The last 13 years have been terrible for ancient African baobab trees.
Nine of the 13 oldest either lost trunks or died altogether after having lived for longer than a millennium, researchers report June 11 in Nature Plants. But just what the demise means for the iconic species is up for debate.
“Whilst we are saddened about the death and collapse [of the old trees], current evidence does not indicate that this is affecting the whole population,” says plant scientist Sarah Venter, who was not part of the new study. Venter, with the University of the Witwatersrand in Johannesburg, does not see an immediate threat to the species as a whole. These trees of extreme age “were probably more vulnerable to dry conditions,” she says. “Tree mortality is complex and can be attributed to many causes, including climate change and droughts.”
The Adansonia digitata species of the baobab group is the longest-living kind of flowering tree. With its mass of skinny branches dividing like rootlets over a fat trunk, the species sometimes gets teased as an upside-down tree. Long-stemmed brown fruits also encourage the nickname “dead-rat tree.” Yet people have long cherished the giant baobabs for food, medicine and spiritual value.
Of the 13 oldest known A. digitata, four have died since 2005, says study coauthor Adrian Patrut, who specializes in inorganic and nuclear chemistry at Babeş-Bolyai University in Cluj-Napoca, Romania. Five others of these ancient trees across the African continent have lost enormous chunks of their multiple trunks, Patrut and his colleagues report.
The oldest to die, the Panke tree in Zimbabwe, had lived about 2,500 years before it collapsed over the course of 2010 and 2011. The other three trees that died, including the Chapman tree in Botswana, were just 1,250 to 1,500 years old.
Patrut does not think mere old age caused their demise. He worries something more troubling may be going on instead. “There were no signs of an epidemic,” Patrut says. He suspects that a warming trend observed in southern Africa may be playing a role.
As world-famous as baobabs are, much of their drought biology and other basic matters remain mysterious. “Does anyone really know the rooting structure of baobabs?” laments plant ecologist Eugene Moll of the University of the Western Cape in Bellville, South Africa. For understanding Africa’s plants, “philosophies and paradigms that originate north of 40° N do not necessarily apply down here in the Southern Hemisphere,” he says. “We humans like symmetry but nature is certainly not always symmetrical.”