Eucalyptus trees contain a host of biologically active chemicals. Many of these chemicals are toxic, designed as natural mechanisms against insect and marsupial folivores and herbivores.
Biologically active products from Eucalyptuses include tannins, polyphenols, terpenes, acids, waxes and formylated phloroglucinol compounds, or FPCs.1, 2, 3. Different species of Eucalyptus exhibit varying concentrations of these chemicals, and correspondingly, herbivores select certain species or even individual trees for consumption based on the level of toxic chemicals present.4
It was previously thought that marsupials regulated their exposure to toxic chemicals in Eucalyptuses through taste recognition; however some researchers have shown that possums can become
accustomed to higher levels of certain toxins in their diet, with gradual exposure.4
There can be a wide variation of the secondary plant metabolites (SPMs) within a given population of eucalyptuses, even within the same species, and it is thought that concentrations are governed by
hereditary traits, although the mechanism by which this happens is unknown.5
Species like E nitens are capable of producing a complex chemical defence in response to fungal attack after being pruned. The trees manufacture FPCs, hydrolyzable tannins, proanthocyanidins,
flavanone glycoside and stilbene glycosides.1
Eucalyptus oil is useful as a medicinal product and as a natural disinfectant; however, you will notice that a bottle of eucalyptus oil is always labelled ‘toxic’. Ingesting just one teaspoon of essential oil
can have fatal effects. Even when applied topically, it is usually recommended that eucalyptus oil, be diluted in a carrier oil to lessen its toxicity.
Clearly Eucalyptuses are capable of producing quite toxic chemicals. While certain species of indigenous animals and insects might have mechanisms to deal with these toxins, we do not.
If plantation E nitens are being ‘genetically improved’, to enhance certain properties, this could have an effect on the concentrations of toxic chemicals produced by the trees. While this might be desirable from a forestry perspective, it could have unexpected environmental outcomes.
E nitens are the tree of choice for the pulp industry, but their fibre requires more severe cooking treatment to break down during processing. This means that, more chemicals are required to treat the pulp.6 Sometimes xylanases are used. This xylanase is an enzyme produced from a genetically modified organism that destroys plant cell walls.7
E nitens are selectively bred for their high kraft pulp content, as this is another hereditary trait.8 Who knows if higher kraft pulp-yielding trees also produce more toxins in the rest of the plant.
Not only are E nitens being planted in a monoculture – some plantations use cloned trees. This means that they are genetically identical – so any negative traits, such as disease susceptibility or
increased levels of certain toxins, is greatly increased. This raises even more questions about the ability of these trees to concentrate certain toxins.
A monoculture of clones is the antithesis of the concept of biodiversity - one of the tenets of sustainability. Eucalyptuses are already toxic, so it comes as little surprise that when planted as a genetically improved monoculture, the increased amount of chemicals entering waterways and soils can have toxic effects.
The toxicity of a chemical is simply a matter of dose. Anything in a large enough quantity can be toxic – even water. Something that is beneficial in a small quantity, might be toxic in a larger quantity. By increasing the concentration of traits in E nitens through monoculture plantations or clones, we are inadvertently increasing the amount of toxins entering the environment from these trees.
1) Barry, K. M., Davies, N. W., Mohammed, C. L., (2001), Identification of hydrolysable tannins in the Reaction Zone of Eucalyptus nitens wood by high performance liquid chromatography – electrospray ionisation mass spectrometry, Phytochemical Analysis, 12: 120-127
2) Rapley, L., Allen G., Potts, B., Davies N., (2007), Constitutive or induced defences – how does Eucalyptus globulus defend itself from autum gum moth larvae feeding, Chemecology, 17, 4, 235-243
3) Eyles, A., Davies, N., Mohammed, C., (2002), Novel detection of formylated phloroglucinol compounds (FPCs), in the wound wood of Eucalyptus globulus and E. nitens., Journal of Chemical Ecology,
Vol. 29, No 4., 881 – 898
4) Lawler, I., Stepley, J., Foley, J., Eschler, B., (1999), Ecological example of conditioned flavour aversion in plant-herbivore interactions: effect of terpenes of Eucalyptus leaves on feeding by common ringtail and brushtail possums, Journal of Chemical Ecology, 25, 2, 401- 415
5) Henery, M., Moran, G., Wallis, Foley, W., (2007), Identification of quantitative trait loci influencing foliar concentrations of terpenes and formylated phloroglucinol compounds in Eucalyptus nitens, New
Phytologist, 176: 82-95
6) Mardones, L., Gomide, J., Freer, J., Ferraz, A., Rodriguez, J., (2006), Kraft pulping of Eucalyptus nitens wood chips biotreated by ceriporiopsis subvermispora, Journal of Chemical Technology & Biotechnology, 81, 4, 608-613
7) GMO Compass website, GMO database, Xylanase, available online at: http://www.gmocompass.org/eng/database/enzymes/96.xylanase.html (cited 23/03/10)
8) Schimlek, L., Kube P., Raymond, C., (2004), Genetic improvement of kraft pulp yield in Eucalyptus nitens using cellulose content determined by near infrared spectroscopy, Canadian Journal of Forest Research, 34, 11, 2363-2370.