Eucalyptus species are widely grown and utilized throughout much of the world, Dr Arne Witt reports. They are a valuable source of timber, fuelwood, paper, nectar, etc. and as such often grown in woodlots and plantations. However, many of these introduced species have escaped cultivation and become invasive.
In South Africa gum species including E. camaldulensis, E. cladocalyx, E. diversicolor, E. grandis, E. lehmannii, E. paniculata, E. sideroxylon, etc. are regarded as being invasive, reducing biodiversity and having a significant negative impact on dwindling water resources.
In fact invasive Eucalyptus species in South Africa are responsible for the loss of 16% of the 1,444 million cubic metres of water resources lost to invasive plants every year. They are thirsty trees! Invaded landscapes also have significantly higher biomass than natural/native vegetation which means that they pose a significant fire risk. Invasive plants like pines, Australian acacias and gums fuel fires.
There are many possible reasons as to why introduced species become invasive – the introduced species is adapted to grow in a wide range of climatic regimes or soil types; it is dependent on generalist pollinators; it has no natural enemies in its adventive range; grows rapidly; establishes easily; etc.
With these attributes invasive plants can outcompete/displace native species for space, access to nutrients and water, etc. Gums have another tool in their arsenal and that is allelopathy – they displace other species through chemical warfare, in other words they release chemicals that other plant species don’t like. In this way they displace native species, improving conditions for themselves – eliminating competitors, etc.
Wetlands in water scarce countries are critical, especially now that climate change is impacting on the frequency and abundance of rainfall. However, in the past, gums were used to dry up wetlands, swamps, marshes, etc. to try to curb the incidence of malaria – the gums suck up the water which is then lost to the atmosphere as a result of evapo-transpiration.
There is anecdotal evidence that a large gum tree growing near water can suck-up 1000 litres of water a day but that is probably an exaggeration – others state that its more like 200 litres of water a day. Either way, they use a lot of water.
There are other reasons for destroying wetlands. How effective this practice was in reducing the incidence of malaria I am not sure but many would believe that it did. However, there are better and more effective ways of controlling malaria.
So, in a nutshell some gum species have become invasive. However, many of these invasive species have costs and benefits – costs include negative impacts on biodiversity and water resources, while benefits include timber, fuelwood, etc.
To try to mitigate their negative impacts it is important not to grow them near any water resources – wetlands, rivers, etc. where they will have a far greater impact on water. Woodlots/plantations should also be well managed – any seedlings which establish outside of woodlot/plantation should be removed as a matter of urgency. Efforts are now also underway to produce sterile cultivars to reduce their invasive potential.
A recent study in South Africa showed that invasive Australian acacias, pines, gums and other woody weeds growing in water catchments that supply Cape Town with water are using up two months of Cape Town’s water supply every year. As such, it is important that these species be managed – without water there can be no life.
This blog stems from an article which Dr Arne Witt contributed to that appeared in the Swiss German language newspaper Neue Zürcher Zeitung.
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