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Africa’s majestic baobab trees are mysteriously dying

Some of the continent’s oldest and biggest specimens collapsed suddenly during a study.

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Baobab trees standing against a dusk sky in front of the flat Makgadikgadi pans, Botswana.

The African baobab is one of the continent’s most recognizable tree species.Credit: Hougaard Malan/naturepl.com

Africa’s iconic baobab trees are dying, and scientists don’t know why. In a study intended to examine why the trees are so long-living, researchers made the unexpected finding that many of the oldest and largest of the trees have died in the past decade or so.

Noah Baker investigates the cultural significance of ancient baobabs’ sudden demise.

The African baobab tree (Adansonia digitata) is the oldest living flowering plant, or angiosperm, and is found in the continent’s tropical regions. Individual trees — which can contain up to 500 cubic metres of wood — can live for more than 2,000 years. Their wide trunks often have hollow cavities, and their high branches resemble roots sticking up into the air.

The researchers — who published their findings1 in Nature Plants on 11 June — set out to use a newly developed radiocarbon-dating technique to study the age and architecture of the species. Usual tree-ring dating methods are not suitable for baobabs, because their trunks do not necessarily grow annual rings.

The trees’ ages were previously attributed to their size, and in local folklore, baobabs are often described as being old, says study author Adrian Patrut, a radiochemist at Babeş-Bolyai University in Romania.

Periodic renewal

Between 2005 and 2017, Patrut’s team dated more than 60 trees across Africa and its islands — nearly all of the continent’s largest, and potentially longest living, known baobabs. To compare ages of different parts of the trees, the researchers collected samples of wood from the inner cavities and exteriors of the trunks and from deep incisions in the stems, which were then sealed to prevent infection.

Patrut and his colleagues say that their measurements suggest the trees live so long because they periodically produce new stems, similarly to how other trees produce new branches. The team says that over time, these stems fuse into a ring-shaped structure, creating a false cavity in the middle.

But, surprisingly, the scientists also found that most of the oldest and largest baobabs died during the study, often suddenly between measurements. Nine of the 13 oldest, and 5 of the 6 largest, baobabs measured died in the 12-year period — “an event of unprecedented magnitude”, says the study. The researchers found no signs of an epidemic or disease, leading them to suggest that changing climates in southern Africa could be to blame — but they stress that more research is needed to confirm this idea.

In one instance, the researchers observed that in 2010 and 2011, all the stems of Panke, a giant, sacred baobab tree in Zimbabwe, fell over and died. The team estimates that the tree was 2,450 years old, making it the oldest known accurately dated African baobab and angiosperm. Other trees across southern Africa also died completely, or had partial stem collapse.

Previous research2 has shown a decline in the number of mature baobabs and a lack of young trees in the region.

Age-old questions

Local experts welcomed the technique for dating baobabs, but some were sceptical of the team’s findings about the die-off. Michael Wingfield, a plant pathologist at the University of Pretoria in South Africa, says that the team’s sample was small and did not provide evidence that baobabs are not afflicted by an epidemic. “We know very little about baobab health,” Wingfield says. “There is much more to this picture than purely the fact that the oldest trees are dying.”

Sarah Venter, a baobab specialist at the University of Witwatersrand in Johannesburg, South Africa, says that her team’s ongoing research shows that baobabs may not be as drought resistant as previously thought — and this could be the cause of the deaths. But lower tolerance for drought would affect all the trees, not just the largest and oldest ones, she says.

doi: 10.1038/d41586-018-05411-7

References

  1. 1.

    Patrut, A. et al. Nature Plants https://doi.org/10.1038/s41477-018-0170-5 (2018).

  2. 2.

    Venter, S. M. & Witkowski, E. T. F. J. Arid Environ. https://doi.org/10.1016/j.jaridenv.2012.12.010 (2013).

Download references

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Transcript

Noah Baker investigates the cultural significance of ancient baobabs’ sudden demise.

[Witness and family singing a traditional song about the baobab]

Interviewee: Witness Konzanayi

Growing up we would name the baobab trees after, you know, maybe after their shape. If it is too ugly, say ‘this ugly one’, if it produces a fruit or sweet foods, we’ll say this one is ‘sweet mama’.

Interviewee: Chris Surridge

The baobab is a completely unique tree in lots of respects, those sort of massive trees that are just about as wide as they are tall. It’s probably the oldest lived, it’s certainly the largest of the angiosperms, these flowering plants.

Interviewee: Witness Konzanayi

It is a tree that every child will identify with.

Interviewer: Noah Baker

If you picture an African savannah, more likely than not you’ll picture a baobab tree. A solemn giant, somehow bulbous and spindly at the same time, often described as growing upside-down. Now, new research investigating the age of baobabs has shown that many of the largest and oldest trees in the world are dead or dying, and scientists don’t know why.

Interviewee: Witness Konzanayi

Once upon a time, a long time ago, the Creator invited all animals to his office so that he could give them trees, you know seedlings for planting.

Interviewer: Noah Baker

That’s Witness Konzanayi from the University of Cape Town in South Africa. He’s just finished his PhD on the governance of the baobab.

Interviewee: Witness Konzanayi

And the hyena of all animals was very lazy and he went to the Creator very late, and when he got there the only seedling that was left was that of a baobab tree. And he didn’t impress the hyena, so in anger the hyena took the seedling, threw it far away. That’s what happened, that’s how the baobab ended up with such an ugly shape.

Interviewer: Noah Baker

There are countless folktales like this one, each surrounding the baobab, many explaining its bizarre shape. Here’s Chris Surridge, the editor of Nature Plants with the scientific version.

Interviewee: Chris Surridge

Normally, trees grow, they have a trunk, and they pretty much have one trunk and they split off further up into branches. Now baobabs are a bit weird in that they through their life can produce additional trunks that come up out of the ground. You sometimes see suckers come out of things like blackberries, but trees do not do this. But the baobab does, it throws up these extra trunks, a ring of stems which then fuse together to form this empty centre. They become sort of circular but with a gap in the middle.

Interviewer: Noah Baker

This is the leading theory for how baobabs have ended up so fat, and also explains why, more often than not, they have huge cavities inside. This bizarre architecture leads to problems if scientists want to find out how old a tree is.

Interviewee: Chris Surridge

Normally, when you try and date a tree you have to chop it down and count the rings into the centre, or you put a bore in and you can do that and check these. But there’s no centre to a baobab tree.

Interviewer: Noah Baker

So, when researchers decided to measure the age of the largest baobabs known across the African continent, they had to turn to another method. Here’s Adrian Patrut from the University of Babeş-Bolyai in Romania.

Interviewee: Adrian Patrut

The only possibility to date a baobab is actually to radiocarbon date samples collected from each stem.

Interviewer: Noah Baker

Radio carbon dating is not uncommon when dealing with very old trees. In fact, trees are often used to calibrate carbon dating methods – because counting tree rings is such a reliable way of measuring age, it can be used as a solid point of comparison. Adrian Patrut’s team started surveying trees in 2005, and some of those they surveyed were truly ancient.

Interviewee: Adrian Patrut

The oldest trees were around 2,000 years old, and we found a specimen in Zimbabwe, the so-called Panke baobab, and we collected samples which were up to 2,450 years old.

Interviewer: Noah Baker

These are trees which sprouted before Aristotle even proposed the division of the sciences. Trees which were already centuries old when Julius Caesar took the throne in Rome.

Interviewee: Chris Surridge

This is just an incredible age. Trees are happy to grow for hundreds of years, but getting up to these sorts of millennial ages is something that the flowering plants and flowering trees just don’t do.

Interviewer: Noah Baker

It’s worth noting here that there are non-flowering trees which are even older. Now, when a baobab gets really large or old, it can take on a particular significance. Here’s Witness again.

Interviewee: Witness Konzanayi

To an African person who identifies with these trees, once a tree becomes this big, it becomes sacred. They become more venues for spirits of the land.

Interviewer: Noah Baker

This significance adds even more weight to another discovery that Patrut and his team made.

Interviewee: Adrian Patrut

It was very unexpected to find that many old and large trees die in a very short timespan.

Interviewer: Noah Baker

Specifically, they found that 9 of the 13 oldest trees measured, and 5 of the 6 largest trees, have all died in the last 12 years. Now, these trees are all spread across Africa, sometimes thousands of miles apart. There was no sign of disease and the revered trees are usually very well cared for – they all even had names. It therefore seems too much of a coincidence that all of these deaths could happen by chance so suddenly. In fact, Patrut claims that it’s impossible.

Interviewee: Adrian Patrut

Scientifically it is impossible for trees which have an age limit of over 2,000 years to die in such a large number over a such short timespan.

Interviewer: Noah Baker

It begs the question then – what’s causing the deaths of these Baobabs?

Interviewee: Chris Surridge

The obvious conclusion is that it’s something environmental, something to do with changes in climate. But again, that’s very difficult to nail down because over 2,000 years these trees have seen a great deal of climate. I mean, they’ve lived through the Little Ice Ages that happened in about the 1400s, 1500s, so they’ve seen much colder temperatures than now. They’ve seen droughts, they’ve seen practically floods, and yet they have carried through that. It is true that as far as we can tell, the temperature in these areas is warmer now than it has often been in the past, and it is also quite dry at the moment so maybe this is going on. But we really don’t know what it is that is killing these trees, if indeed this is an unusual amount of deaths.

Interviewer: Noah Baker

It’s an interesting scientific mystery. As an academic who works with the baobab, Witness too was intrigued, but for him speaking as a Zimbabwean, the findings also represented something else.

Interviewee: Witness Konzanayi

To ecologists it is just the dying of trees, but to an African person the death of such big trees means the death of culture, it means the death of identity, it means the death of spirituality.

[Witness and family singing a traditional song about the baobab]

Interviewee: Witness Konzanayi

Increasingly I think people are getting to know about climate change, even in the modest areas, in fact you don’t need to be told – you live it because you see your leaves are drying, we experience floods every year. But what I’m not sure of is if people are able to relate the deaths of these trees to climate change. If the big trees are dying I think what we need to do is to quickly establish what the cause is, because for some communities, the baobab tree defines who they are.

[Witness and family singing a traditional song about the baobab]