Researchers fear the trees are parched by drought and rising temperatures linked to climate change.
On January 7, 2016, a group of tourists set out to visit Chapman’s Baobab, one of the oldest and mightiest trees in Africa.
Visible from miles away, it had long served as a landmark for travelers and explorers, including David Livingstone. The cavity inside its trunks — with an outer circumference of more than 80 feet — reportedly served as one of the continent’s first post offices. Botswana considered the tree a national monument and promoted it as a sightseeing attraction.
As the visitors neared that day, they heard a cracking boom like thunder. A cloud of dust obscured the site: Chapman’s Baobab had collapsed.
Across Africa, the oldest and largest baobabs have begun to fall and die, according to new research in the journal Nature Plants. Scientists believe that prolonged droughts and increasing temperatures may have parched the trees, leaving them unable to support the weight of their massive trunks.
“The largest and oldest trees are more sensitive to changing climatic conditions because of their large dimensions,” said Adrian Patrut, a chemist at Babes-Bolyai University in Romania and lead author of the new study.
After Chapman’s Baobab collapsed, for example, Dr. Patrut found that the tree’s water content was just 40 percent, compared to 79 percent for healthy baobabs.
Dr. Patrut and his colleagues did not set out to document the death of Africa’s “wooden elephants,” as the species are sometimes called. Instead, they wanted to date them.
“There were some fairy tales and folklore that these trees could be as old as 6,000 years,” said Karl von Reden, an oceanographer at the Woods Hole Oceanographic Institution and co-author of the new paper. “We were interested in finding out if there’s some upper age limit, at least for the existing ones.”
Chapman's Baobab in Botswana in 2015. News of its collapse the following year was treated as a national tragedy.CreditJJ Vos
Baobabs do not regularly produce tree rings, so the team turned to radiocarbon dating. The scientists compared carbon-14 levels from small samples taken in the oldest parts of the trees to samples from other tree species whose age had been determined by counting their rings.
In 2005, the researchers began collecting samples from more than 60 of the largest African baobabs — those with trunk circumferences of at least 65 feet. The oldest trees, they found, were around 2,500 years old.
The scientists also confirmed that baobab’s unique structure — they often have hollow centers — are formed when the trees generate new stems in a ring-shaped pattern. Over time, those stems may fuse together, creating an open or closed circle.
As the study progressed, though, the researchers were shocked to find that a number of their subjects — the largest and oldest trees — began to fall. Eight of the 13 oldest and five of the six largest have died or partly collapsed in the past 13 years.
The trees did not simply succumb to old age, the researchers believe. “The fact that these trees just suddenly died in the early part of this century is to me a canary in the mine,” Dr. von Reden said.
While he and his colleagues have yet to determine what is causing the deaths, they have largely ruled out disease. Instead, they suspect climate change.
“The new paper nicely brings together information showing that the death of the millennial baobabs is likely due to an unprecedented combination of temperature increase and drought,” said Jens Gebauer, a horticulturist at the Rhine-Waal University of Applied Sciences who was not involved in the research.
Baobabs do not regularly produce tree rings, so scientists must use radiocarbon dating to determine how old they are. The most ancient are around 2,500 years old, and often have hollow centers.CreditArterra/UIG, via Getty Images
“This information is valuable to the scientific community and the public, as the baobab is an outstanding and very important species in many African countries.”
While younger baobabs so far seem to have been spared, the largest trees often host rich communities of animal life, including bats and bees that nest inside their cavities and birds adapted to building nests in the tree’s branches.
People value them, too. Ancient baobabs often are staples of local lore and frequently are the site of ceremonies and meetings. In times of famine, their nutritious seeds feed humans and wildlife, and the tree’s bark — which can be stripped away without killing it — is a source of both nutrition and hydration for elephants.
In addition to impacts on people and the environment, there is also a symbolic loss, said Jack Pettigrew, an emeritus professor of physiology at the University of Queensland who has studied baobabs.
“This is a sad story,” he said. “It is upsetting to face the loss of some of the oldest, biggest trees in the world.”