home | glossary | about eucalypts | about EUCLID | browse species

About eucalypts


 
About eucalypts See also


Introduction

Eucalypts are almost a defining feature of Australia. They are the dominant tree of the developed areas of the country, although only sparsely represented in the driest regions. There are over 800 species which have adapted to nearly every environment. In EUCLID we include (because of its close affinities to Eucalyptus) the long-standing genus Angophora, which is exclusive to eastern Australia excluding Tasmania.

Eucalypts must have been known from the early 16th century when the Portuguese colonised Timor. There are at least two indigenous species, E. alba and E. urophylla on the island. Following the Portuguese occupation, it is probable that eucalypts were established from seed in Brazil which was colonised about the same time, although records are too hazy to confirm this. Eucalyptus came into recorded history in 1788 when the French botanist, L'Héritier de Brutelle, published Eucalyptus obliqua, the well known Messmate of widespread distribution in the wetter regions of the south-east of the continent and Tasmania and Kangaroo Island. This species was named from a specimen collected on Bruny Island south of the island of Tasmania by one of the botanists on Captain James Cook's third voyage in 1777.

TOP


Evolution and distribution

Eucalyptus species are likely to have evolved from rainforest precursors in response to great changes in the landscape, soils and climate of the continent. No point of origin is possible to determine but it is assumed to have been on the Australian landmass from which several species have migrated probably by land bridges to islands north of the continent.

One species, E. deglupta, is distributed as far as the island of Mindanao, in the southern Philippines which places one eucalypt naturally in the northern hemisphere. However, the genus is now cultivated world-wide in tropical and temperate countries and in some places has become naturalised.

Eucalypts are now of great importance commercially in other countries, particularly South Africa, China, India and Brazil and to a lesser extent in central and northern Africa and in Mediterranean countries. They have many advantages apart from the timber and fibre which are the basis of huge paper industries. Eucalypts are also notable for their oils, use in lowering water tables, horticulture, shade and simple ornamentation, largely for the colourful flowers in many species.

 

TOP


Identifying eucalypts

Innumerable books have been published on eucalypts. Some of these are packed with all aspects of information, others concentrate on the more spectacular flowering species while others specialise in identification. Identification has always been regarded as difficult but this has been largely due to the lack of instruction on the botanical characteristics. Understanding the eucalypt plant is a vital element in attempting the identification process.

It is a fact that, to the uninitiated, most Eucalyptus species tend to look the same. People may think of well known wattle species and other plant groups and make interesting comparisons with eucalypts. Consider the Cootamundra Wattle (Acacia baileyana) and Golden Wattle (Acacia pycnantha) of eastern Australia, which are so clearly different with their greatly contrasting types of foliage. Only a few eucalypts, on the other hand, have very odd leaves and these are mainly the more bizarre but beautiful ornamentals from Western Australia.

In eucalypts there is a striking array of juvenile or seedling leaf types from opposite and completely connate pairs of leaves (e.g. E. uncinata), to crowded and spirally arranged short linear leaves (e.g. E. brockwayi), to disjunct petiolate ovate leaves (many species). What can be equally confusing is the change from seedling to juvenile to sapling to adult leaves that takes place in the majority of species. When identifying a eucalypt, this must be remembered. Attention is drawn to this in the descriptions that accompany every species in EUCLID.

  Some species never, or seldom, develop true adult leaves in the mature crown but instead retain their juvenile leaf phase where the leaves are commonly glaucous and rounded. This condition is rare in eastern Australian species but is notable in E. risdonii an endemic to Tasmania and in E. cinerea of New South Wales and Victoria. In Western Australia many more species have the glaucous crown, probably the most spectacular being the glaucous-leaved E. macrocarpa which produces large red flowers.

The species that produce the colourful flowers, however, are not restricted to those with juvenile crowns, particular examples being the brilliantly flowered E. ficifolia, E. erythrocorys and E. caesia which all flower with apparently adult leaves.

In contrast, it is clear to the average observer that in south-eastern Australia, nearly all Eucalyptus species have green leaves of roughly similar size and fairly inconspicuous white flowers. Only two species in south-eastern Australia, E. sideroxylon and E. leucoxylon, can have strongly coloured flowers. A few tropical species have brilliantly coloured flowers, e.g. E. miniata, E. phoenicea, E. ptychocarpa and E. cadophora subsp. pliantha. So the problems of identification in EUCLID for south-eastern Australian species usually fall back on the less conspicuous and accessible but highly diagnostic characters, often ones that may be less relevant in other plant groups. In Western Australia, however, if the tree or mallee is in flower, identification is made easier.

The first assessment of a eucalypt will obviously be made on approach to a tree or forest. An important decision that has to be made is to recognise whether the trees are cultivated, or in a natural stand. If cultivated, they could be from anywhere in Australia and the identification cannot take into account the regional break-up in EUCLID.

The observer also needs to take into account other aspects of the specimen, viz. the height of the plant, the number of stems or trunks, the colour of the crown, the overall appearance of the crown to determine if it is composed of juvenile or adult leaves, general size of the leaves (very small, e.g. E. parvula or E. kruseana, or very large, e.g. E. globulus) and the type of bark, basically, whether rough or smooth. It is worth mentioning here, that if a photographic record is wanted, trees should always be photographed in early morning or late afternoon, although in winter, photography is usually successful at most times of the day if the sun is out. This timing is to ensure that side light is used such that the trunk or stems are in sunlight. Crown colours can be distorted or misinterpreted by top light near the middle of the day.

TOP


Inspection of specimens

The "internal" features of the eucalypt plant are more reliable than the "external" features. They are relatively protected from the elements and from various forms of predation. They are the parts that require handling and close inspection or even dissection, as opposed to mere observation. Specimens for study may be obtained in several ways from a living tree. Sampling mallees is usually easy because the leaves and flowering structures are often at about head height and no sophisticated methods of collection are needed. For most trees, however, a weighted length of rope can be thrown over a low branch which can then be broken off with a sharp tug and pulled to the ground for close inspection of the parts. For tall trees it is a curious fact that the flowers and fruits are small and scarcely visible to the unaided eye, e.g. E. regnans. Then the canopy needs to be inspected with binoculars and a useful branch selected. If it is above rope-throwing height, the branch may be reached with the use of a shanghai by shooting a lead weight attached to a fine, light line over the branch and then attaching a thicker, stronger rope to one end of this line and then pulling this line up over the branch. For large branches which are often dropped for commercial seed collecting, high-powered rifles are used. Often the smallest trees or mallees have the largest buds and fruits, e.g. E. pyriformis. These plants are the easiest to sample, examine and assess.

The whole process of identification begins in the field with broad external assessment and ends with microscopic examination. The characters in this sequence of investigations have reliabilities that vary from very low to high and finally absolute. Only experience allows the user to weigh up these relative values and apply them with confidence.

In summary it might be said that the number of opercula on the developing flower bud is of absolute reliability, staminal inflexion and ovule row numbers of high reliability, bud numbers, flower colour and bark type of medium reliability, leaf colour of low reliability, bark colour of very low reliability. External features are very susceptible to seasonal and intra-population variability.

When choosing a specimen for identification there are some things to be avoided. For example always choose "typical" leaves on the specimen for assessment, avoiding the largest and the smallest. Similarly avoid using fruit that are lying on the ground, especially if in a mixed eucalypt species stand, for they may not belong to the tree under which they are lying. When looking around for juvenile leaves to assess then make sure they belong to the tree or mallee you are investigating – if there is any doubt do not use them. A mixed species stand may produce a variety of juvenile leaves. Time spent looking at both adult and juvenile growth in a stand will be very rewarding.

If an identification is proving difficult then growing of seedlings may be a help in resolving it. Obviously this slows down the process but valuable information can be obtained from observing seedling growth – firstly the shape of the cotyledons and secondly whether the leaves become disjunct early in growth or persist as opposite for many pairs. The shape of seedling leaves, whether they are stalked or stalkless and other leaf features can help also.

TOP