"Something obviously is going on in nearly selectively affecting the largest and oldest", Thomas Lovejoy, an environmental scientist and Amazon rain forest expert at George Mason University, wrote in an email comment on the study.
Despite these trees having many stems and trunks of different ages, the stems of many had died suddenly. But during their study period, the researchers discovered that the oldest and largest had died. The work also addresses the mystery of why so many of these unusual trees are dying. While baobabs typically begin growing as single-stemmed trees, they produce new ones over time, developing increasingly complex structures. Found in savanna regions of Africa, Madagascar and Australia, the trees form a very thick and wide trunk and mainly branch high above the ground.
Some of the largest trees have died over the last 12 years. Baobobs grow in unusual ways, often with hollows, making it hard to gauge precise ages, but the research team says the trees in the survey range in age from 1,000 to 2,500 years, reports NPR.
All of the dead or dying baobabs detailed in the latest study are located in Zimbabwe, Namibia, South Africa, Botswana and Zambia, and all are between 1,000 and more than 2,500 years old.
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Adrian Patrut, a co-author of the study and an academic at the Babes-Bolyai University in Romania, said: "It is definitely shocking and dramatic to experience during our lifetime the demise of so many trees with millennial ages".
The tree named after South African hunter James Chapman, who visited it in 1852, saw all six its stems topple simultaneously on January 7, 2016 where it had stood for some 1,400 years.
(They do refer to other baobab mortality but dont have real data on it), Lovejoy continued. This included the Platland baobab and a few trees that appeared, by Patrut's calculations, to be more than 2,000 years old. Increased temperature and drought are the primary threats, Patrut told BBC News.
While the reasons behind the trees' sudden and apparently concurrent difficulties remains unclear, the researchers said they suspect the demise "may be associated at least in part with significant modifications of climate conditions that affect southern Africa in particular". They have been surveying the trees since 2005 and have developed a theory of how they grow, while also documenting the losses.
The southern African countries where the trees died are warming faster than the global average.
"The decline and death of so many large baobabs in recent years is so tragic", Baum says.