Meet Eucalyptus grandis, called rose gum in English, the subject of the research described in the story above and a stalwart of the South African forestry industry.
We thought it would be interesting to explore the traits of this tree and the story of its use in South Africa. As it turns out, E. grandis is the mother of a large clan of hybrids and their clones.
During interviews for this story, we discovered a larger one: the intriguing tale of tree breeding in South Africa. This science is only a few decades old but is developing fast and is in “a constant state of flux,” as Prof Ben du Toit, Associate Professor of Forest and Wood Science at Stellenbosch University, put it.
Our investigation also led us to realise just why the first South African foresters grew Australian trees – and why forestry is far more environmentally friendly than its detractors realise. (For more about this, please read our next story “Why Forestry Is Environmentally Sustainable)
Huge strides, in just a few decades
Tree breeding is a recent science in South Africa. It officially took off in 1947, when the South African Forestry Research Institute was formed. Private forestry companies launched their own programmes in the 1980s – just 45 years ago.
Huge strides have been made in that time. Ninety species of eucalyptus, 50 species of pine and 30 species of wattle have been tested, according to information provided by forest genetics lecturer and Sappi Tree Breeding Programme Leader Dr Andre Nel*, following an interview.
The South African Forestry website states that over 40 species of forestry trees are now grown in South Africa.
E. grandis was brought to South Africa from eastern Australia in 1890 and became one of the five main eucalyptus varieties grown here. Its timber has been widely used in the pulp and timber industry due to the tree’s fast growth (reaching heights of about 50m), its strength, and durability − which is why it’s good for poles.
Foresters also grow a host of E. grandis crosses, also known as hybrids. For a little more detail, please read the Did You Know section at the bottom of this story.
Interestingly, most hybrids (and their clones) are not invasive. “I have no evidence of a single eucalyptus hybrid that’s invasive, and we’ve planted far and wide.”
Travels to far-flung places
Why Australian trees? South Africa’s first foresters needed trees that would grow quickly in our country’s dry conditions – and indigenous trees grow far too slowly. They travelled to places with similar climates to seek them out.
They found what they were looking for ‘Down Under’. The eucalyptus species like E.grandis now grown locally were found in Australia and some islands to its north: Timor-Leste, Borneo and even the Philippines. Pines came mostly from southern Europe, Mexico, and Central America, while the Pinus radiata grown in the Western Cape comes from California.
Finding the right trees took great care. The foresters didn’t only need only hardy and fast-growing trees; they also needed superior specimens in specific “provenances” – a forestry term that means with local genetic variations, such as the E. grandis from Coff’s Harbour.
At home in South Africa, these trees were tested and certain specimens were found to be best suited to various local sites. (Foresters use the term “forestry site types”, which encompasses all the attributes of the place, from rainfall, temperature, windiness and other aspects of climate to soil and terrain.)
Hybrids – how children outperform their parents
Then came the next challenge: cross-pollinating the best provenances and species to create even better varieties for local conditions.
Hybrids have long been used in forestry. They are created by cross-pollination, with the aim of capturing the positive qualities of both parents. This may be resistance to pests or diseases, frost- or drought-tolerance, and always of course, high-quality timber. Happily, the offspring often outperform both parents.
Crosses have been made throughout the history of agriculture. “This is just good old plant breeding,” pointed out Ben.
However, in nature, different species usually cannot procreate. In the world of forestry, this changed when French researchers working in Pointe-Noire, Congo, managed to cross two different eucalyptus species. “This took the world by storm,” said Ben. “Their work enabled us to create what we call interspecific hybrids.”
This is now common practice in forestry across the world and is possible to achieve when two species have not evolved too differently to produce offspring.
Hybrid trees are created in large numbers – as many as 1000 of each tree – and tested. “Some are good, some bad and some average,” said Ben. “We select 10 or 20 of the best with the traits we need, such as good resistance to pests or diseases, tolerance of drought or frost, and other qualities such as the ability to grow on very steep slopes. Of course, we also want good timber.
“Really, we’re looking for trees that have all these traits, but you can’t always have everything.”
The hybrids are matched to a specific site where they’re likely to do best. “We end up with some individuals that are extremely strong, robust, fast-growing and have good wood properties. These are the ones we select.”
Success requires a lot of experimentation across many site types. Eventually, the foresters know the growth and wood properties they can expect from their hybrids.
Hybridisation is not just a way of boosting trees’ productivity; it has become essential to future-proof the industry. In the past ten to 15 years, many of the commercially important species of the South African forestry industry – including the hero of our story, the pure E. grandis – have been impacted by climate change and the increase of pests and diseases. Along with Eucalyptus nitens and Pinus patula, these pure species together constituted up to 70% of commercial species, observed Andre Nel.
“The survival of such species became very difficult to achieve, so we’ve had to move to hybrid breeding.”
The next step is mass production.
Clones vs genetic modification
Have you ever seen large stands of forest in which the trees look identical? They’re probably clones. These are shoots which are taken from the best trees, then cultivated as rooted cuttings and finally planted in “clonal blocks”.
Clones are perfect replicas of the original tree, with an identical genetic make-up. This makes them different from genetically modified organisms, in which scientists modify the genes of the original plant or animal. Genetic modification is banned by the Forest Stewardship Council and does not take place in the South African forestry industry.
Interestingly, most hybrids (and their clones) are not invasive, said Ben. “I have no evidence of a single eucalyptus hybrid that’s invasive, and we’ve planted far and wide.”
The virtue of the donkey
Happily, through hybridisation foresters can end up with a generalist tree that grows very well across several site types. However, it’s not all black and white. Some trees grow extremely well under good conditions but in poor conditions, fail miserably.
“For this site another species, a real donkey, might outgrow it,” said Ben.
He explained: “A tree that grows fast might not be drought tolerant, because it puts all its nutrients into the growth of the stem without keeping many reserves. Another tree that doesn’t grow fast, may have better root growth and reward you more on a dry site.”
This is an example of what foresters mean when they talk about “genotype by site interaction”.
On most plantation sites in South Africa, either pine or eucalyptus can be grown. The choice depends not only on the conditions of the site, but on the purpose of the timber.
The point is that every tree has its use. And in the evolving field of forestry, foresters are continuing to expand and finetune their options.
*Our thanks to Dr Andre Nel and Prof Ben du Toit for their substantial input for this story.
Did You Know?
For agricultural poles these days, E. grandis and her hybrids are the most popular. Eucalyptus varieties are also grown for mining and transmission poles; board, furniture, cellulose and wood chips.
The five main eucalyptus species grown in the country are E. nitens, E. macarthurii, E. Dunnii, E. smithii and E. grandis, states the Forestry SA website. (We struggled to read these names – then it was pointed out that that the double ‘i’ comes after the name of the person the tree is named after. Phew!)
The most common E. grandis and other hybrids grown in South Africa are:
- A cross with E. urophylla, or Timor white gum, has largely replaced E. grandis in the sub-tropical areas of KwaZulu-Natal, as well as the warm temperate warm areas of KwaZulu-Natal and Mpumalanga. This hybrid has tolerance to some of the pests and diseases that attack E. grandis.
- E. grandis crossed with E. nitens has largely replaced E. nitens and E. macarthurii in the cool temperate areas of KwaZulu-Natal and Mpumalanga. This hybrid has tolerance to the cossid moth, which has decimated E. nitens.
- P. patula crossed with P. tecunumanii (low and high elevation) has replaced P. patula, mainly due to a fungus (Fusarium circinatum) which attaches to pure P. patula, The hybrids are not as cold tolerant as P. patula, so some P. patula is still planted in very cold sites.
- P. elliottii crossed with P. caribaea is planted instead of P. elliottii, as the hybrid is substantially more productive and has more favourable wood properties compared to P. elliottii.
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