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Bluegum trees – well-known immigrants from Australia

The bluegum tree Eucalyptus globulus has become a common sight on many South African ranches and farms but it is now being regarded as a thirsty invader. It is one of more than 700 species of gum tree that occur mostly in Australia. The name Eucalyptus is of Greek origin and consists of the words eu (well) and kalyptos (covered). It refers to the calyx that covers the flowers of this genus. Eucalyptus globulus that has been introduced to South Africa was first described scientifically in 1800 by the botanist Jacques Labillardiere from Recherce Bay in Tasmania, Australia. The name globulus is Latin for a globe and refers to the shape of the fruit.

Eucalyptus shrubs and trees of the family Myrtaceae are almost entirely limited to Australia with only some 15 species occurring in the adjacent regions and with Corymbia and Angophora being related genera. Eucalyptus developed from Gondwana tropical rain forest plants some 35 to 50 million years ago in a land mass that still contained Australia and New Guinea, the last two tectonic plates to break away from Gondwana 66 to 55 million years ago. The current Antarctic Ice Cap only developed some 5 to 15 million years ago. At the time of breaking away from Gondwana, Australia and New Guinea were humid and covered in tropical rain forests because they had been located near the equator 452 million years ago. Over time, Gondwana slowly rotated until Australia and New Guinea were near the South Pole before they broke away to drift north-eastwards towards Indonesia. Some 20 million years ago Australia started to become arid and lose its soil nutrients which led to vegetation that was dominated by Casuarina and Acacia in open forests. When the Australian climate became more arid 15 million years ago Eucalyptus species became the dominant plants in Australia. This process became more advanced in central Australia when the ice sheet over Western Australia started to melt some 5 to 3 million years ago. Because Australia has no effective geographical barriers such as large rivers and great mountain chains to prevent the spread of fire, Eucalyptus species gradually displaced much of the original Gondwana rain forests except in some localized places.

The genus Eucalyptus was first described scientifically in 1789 by L. Heritier. Various species of Eucalyptus have become widely cultivated in the world after the genus was first introduced elsewhere by Sir Joseph Banks who was the botanist on the expedition of Captain James Cook to Australia in 1770. Since then it has been spread to the Americas, Israel, northern and southern Africa and southern Europe. The trees grow rapidly, exude copious amounts of sap, and are a good source of wood and oil. The Australian didgeridoos are traditional Aboriginal instruments that are made of gum tree logs that have been hollowed out by termites. The oil is used for cleaning and is a natural insecticide that is toxic in large quantities. It also protects a gum tree against browsers to some extent but the leaves are especially eaten by the koala Phascolarctos cinereus and some other marsupials. The oil also makes the trees highly flammable and some trees are known to explode spontaneously and cause devastating bush fires. On warm days the Eucalyptus forests are sometimes shrouded in a smog-like mist of vaporised, volatile organic compounds which have given the Blue Mountains of eastern Australia their name. Different parts of various Eucalyptus trees are also used as natural yellow, orange-green, tan, chocolate and deep rust dyes, while the flowers produce high-quality monofloral honey. The mulched pulp can be used as a fertilizer. In some regions of the world, Eucalyptus trees were planted in swamps to drain them in an effort to reduce the occurrence of malaria.

Eucalyptus trees seldom have tap roots that extend deeper than 2.5 into the soil but they do have extensive secondary root systems that spread widely to tap moisture from large areas of topsoil around a tree. The various species vary from being low shrubs to the tallest flowering plants in the world. Only the coniferous redwood trees of the subfamily Sequoioideae of the USA are larger than the tallest Eucalyptus ones. The flowers are distinctive but eucalypts only flower when a plant is adult, while the fruits are cone-shaped. The bark sheds in various patterns and intensity, sometimes forming long strips of loose bark.  The tallest known flowering plant is a Mountain ash tree Eucalyptus regnans of south-eastern Australia that is 114.4 m tall.

Eucalyptus species are mostly intolerant of frost although some degree of cold in the winter can be tolerated. In South Africa the bluegum tree has therefore often been planted in arid, treeless regions that can become fairly cold in the winter. In Africa, Eucalyptus was first introduced to Addis Abada in Ethiopia in 1894 or 1895 by either the French advisor Mondon Vidailhet of Emperor Monelik II or by a captain O’Brian of England. The objective was to supply wood for fuel to Addis Abada which had become massively deforested by firewood harvesting. In addition to the bluegum tree, numerous species of Eucalyptus have been introduced to South Africa, and it and four other species have become naturalized there. However, in South Africa they are usually unable to compete with grasses of a good quality but they will become established after severe veld fires have devastated the seed beds of such grasslands. Currently Eucalyptus is the most widely planted tree in plantations world-wide.

 

References:

Anon. 2012. Eucalyptus regnans. http://en.wikipedia.org.wiki/Eucalyptus_regnans - last modified on 31 December 2012.

Anon. 2013. Eucalyptus. http://www.en.wikipedia.org/wiki/Eucalyptus - last modified on 20 January 2013.

 

By: Prof J du P Bothma

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