THE EUCALYPTUS TREE

This blog entry is dedicated to our daughter Fran. Fran, as some of you may or may not know; graduated last year with a Bachelor of Science in Forestry and is currently enrolled in a Masters Program in Forestry at the University of Alberta. As you can imagine, Fran likes trees. In our travels and day to day life in Australia, we have found the Eucalyptus Tree to be a most interesting species and know that Fran would equally enjoy what we have learned about this plant. Hence this blog is for Fran. Our apology as some of the information in this blog contains some scientific terminology. For those readers who do not have a biological or scientific background, we have attempted to clarify terminology with the hopes that everyone reading our blog will find this entry interesting and informative. We suspect that you will as the Eucalyptus Tree is one of Australia’s icons.

As you may recall from high school biology classes, plants are classified in the following manner: Family, Genus and Species. So in accordance with proper plant classification, the Eucalyptus Tree is in the Myrtaceae Family, which is commonly referred to as the Myrtle Family. Eucalyptus Trees belong to the Genus Eucalyptus. There are reported to be between 700 and 800 species of Eucalyptus in Australia. The exact number of species has yet to be determined. The reasons for not knowing the exact number of species is discussed later in this blog entry.

The Genus Eucalyptus is characterized by its distinctive flowers and fruit. Floristically, the stamens (i.e., the male parts of the flower) are enclosed in a cap of fused flower petals that are forced off and drop to the ground as the stamens expand. When discovering this distinctive floristic characteristic, Gail was quick to observe that even male parts of a flower expand. She was even quicker to point out that it is not the size of the stamen that matters; however, we digress. The flowers have no petals, but are decorated by many showy stamens which may be white, cream, yellow, pink or red in colour. The absence of showy flower petals is one of the key features of the genus. As the ovary is pollinated and matures, the fruit forms a woody cone-shaped capsule or “gum-nut”, another key characteristic of the genus. A further striking characteristic of the genus is the eucalyptus odour that is emitted when the leaves are crushed between your fingers. Some species are particularly fragrant and you can smell eucalyptus as you walk by a big old tree. It is an intoxicating experience!

Flower and fruiting bodies of a Eucalyptus Tree

Red Stamens of a Eucalyptus species with fused orange-brown petals below the stamens.

Characteristic fruit which is a cone-shaped capsule or gum-nut.

Eucalyptuses or Eucalypti are either commonly called “Gum Trees” or “Stringy-bark Trees”. They are referred to Gum Trees because they profusely ooze sap from any break or deep cut in the tree. Similarly they are often called Stringy-bark Trees because like the Arbutus Tree in British Columbia, they will shed their bark; with the difference being that the Eucalyptus will shed its bark in either long strips or variably-sized flakes. The shedding of the bark results in the trunk of the trees exhibiting remarkable colours and beauty.

In addition to the beautiful colour of the trunk, note the thick mulch layer created by the shedding bark and leaf litter. The purpose of this mulch layer is discussed later.

Another natural beauty!

As you have probably surmised, many of the Eucalyptus species are, botanically speaking, difficult to identify and classify. In addition to the vast number of species, classification of this genus is clouded because the species has evolved both morphologically (i.e., the structure of plants) and physiologically (i.e., plant function) to adapt to the many different environments in which it inhabits. For example, gum trees grow in a diverse range of habitats from extremely harsh locales such as alpine regions and granite rock outcrops to lush rainforest environs. Depending upon the particular environ, they can grow to be hundreds of feet high or merely a few feet in height. In addition, many species have an overlapping geographical distribution range, with gene exchange still occurring. As a result intermediate forms of the two species are common and hence the difficulty in classification. Similarly in Canada, we can experience gene exchange between Engelmann Spruce and White Spruce.

In South Australia, it is reported that there are approximately 80 Eucalyptus species. They inhabit all types of climatic regimes in the state from temperate to extremely arid. Given that we presently reside in an environment with an annual rainfall of approximately 15 inches per year; our observations with respect to the Eucalyptus Tree are predominantly related to its growth form in an arid environment. The following series of photographs and descriptions highlight the survival mechanisms that the Eucalyptus Tree has adapted to live in an arid environment.

In arid environments, the Eucalyptus Tree typically has a multiple stem growth configuration starting at ground level. This grow form is referred to as “mallee”. A mallee growth configuration is optimal in arid environments because this stem formation allows rainwater to collect along the various branches and funnel its way down to the main trunk system, thus maximizing moisture collection and retention. During times of drought, the tree will literally drop large branches from its trunk so as to enhance its chances for survival. By losing branches, the plant reduces its overall immediate need for water because the plant’s surface mass has dramatically decreased.

Mallee growth formation is typical for Eucalyptus Trees growing in arid environments

Typically the Eucalyptus Tree is an ever green tree meaning that it does not lose its leaves after the growing season. However, the Eucalyptus Tree readily and periodically sheds its bark and leaves to effectively create a mulched surface environment around the base of each tree. This mulch surface serves two purposes: (1) The mulch restricts the germination of other vegetation beneath the tree. Thus, the tree ensures that other vegetative growth is limited so that the much needed soil moisture is not used up by a competing species. (2) The mulch creates a surface layer capable of retaining soil moisture and reducing moisture loss due to evaporation.

Note the absence of understory vegetation.

The leaves of Eucalyptus Trees growing in arid environments have also adapted to its environment. The leaves are typically long and narrow with a waxy – oily cover. The leaves are narrow so that there is minimal surface area exposed to the direct sunlight. On the leaf, a eucalyptus smelling oily resin is emitted to form a thick cuticle on the leaf surface to create a protective cover so as to minimize water loss. Essentially the leaves are hydrophobic (i.e., tending to repel and not absorb water) as the oily/waxy surface prevents the leaf from drying out. Also, as the sun shines down on the tree, the long narrow leaf surface will rotate and hang vertically downwards away from the direct rays of the sun. By doing this, the surface area of the leaf that is exposed to the direct sunlight is minimized, thus minimizing water loss due to evapo-transpiration (i.e, water that is evaporated through the leaf tissue).

Long and narrow leaf surface capped with a waxy – oil resin cover

In an arid environment, forest fires from lightning strikes are common occurrences. Unfortunately those plant survival mechanisms described above that minimize water loss also are ideal sources of fuel for fire. The oily-waxy leaves and mulch litter on the ground surface are highly combustible materials. In response, the tree has also developed a well-established root system that is capable of rapidly regenerating plant shoots after a fire event. The Genus is noted as being extremely fast growing, thus minimizing long term impacts from fire on its ecosystem.

Note the rapid generation following a forest fire. Area was burned December 2007 with photo being taken in March 2010.

Some times when we are driving and scanning the passing landscape, we periodically ask ourselves whether we are in Africa or Australia? This is because the growth form of the Eucalyptus Tree dotted on the landscape appears to be akin to many “Out of Africa” travel photos. The Eucalyptus Tree is an excellent example of a plant species that has adapted to the environmental conditions in which it lives.

That “Out of Africa” or is it “Out of Australia” feeling?

Easter is coming, the summer season has gone and the weather is starting to cool. School holidays here in Australia are around the corner. Our blog entries will not be forthcoming until we return from holidays. May you have friends and family close to you over the Easter time and enjoy plenty of delicious meals with your people. HOPPY EASTER!