Baobabs are some of the most distinct and revered trees in Africa, living for hundreds, often thousands, of years and containing up to 500 cubic meters of wood. But scientists have discovered that an alarming number of these trees are dying — and they don’t know exactly why.

According to a new study published this week in the journal Nature Plant, nine out of Africa’s 13 oldest baobabs, and five of the six largest, have died over the past 12 years. Although the researchers don’t have a definitive reason for the deaths, they suspect a changing climate is playing a role, particularly Africa’s hotter, drier conditions in recent decades.

Baobabs are comprised of multiple stems and trunks that fuse together to form a single, ring-like plant. This structure often leaves a hollow center, one of which was so large that residents in South Africa’s Limpopo province built a pub inside of it. Adrian Patrut of Romania’s Babes-Bolyai University and his colleagues set out in 2005 to study how the trees are able to live so long and grow so large. Instead, they found a startlingly high mortality trend among the species’ oldest trees.

“It is very surprising to visit monumental baobabs, with ages greater than a thousand to two thousand years, which seem to be in a good state of health, and to find them after several years fallen to the ground and dead,” Patrut, lead author of the new study, told National Geographic. “Statistically, it is practically impossible that such a high number of large old baobabs die in such a short time frame due to natural causes.”