Gum trees (eucalypts) are the essence of the Australian flora. Their range extends from sub-alpine areas to wet coastal forests, temperate woodlands and the arid inland. In fact, the only major environment where eucalypts are absent is probably rainforest. There are about 12 species which occur naturally outside of Australia but around 700 are Australian endemics. Only 2 species are not found in Australia. One of these, Eucalyptus deglupta, is the only eucalypt to be found growing naturally in the northern hemisphere, occurring in the southern Phillipines (as well as in New Guinea and parts of Indonesia).
Even Australians who wouldn't know a Banksia from a Begonia know what a gum tree looks like and smells like. Soldiers returning by ship from the first and second world wars are rumoured to have been able to smell the aroma of the eucalypt before land was visible on the horizon. Most Australians may not be able to identify a particular species (there are hundreds of them, after all!) but they will know "that's a eucalypt".
Although usually regarded as applying to the genus Eucalyptus, the term "eucalypt" really encompasses the closely related genera Angophora and Corymbia as well. Corymbia is, in fact, a new genus (1995) which contains 113 species, 80 of which were formerly within Eucalyptus. Angophora consists of about 13 species.
There are a number of differences which serve to distinguish the three "eucalypt" genera. The following are the most obvious features which separate them but they are only general and there may be exceptions:
The term "gum tree" is derived from the habit of some eucalypt species to exude a sticky, gum-like substance from the trunk. This is by no means a general characteristic but "gum tree" has become a common generic term for most eucalypts. A number of other common names have been applied to certain groups of eucalypts based on features such as bark type, timber characteristics or growth habit. Some names in common usage are:
|Some Eucalypt Bark Types
From left - Angophora bakeri (Narrow-leaved apple), Corymbia maculata (Spotted gum)
Eucalyptus saligna (Sydney blue gum), Eucalyptus eugenioides (Thin-leaved stringybark).
Photos: Brian Walters
The eucalypts are within the plant family Myrtaceae and can number among their relatives such well known Australian genera as Callistemon (bottlebrushes), Melaleuca (paperbarks), Leptospermum (tea trees) and Syncarpia (turpentine).
|Eucalyptus gillii, showing juvenile foliage, buds
and flowers. Photo: Brian Walters
The adult leaves of the majority of eucalypts are sickle-shaped and arranged alternately on the branches. They have special glands containing volatile oils which, in some species, are exploited commercially. The leaves of certain species (eg. E.camaldulensis, River red gum; E.punctata, Grey gum; E.viminalis, Ribbon gum) are important as the food of the Koala, a tree-dwelling marsupial.
The juvenile leaves are usually quite different from the adult ones, are often greyish in colour and opposite rather than alternate. These leaves are usually superseded by the adult leaves early in the tree's development but, in a few cases, they may persist into the adult stage of growth. E.cinerea (Argyle apple) is perhaps the best known of these and its round, silvery foliage is sought as decorative foliage in cut flower arrangements. The Tasmanian blue gum (E.globulus) and the Curly mallee (E.gillii) are two others that retain the juvenile foliage for a considerable time.
Like many related plants, the conspicuous parts of the flowers of the majority of the species are the stamens, the male parts of the flowers. These surround the central stigma which arises from the flower's ovary. In Eucalyptus and Corymbia the petals and stamens are fused into a cap called an operculum which covers the flowers at the bud stage. As the flower opens the cap is shed. It is, in fact, the operculum which gives Eucalyptus its name (eu, well and calyptos covered). In Angophora the cap is not present and the flowers develop with small petals.
After flowering, hard, woody seed pods develop. The shape and size of these pods are important aids in identifying species. It is often possible to identify a tree, even when foliage and flowers may be many metres above the ground, by collecting pods lying around its base. The pods usually contain a number of true seeds and an amount of infertile "chaff". The seeds are generally larger and more deeply coloured than the chaff. In most cases seed matures within 12 months of flowering but is retained on the tree until stimulated to be shed by the heat from a fire or by the death of the plant.
|Some Eucalypt Buds and Fruits
Clockwise from top left - buds of Eucalyptus lehmannii showing the long, narrow operculum, fruit of Eucalyptus lehmannii, buds of Eucalyptus amplifolia and the fruit of Eucalyptus erythrocorys. Photos: Brian Walters
Another well-known feature of many eucalypts is their ability to recover quickly from the effects of bushfires. There are three mechanisms of recovery:
|Regeneration from a lignotuber.
Photo: Brian Walters
Some species, notably those like Mountain ash (E.regnans) from wetter forests, rely solely on seed to regenerate. The long-term survival of these species can be at risk if fires occur too frequently (i.e. at intervals that do not allow sufficient time for seedlings to grow to maturity, flower and set seed. Those species that regenerate vegetatively from the trunk or lignotubers are more readily able to cope with more frequent fire. However, even with these species, regeneration from seed is important to ensure genetic diversity of the plant population.
The most important commercial use of eucalypts is in forestry and this is an area where there has been considerable conflict between conservation and timber interests in the last 20 years or so, particularly as resistance to woodchipping and the move to preserve old growth forests have gained momentum.
Timber production from eucalypts is carried out in Australia and overseas. Many different species are used both from natural forests and from plantations. Eucalypt plantations can be found in more than 90 countries with the largest overseas plantations being in Brazil which has over 1 million hectares. Some of the uses for eucalypts are:
Another commercially important feature of eucalypts is the extraction of the oils contained in the foliage. Surprisingly, production of eucalypt oil is greater overseas than in Australia. Portugal and Spain together produce more than 50% of the world's supply with Australian production being around 5%. Eucalypt oil has been used in medicine, industry and for perfumes and this is outlined in Volatile Oils from Eucalypts.
The flowers of all eucalypts contain nectar and many species are important in the beekeeping industry. Honey is often marketed under the name of the main species involved in its production (eg. Yellow box, Red ironbark).
Large scale planting of Eucalypts overseas is not always welcomed by the local population. The following extracts illustrate the extent of the problem (from "Science Comes to the Gum Tree", Dr Anne-Maria Brennan, University of Kent, Canterbury, in Waste Disposal and Water Management in Australia, April 1992).
"In some parts of the world eucalypts have been heralded as the saviour of forests, for their rapid growth makes them a naturally renewable resource of timber and wood for fuel, helping to obviate the need for large-scale deforestation. But things are different in other places where the non-native trees are regarded as little more than weeds which threaten to oust the natural vegetation of the area."
........As a result of this, eucalypts have in recent times become the focus of riots and open hostility between farmers, forestry officials and funding bodies. ..........In Spain, a Society which promotes the cause against non-native introduced species has even called itself the Phorocantha Club, after the name of the bark beetle which infects and kills the much-hated eucalyptus!
Those who campaign against eucalyptus do so on the basis that the trees are aggressive non-natives that lower the water table, poison the soil and deplete it of nutrients."
The situation is a little less volatile where eucalypts have been planted mainly for landscaping. In California, for example, the eucalypt is so common that many people believe that it is a Californian native. With the exception of E.globulus, which is listed among the exotic pest plants of greatest concern by the California Exotic Pest Plant Council, eucalypts seem to have been generally accepted in California for their intrinsic qualities but there are understandable concerns about the widespread use of what is, after all, an alien species. An excellent account of the history, uses and environmental issues of Californian eucalypts can be found in The Eucalyptus of California: Seeds of Good or Seeds of Evil?