesmia Etymology: from the Greek eu (well), and desmos (bundle), referring to the grouped stamens in some species. The sub- genus has a heirarchy of 4 sections, 4 series, 6 subseries, and 20 species, mainly from the northern, central, and western regions of Australia. In all species only one operculum (the corolla) is apparent, but some species have persistent sepals that show up as small calyx teeth. This group includes some of the most spectacularly beautiful flowering trees in the whole genus Eucalyptus.Eight species have been introduced into Zimbabwe; three of these were planted in Harare and its environs, but none has survived (see Appendix 1).
Acerosa Etymology: from the Latin acerosus (needle-shaped), alluding to the shape of the seeds. This subgenus was formerly included in Gaubaea by Pryor and Johnson. There is only one species,
but this has not been introduced into Zimbabwe.
Cruciformes This is also a monotypic subgenus, and the name is derived from the Latin cruciformis (cross-shaped), referring to the cross pattern on the operculum. The single species was
formerly included in Symphyomyrtus by Pryor and Johnson. It has not been introduced into Zimbabwe.
Symphyomyrtus Etymology: from the Greek symphyo (joined), and myrtos (myrtle), referring to the fruit clusters of E. lehmannii, which was originally given the generic name Symphyomyrtus, but subsequently placed in Eucalyptus. Symphyomyrtus is by far the largest subgenus in Eucalyptus, and it is from this group that the most successful exotic plantations have been developed outside Australia. The subgenus is botanically complex, and comprises 15 sections, 8 subsections, 75 series, 45 subseries, 10 supraspecies, and 488 species. All species have double opercula (calyx and corolla), and the outer (calyx) operculum is shed early in the bud development of most species, leaving an obvious scar which is retained until flowering. Some species retain both opercula up to the flowering stage. In all, 131 species and 5 subspecies of sub-genus Symphyomyrtus have been introduced into Zimbabwe, of which 56 taxa are known to have been planted in Harare and environs (see Appendix 1).
Minutifructa Etymology: from the Latin minutus (very small), and fructus (fruit), referring to the very small capsules found in all species. In the Pryor and Johnson classification the subgenus was named Telocalyptus. This is a small group comprising 2 sections and 4 tropical to equatorial species, one of which is non-Australian. All have a double operculum, and the outer one is shed early in bud development. Two species, have been introduced into Zimbabwe, but neither has been planted in Harare.