Africa's ancient baobab, with its distinctive swollen trunk and known as the "tree of life," is under a new and mysterious threat, with some of the largest and oldest dying abruptly in recent years.
Nine of the 13 oldest baobabs, aged between 1 000 and 2 500 years, have died over the past dozen years, according to a study published in the scientific journal Nature Plants.
The sudden collapse is "an event of unprecedented magnitude", the study says.
Climate change, with its rising temperatures and increasing drought conditions, is a suspected factor but no definite cause is known. The deaths occurred in the southern African countries of Botswana, Namibia, South Africa, Zambia and Zimbabwe.
Researchers are seeing very few juvenile trees in the affected region while the mature trees are dying off.