Baobabs are the oldest and largest flowering plants in the world, yet some of the oldest have started dying suddenly over the last 12 years. LouieLea/Shutterstock

The baobab is possibly the most distinctive species of tree on Earth, and the longest-living member of the flowering plants. Yet trees that have survived for 2,000 years have suddenly started dying all at once. We can't yet be sure of the cause, but it's almost certainly human-induced, and climate change is the prime suspect.

Baobabs' unique shape makes them an iconic symbol of Africa. Although far from the tallest of the angiosperms – the group that includes all flowering plants – their enormous trunks make them the largest. Baobabs can contain 500 cubic meters (17,000 cubic feet) of wood. The enormous central spaces that often form inside their trunks have been used as homes, jails, and drinking venues.

An international team of botanists set out to study Africa's largest and oldest baobabs finding more than 60 outstanding examples. They collected and sampled the wood extensively, including its inner cavities, and used accelerator mass spectrometry for carbon dating, revealing the trees' growth history. This technique reveals the growth of multi-stemmed trees in a way that studying tree rings – appropriate for trees with a single stem – cannot. The team did indeed learn some interesting things about the way these mighty trees grow, which they have reported in Nature Plants.

However, the same paper reveals something far more unexpected and disturbing – five of the six largest trees recorded, and nine of the 13 oldest, have either died over the last 12 years or suffered the collapse of their oldest stems. Given the vast age of these trees, so many dying at once can't be a coincidence, but the authors admit they are not yet sure of the reason.

All the oldest trees were from southern Africa, but spread out over thousands of kilometers, so the cause cannot be local. Moreover, there are widespread reports of other old baobabs dying, and no signs of a fungal or insect epidemic, making climate change the most likely culprit.

In June 2015, the famous Chapman's Baobab was 1,400 years old and apparently in good health. By January 2016 all six stems were dead. Patrut et al/Nature Plants

When it comes to the things the research originally intended to discover, the authors found baobabs usually start growing as single-stemmed trees. However, the paper observes; “Owing to the baobabs’ ability to periodically produce new stems, in much the same way other tree species produce branches,” all large baobabs are multi-stemmed, developing increasingly complex architecture as they grow.

The authors discovered baobabs use what they call a “ring-shaped structure” to achieve their mighty sizes. This is obvious when they are poorly fused at the base with shapes resembling a cylinder. Frequently, however, the stems have fused so well that only carbon dating can reveal that they are separate stems of different ages. Sometimes the tree's center hollows out from fire and fungal attack, but more often the central space was never filled, with stems instead having sprung up around it.