Image caption Spreading twice as fast as Delta, Oxalis devours low-lying pines over the Namib Desert
A fungus is eating through South Africa’s iconic pines, taking advantage of a warming continent by increasing its rate of spread, and destroying a once-mighty industry.
Researchers at the University of South Africa’s (Unisa) Adaptive Tree Restoration Centre have been monitoring the spread of Oxalis oxalis, also known as Syringomyces longbranch and Ketito.
It started in dry areas to the west of Cape Town and spread eastwards, concentrating on the communities living near high-altitude pines.
Previous estimates suggested it was 85% of all pines in Namibia, but new data shows its death rate in the country has almost doubled in just one year.
“There’s definitely more of it, they have spread massively,” said Marijke Postmans from Unisa’s Adaptive Tree Restoration Centre (ABRC).
Ineffective pesticides
The burning, dying and rotting of pines is not the only threat but it is often the one we see the most. Pines are also used for chopping wood and drying tyres, and to stop the spread of insects.
“Pines are an important part of the landscape,” said Dr Pierre Mpiena of University of Cape Town’s School of Agricultural Sciences.
“They create a climate sensitive forest and the elderly and children depend on this structure to live.”
Pine forests were once largely non-native, but now account for about 20% of the Namibian landscape.
There is a chance that fire-resistant pines could emerge to replace the dying pines, but it’s not yet clear how they will work with the region’s existing ecosystem.
Pollution
Oxalis is spread through soil by the vacuum effect. Rainwater runs into the soil and away from the pines, carrying with it the fungus.
The fungus also moves through drains, drainage and watercourses. But the effects of the fungi are less apparent.
“The most serious effect of Oxalis is on our forests that are a bit more shallow, and that includes the savannah plains,” said Dr Mpiena.
“[T]he only population that has been affected is the highland parts of the region.”
Not only does rain increase the spread of Oxalis, it also helps it spread more quickly.
“It was initially thought that there would be a ‘no fly zone’ around pines, but once the fungi started spreading, there was no such barrier,” said Dr Mpiena.
Big business
Oxalis has devastated forests in the Omaheke and Kavango regions of northern Namibia. This is a high-altitude region made up of deserts and savannahs, which is prime for spreading a fungus that thrives in warm, dry conditions.
Alongside the pines, it has also spread along river and drainage channels, which link together the savannahs, and as the dry season begins there is no buffer of vegetation to stop it.
The affected areas host animal and plant species with significant economic value. They include the tiniest seedlings that are not firm enough to grow in the bush, and they also provide livelihoods for many people who depend on growing trees for firewood.
Image caption As the leaves fall, some of the timber is collected to be dried and recycled
They also provide an escape route for pests and disease. The University of Cape Town found that Oxalis spreads slower when a disease is present, which would contribute to the sustainability of trees, but it has not been known if such diseases exist in the region where the fungus is affecting pines.
Droughts and extreme weather will only exacerbate the problem.
“We need to introduce some strategic interventions to slow its spread,” said Dr Mpiena.
“They could be adopting pesticide management or fruit harvest during dry season [but] the epidemic will definitely continue as long as there is rain.”
Oxalis also claims other victims outside of the pines.
“The epidemic is affecting honey bees, does this have an impact on honey harvest?” asked Dr Mpiena.
Other studies have shown that Oxalis causes honey to ferment as well as damaging the honey beehive.
It does not however affect the nutritional composition of the honey.