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Africa’s ‘Tree of Life’ is dying
2 July 2018

Baobabs have graced the African landscape for thousands of years, reaching great heights of over 30 metres, stretching across 32 countries on the continent and its islands. These incredible giants serve their environment and local communities, having over 300 uses. But a 12-year international study that the University of Pretoria (UP) has been a part of has found that these ancient trees, described as the ‘Tree of Life’ are dying.

Researchers focused their investigation on superlative individuals – which included some of the oldest and largest African baobabs across the continent. Nine of the 13 oldest and five of the six largest individuals died during the 12 years that the study was conducted in, in southern Africa. These were some of the most well-known trees in southern Africa, not only prized by local communities, but also popular tourist attractions and top rankers on the Champion Tree Lists. Their deaths are a fairly recent phenomenon, and while the cause still needs to be confirmed, researchers believe climate change is a major contributing factor.

During the study period, researchers obtained records of climatic conditions for the past 1 000 years, particularly focusing on rainfall patterns. These records provide evidence that the southern African climatic conditions have and continue to change. While these changes affect general biodiversity, there are some species that are most sensitive to climatic changes, such as baobabs. ‘We did not find much evidence for other causes that could be affecting baobabs, such as bacterial or fungal infections[1],' said Dr Grant Hall of UP’s Stable Isotope Laboratory in the Mammal Research Institute, who has been part of this study. An important factor to note is that the southern African baobabs are growing on the southern-most edge of their distribution in Africa and as such are more likely to be affected by any climatic changes. ‘As distribution ranges are altered, and in this case shrinking, there is a higher likelihood that the individuals existing on the edges are dying off because they are at the limits of their ecological tolerances,’ explained Dr Hall.

The in-depth study started in 2005 to gain more information about the poorly understood aspects of the architecture, growth and age of the African baobab, particularly focusing on baobabs in Namibia, Botswana, South Africa and Zambia, as well as southwestern Madagascar. Using a new approach, the study was not limited to fallen trees, but also investigated living trees. ‘The approach consists of accelerator mass spectrometry (AMS) radiocarbon dating of small wood samples collected from inner cavities and/or from different areas of their trunk/stems,’ explained Dr Hall. This is a method for determining the age of the tree by using properties of radiocarbon.

Baobab tree in Thuli Block, Botswana. Photo: Dr Grant Hall

This international collaboration relied heavily on the Stable Isotope Laboratory to conduct tests on small wood samples of the trees, gaining insight into records of past rainfall and other climatic conditions for southern Africa for the past 1 000 years, using stable carbon isotopes. These analyses suggest changes to rainfall patterns over the past 1 000 years, while the increase in average temperatures may be playing a significant role in their demise.

On studying the architecture of the African baobabs, other incredible findings were revealed. Dr Hall and his colleagues have come to better understand the structure and growth patterns that allow these trees to become so old and so large. These life-giving trees grow as they do, reaching ages of over 2 000 years because of their ability to shift from being a single-stemmed tree to a multi-stemmed tree by the growth of new stems. ‘The production of extra stems provides additional support, as these stems often fuse together at the base of the tree. They also produce additional tissue in their branches to provide buttressing or support for their heavy limbs, explained Dr Hall.

While greater insights have been discovered, the deaths of most of the oldest and largest African baobabs over the past 12 years have been described by the research team as an event of ‘unprecedented magnitude’. While it may be that these magnificent giants have come to the end of their lifespan, the fact that there have been reports of a rapid increase in deaths in many other mature baobabs in recent times, suggests an external factor must be the cause of southern Africa’s baobabs dying.

As their analysis on climatic conditions suggests, there are some serious implications for so many baobabs dying. A worst-case scenario may be that in the future, southern Africa may lose its baobabs completely as their distribution range is forced to shift more northwards. What does this mean for the southern African environment? Dr Hall warns, ‘the loss of these trees may serve as a warning, global climate change will not only affect baobabs in southern Africa, but other iconic long-lived tree species which may be lost or reduced in their distribution as well. We need to begin to plan for such changes as these will not only affect biodiversity, but have a huge impact on society in general.’

[1] Dr Hall said there is a type of black mould that can infect baobabs, but this mould does not result in the death of baobabs. Research into this has been conducted by UP’s Forestry and Agricultural Biotechnology Institute.

 

- Author Louise de Bruin
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Last edited by Primarashni GowerEdit
Dr Grant Hall