On January 7, 2016, a group of tourists set out to visit Chapman’s Baobab, one of the oldest and mightiest trees in Africa.

It had long served as a landmark, used by David Livingstone. The cavity inside its trunks — with an outer circumference of more than 25 meters — reportedly served as one of the continent’s first post offices. Botswana considered the tree a national monument and promoted it as an attraction.

As the visitors neared that day, they heard a cracking boom. Chapman’s Baobab had collapsed. The tree was around 1,400 years old, and its loss brought mourning. But it was not an isolated event. Across Africa, the oldest and largest baobabs have begun to die, according to research published in June 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 study. After Chapman’s Baobab collapsed, Dr. Patrut found that its water content was just 40 percent, compared with 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 trees are 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, co-author of the paper.

In 2005, the researchers began collecting samples from more than 60 of the largest baobabs — those with trunk circumferences of at least 20 meters. The oldest, they found, were around 2,500 years old. The team compared carbon-14 levels from samples from the oldest parts of the trees to samples from other tree species whose age had been found by counting their rings.

The researchers were shocked when some of their subjects began to fall. Eight of the 13 oldest and five of the six largest have died or partly collapsed in 13 years. «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.

Baobab trees often host rich communities of animal life, including bats and bees. People value them, too. Ancient baobabs often are staples of local lore and are the site of ceremonies. In times of famine, their seeds feed humans and wildlife, and their bark — which can be stripped without killing the tree — is a source of 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 at the University of Queensland who has studied baobabs. He said, «It is upsetting to face the loss of some of the oldest, biggest trees in the world.»

Researchers fearclimate change is killing baobabs.

© 2018 The New York Times