Trees as weathermen?

Vachellia and Welwitschia, weathermen of the past:
QRS has begun a project to evaluate the radiocarbon dating potential of Vachellia and Welwitschia plants in the Central Namib Desert for use as recent palaeoclimate proxies.

Norman Green and staff from Swakop Uranium gathered at QRS headquarters on Sunday, May 13th, 2012 to test two different drill designs for extracting heartwood from an old  camelthorn tree (previously Acacia erioloba, now Vachellia erioloba. For an excellent explanation of the taxonomic reasons for the name change, follow this link: safari-ecology.blogspot.com ).

This experiment prepares for a pilot study run to evaluate the potential of  dating Vachellia and Welwitschia plants as climate proxies in the Central Namib Desert. Climate reconstructions based on speleotherm studies provide a sequence of wet and dry periods: if the trees are dated, their germination may be related to rainfall patterns over the past 1 000 years. Human settlement patterns in the Namib are also tied to this climatic sequence.

Two previous studies have explored facets of this idea: John Vogel, who established the radiocarbon dating laboratory at the CSIR in South Africa, dated 12 trees from both Sossus and Tsondab Vleis (Vogel 2003). These dates suggested that the oldest trees became established in the Medieval Warm Period (11th to 12th centuries), and began to die off when the more arid conditions of the Little Ice Age set in during the 14th century.He suggested that the younger trees represented a brief climatic amelioration in the 17th century.

A second study (Kinahan & Kinahan 2006) dated four Vachellia trees in the Awasib-Gorassis basin, and related the germination of the trees to periods of above average rainfall and to stratigraphic dates from an excavation of archaeological deposit in a rock shelter from the same locality. Periods of higher rainfall allowed hunter-gatherer communities to move into this arid area to hunt oryx and harvest grass seed for making porridge and beer.

Although there are many claims as to the antiquity of the Welwitschia, its dating has proved inconclusive. Direct radiocarbon dating could, for example, resolve whether the Giant Welwitschia, a National Heritage monument in the vicinity of the uranium tenements, really is 2500 years old, or less than 1000, as the other tree dates suggest.

The implications of the new study, supported by Swakop Uranium, are important and exciting for our knowledge of climate change, re-population of the Namib in the Holocene, and conservation: the ancient trees may be living relics of shifts in rainfall patterns in the desert over the past millenium – weathermen of the past, as it were.

Direct dating of the tree requires a few grams of its oldest dead wood. The best and most accurate method chosen for the project is Accelerator Mass Spectrometry (AMS) dating, as very small samples (300 mg after cleaning) are required.  In the case of the Welwitschia, the oldest part of the tree is thought to be dead tissue forming a rough layer of bark on the outside of the plant, or within the trunk lobes. Samples can be removed without damaging the plant. However, the oldest wood of the Vachellia is the heartwood, and requires a drill bit of at least 50 cm to extract a sample from the core of the tree. Where possible, fallen trees with exposed cores can be sampled, but standing trees will have to be drilled. To avoid destruction to the tree, the drill holes will be filled afterwards with polyurethane foam and sealed  with silicon.

One drillbit design is based on the screw principle whereby cortex from inside the trunk is corkscrewed to the outside in fragments, the drilled hole being cleaned each time until the centre is reached.

The alternative is a dowel plug cutter, extended to 50 cm to extract a single cylindrical core. The nail-biter with this was whether the core could be extracted from the trunk in one piece – or at all.

 

The results of drilling: top right is from John’s ‘corkscrew’ bit; bottom left is Norman’s, showing the cylindrical core still in place

The drilling efforts were lubricated with generous amounts of food and drink sponsored by Swakop Uranium. Engineers braaied succulent wors, chops and steak alternately with advising, commenting on or assisting with the drilling. Enthusiasm was high and the sequence of samples were rigorously bagged and labelled. Swakop Uranium Environmental Manager Michelle Kilbourn-Louw handed these to John with due ceremony punctuated with ribald comments. Company photographer Charles Corbett recorded the process with interest.

 

The next stage in the project is the collection of samples for dating from the lower Khan River valley and Welwitschia Plains, and a permit to carry out the sampling has been issued by the Ministry of Environment and Tourism.

References:
Kinahan, J & Kinahan, J. 2006. Preliminary report on the late Holocene archaeology of the Awasib-Gorassis basin complex in southern Namibia. Studies in the African Past (Dar es Salaam) 5: 1-14. This article may be downloaded in pdf from “Downloads”.

Vogel, J.C. 2003. The age of dead trees at Sossus and Tsondab Vleis, Namibia. Cimbebasia 18:247-251.

 

Jill

About Jill

Jill is a historical archaeologist interested in contact studies. She has worked with John in Namibia for more than thirty years, sharing an interest in the history of nomadic desert communities and delighting in raising their son Tim. She assists with running the Namibia Archaeological Trust and Quaternary Research Services. After being the copy-editor for the South African Archaeological Bulletin, she now works for UNAM Press as Editorial and Production Manager. Through this web site they hope to share research results and ideas.
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