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Dating Africa’s giants reveals far more than just age
Dating a fallen baobab giant from the Pafuri Section of Kruger National Park has led CSIR senior researcher Dr Stephan Woodborne to two surprising discoveries.
First, at 1 000 years the tree was surprisingly old considering its stature: a mere 14.2 m in circumference and 4.2 m in diameter. While some specimens are many times larger, the oldest specimen to date seems to have been no more than 1 300 years old. It would seem that, despite their size, most baobabs are not many thousands of years old as is sometimes suggested. On average they live to about 1 500 years, which is more than three times the age of most species.
The second discovery was that the stem of the tree stopped growing about 530 years ago. According to Woodborne, it is highly unlikely that baobabs fluoresce each year for over 500 years, but simply cease to grow. ‘It could be that, while annual growth rings are not formed on the stem in the tree’s latter years, growth rings may well be formed on the upper limbs of the tree.’
This was not Woodborne’s first venture into dating Africa’s giants. In an article in <i>Applied Geochemistry</i> published in 2006, he described scientist’s fascination with one of Africa’s most recognisable tree species.
‘In 1749, the French naturalist Michel Adanson examined two living trees on the Magdalene Islands off Cape Verde on the coast of Senegal, which he estimated to be 5 150 years old. However, the African missionary and explorer, David Livingstone, questioned these calculations as a tree of this age would pre-date the Great Flood of the Old Testament. Assuming annual growth rings, Livingstone calculated that a baobab 26 m in circumference was 14 centuries old and consequently within the Christian era.’
Yet the genus is poorly known. Up to the 1970s, just about every authority on the subject stated categorically that baobabs do not form annual rings and could therefore not be dated by ring counting (dendrochronology). Even if the rings were assumed to be formed annually, baobabs retain a great deal of water in their trunks, making the wood prone to shrinking and deformation, which in turn makes the analysis of ring width as a proxy for environmental conditions difficult, if not impossible.
With the advent of modern chemistry and the introduction of radiocarbon dating methods, dating became easier. By using this method of dating, Woodborne demonstrated that the baobab’s rings were indeed annual and he demonstrated that 500 of the rings in the Pafuri baobab appear to be missing. ‘If every ring represents one year of growth and we know when the tree died, then, according to conventional ring counting, we know what the age of Ring 40, for example, should have been. However, with carbon dating we were quite surprised to find that Ring 40 was 490 years older than it should have been.’
Another bonus of employing radiocarbon dating is that baobabs can now be used to provide an understanding of past changes in the Earth’s climate systems. If a baobab is over 1 000 years old, every annual ring stores information on the climatic conditions of the environment during that year’s growth cycle. Was it drier than usual? Wetter than usual? Warmer or colder?
Woodborne measured carbon isotope in each ring to determine the relative rainfall during the year that the ring formed. According to Woodborne, trees have developed quite a neat process to control carbon isotopes. In the 2006 study of baobabs from Kruger National Park, carbon isotopes were shown to reflect the ability of trees to regulate water loss through their leaves. ‘When water is abundant they transpire a great deal of water. When it is dry, they conserve water and transpire less through their leaves. The mechanism that controls the water flow is called stomatal regulation and this has a side effect: it affects the isotope ratio of the carbon dioxide that flows into the leaves in the opposite direction to that of the departing water. Therefore, the carbon isotopes in the growth ring reflect the degree of transpiration that took place. It is quite neat actually.’
Woodborne sampled some 300 rings of the Pafuri baobab. This provided an inter-annual record of rainfall from the year 1165 to 1475. ‘This record is critical in understanding the environment in which our Iron Age ancestors lived. The record shows, for example, the occurrence of a succession of droughts at the time that the Kingdom of Mapungubwe went into decline,’ says Woodborne.
Woodborne and his co-workers from SANParks presented the latest findings of their research at the Savanna Science Network Meeting in the Kruger National Park on Wednesday 10 March 2010.
Source: CSIR Strategic Communication and Stakeholder Relations
Caption: CSIR Isotope technician Grant Hall putting a sample of one of the annual rings from the Pafuri baobab in a tube in order to extract the carbon from it.
[insert baobab incinerated]
The wooden sample from the baobab is incinerated at a very high temperature in these tubes in order to capture the carbon dioxide.
[insert baobab pure carbon]
Pure carbon – the carbon dioxide goes through various filtration process until the pure carbon is captured in this tube.
[insert baobab-4-sep rings and baobab fallen]
The annual growth rings of the fallen Pafuri baobab was carefully separated with a spatula and then numbered, the outer layer being the most recent and the inner rings the original core of the tree.
Yes – as long as you also acknowledge the original source.