AMS radiocarbon dating of the largest and oldest African baobabs of Senegal

Adrian Patrut¹, Roxana Patrut², Stephan Woodborne³, Karl von Reden⁴, Oumar Ka⁵, Sébastien Garnaud⁶, Daniel Lowy⁷

¹ Babeş-Bolyai University, Faculty of Chemistry, Cluj-Napoca, Romania.

²Babeş-Bolyai University, Faculty of Biology and Geology, Cluj-Napoca, Romania.

³ iThemba LABS, Private Bag 11, WITS 2050, South Africa.

⁴NOSAMS Facility, Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution,

  Woods Hole, MA 02543, U.S.A.

⁵ Faculty of Sciences and Techniques, Cheikh Anta Diop University, Dakar, Senegal.

⁶INECOBA, Aulnay-sous-Bois, France.

⁷Nova University, Alexandria Campus, Alexandria, VA 22311-509, U.S.A.

Abstract. The scientific name Adansonia digitata honours Michel Adanson, who documented and described the African baobab in its native habitat after his voyage to Senegal (1749-1754). Even if Senegal hosts more baobab specimens than all countries from southern Africa combined, the number of significant investigations and surveys of Senegalese baobabs was scarce over the past 50 years.

The aim of our investigations by AMS radiocarbon was to determine the architecture and age of the largest and potentially oldest Senegalese baobabs.

Senegal hosts 6 very large baobabs with girth values over 23 m, i.e., the baobab of Warang (28.69 m), of Lalam, called “Ngokole” (28.10 m), of Sinthiou-Keita (27.10 m), of Diyabougou/Samba Dia (26.31 m), of Nianing (24.76 m) and of Iwol (23.52 m). Our investigation has revealed that they are all multi-stemmed, with up to 14 (Ngokole) and even 16 stems (Warang). Some of them exhibit closed ring-shaped structures, with one ring and one false cavity (Diyabougou, Warang) and even 3 rings and 3 false cavities (Nianing). The large Senegalese baobabs have often false stems, which emerge from another adjacent stem, and act as anchor in sandy soils. The false stems have a peculiar aspect, have a triangular or trapezoidal horizontal section and may reach lengths up to 5 m. Their oldest age can be found toward the upper contact area with the larger adjacent stem. According to radiocarbon dates, the 6 large baobabs exhibit ages between 400-700 yr.

The oldest baobab of Senegal and of the Northern Hemisphere is the historic “Gouye Ndiouly” of Kahone, the old capital of the Saloum kingdom. The baobab toppled ca. 200 yr ago and only one old stem survived, from which 7 new stems emerged. The oldest dated sample had a radiocarbon date of  833±25 BP. This value suggests an age of 1000 yr in the missing pith.

The research was funded by the Romanian Ministry of National Education CNCS-UEFISCDI.

AMS radiocarbon dating of baobabs from dwarf baobab groves

Adrian Patrut¹, Stephan Woodborne², Roxana Patrut³, Laszlo Rakosy³, Grant Hall⁴, Karl von Reden⁵, Pascal Danthu^6,7, Jean-Michel Leong Pock-Tsy⁴, Dragos Margineanu¹

¹ Babeş-Bolyai University, Faculty of Chemistry, Cluj-Napoca, Romania.

² iThemba LABS, Private Bag 11, WITS 2050, South Africa.

³Babeş-Bolyai University, Faculty of Biology and Geology, Cluj-Napoca, Romania.

⁴Stable Isotope Facility, Mammal Research Institute, University of Pretoria, South Africa.

⁵NOSAMS Facility, Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution,

  Woods Hole, MA 02543, U.S.A.

⁶Cirad, DP Forêt et Biodiversité, Antananarivo, Madagascar.

⁷Cirad, UPR  BSEF, Montpellier, France.

Abstract. The African baobab (Adansonia digitata) and the Grandidier baobab (Adansonia grandidieri) are the biggest of the nine Adansonia species. The mature specimens typically have heights of 15-25 m and girth values of 7-25 m.

There are 3 isolated groves in the world consisting exclusively of dwarf baobabs, with peculiar aspect and heights lower than 10 m. Two groves, located on Magdalene Island (Senegal) and Mannar Island (Sri Lanka), are populated with dwarf African baobabs. The third grove, near the village of Andavadoaka (Madagascar), is the Forest of bottle Grandidier baobabs. All 3 groves are very close to the ocean shore.

Our research is the first investigation of the architecture and age of dwarf baobabs.

The baobabs of Magdalene Island, in front of Dakar, were first investigated in 1749 by Adanson. In 1797, British sailors vandalised the island and blew up almost all dwarf baobabs. Today, the Magdalene Island (15 ha) hosts 60 baobabs, mostly multi-stemmed with heights of 4-7 m. Some baobabs combine elements specific to trees and shrubs. We dated the two biggest baobabs, the “Parasol” and “Lébou baobab”, which are 250 and 375 yr old. 

There are only 22 remaining dwarf baobabs in the E part of  Mannar Island (117 km²). Most of them are considerably bigger, in terms of girth and volume, than those of Senegal. The largest is the “Big Biobab” of Mannar Town (height 9.0 m, girth 20.01 m, volume 100 m³). It consists of 7 fused stems, that build 2 rings which close 2 false cavities. The oldest sample had a radiocarbon date of 607±22 BP. This value suggests an age of 750-800 yr for the Big Biobab.  

The baobab forest near Andavadoaka consists of up to 200 fat dwarf trees, with heights of 5-8 m. The old specimens are multi-stemmed and have many hollow parts inside, resembling Swiss cheese pieces. The largest, called “The jar” (7.1 m, 13.52 m, 55 m³), is 850-900 yr old.

The research was funded by the Romanian Ministry of National Education CNCS-UEFISCDI.

Millennial rainfall variability in southern Africa from trees dated by AMS

Dr. WOODBORNE, Stephan (iThemba LABS); MULLINS, Simon (iThemba LABS); Dr. MBELE, Vela (iThemba LABS); Ms. HAMILTON, Tamryn (University of the Witwatersrand); Mrs. RAZONATSOA, Estelle (University of Cape Town); Prof. PATRUT, Adrian (Babeș-Bolyai University); Dr. WINKLER, Stephan (iThemba LABS)

Abstract. The instrumental record of rainfall in southern Africa is short and fragmented, and is of little use in determining the drivers of climate variability in the region. In other regions tree ring widths have been useful in reconstructing past climate, but there are few chronologies of centennial duration for southern Africa. This is because subtropical tree rings are formed sub- or super-annually and do not necessarily form rings with widths that proxy meaningful climate variables. Trees can nevertheless be used as a climate proxy using alternative approaches. In baobab (Adansonia digitata) and Camelthorn (Vachellia erioloba) trees, the leaf-level physiology that regulates water uptake also regulates carbon assimilation, and as a result the stable carbon isotope ratios (¹³C/¹²C expressed as δ¹³C) are a proxy for rainfall. The chronology for the δ¹³C proxy rainfall records must be derived using AMS radiocarbon dating because of the lack of ring resolution. “Wiggle matching” of both radiocarbon and the climate record allows the age model resolution to be reduced to approximately 5-10 years. The approach has yielded several proxy records spanning the last 1000 years, and multiple shorter records. The temporal and spatial variability in southern African climate is linked to global-scale climate forcing such as the el Nino Southern Oscillation, the Southern Annular Mode and localized sea-surface temperatures. The record also provides important insight into the archaeological trajectory of the region.

Comparing a high spatial/temporal resolution rainfall proxy dataset from southern Africa

with a last millennium simulation

Session: Benchmarking & cross-cutting Group 1 (Isotope modelling, COMPARE)

Authors:

-Stephan Woodborne, iThemba LABS, Johannesburg, South Africa; Stable Isotope Laboratory, Mammal Research Institute, University of Pretoria, Pretoria, South Africa;
-Qiong Zhang, Department of Physical Geography and Bert Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden;
-Grant Hall, Stable Isotope Laboratory, Mammal Research Institute, University of Pretoria, Pretoria, South Africa;
-Tamryn Hamilton, Department of Animal, Plant, and Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa;
-Adrian Patrut, Babeş-Bolyai University, Faculty of Chemistry and Chemical Engineering, Cluj-Napoca, Romania;
-Roxana Patrut, Babeş-Bolyai University, Faculty of Biology and Geology, Cluj-Napoca, Romania;

-Estelle Razanatsoa, Plant Conservation Unit, University of Cape Town, Cape Town, South Africa;
-Christiaan Winterbach, Tau Consultants, Maun, Botswana;

-Stephan Winkler, Tau Consultants, Maun, Botswana;

Abstract. Edaphic soil moisture potential is the main determinant of leaf-level carbon isotope discrimination in savanna trees of southern Africa, and a proxy rainfall record can be obtained from radial (time series) analysis of carbon isotope ratios in trees in the region. The approach has been tested in baobab trees (Adansonia digitata, A. za, and A. grandidieri), Black Monkey Thorn (Acacia burkei) and Camelthorn (Acacia erioloba) trees. This diversity of species allows the proxy to be applied across a range of xeric conditions from the arid Namib Desert in the west, through the Kalahari Desert to Madagascar. In the mesic regions a record has been generated from Yellowwood (Afrocarpus falcatus) trees. The lack of annual rings in the stems of most of these tree species makes it necessary to generate age models using radiocarbon dates which introduces a degree of error in the age assigned to each sample. In general the sampling resolution is sub-annual, but the age error only allows decadal to centennial trends to be inferred. The outcome is a time/space matrix of rainfall variability in the region over the last 1000 years. For each sample site a composite record is generated from multiple trees. Comparison between the carbon isotope ratio proxy and the short, patchy coverage with instrumental records provides strong support for the authenticity of the proxy record. The tree record indicates synoptic scale variability in response to climate forcing. In some instances the rainfall anomalies have the same sign across the entire region, and in others there is a clear dipole response with opposite sign anomalies in different regions. The underlying forcing is attributed to north/south and east/west displacement of the main rainfall systems. These dynamics provide a tangible basis for testing model climate simulations. The EC-Earth last millennium simulation of these displacements in response to the inferred climate forcing well matches the pattern observed in the tree records. The result suggests that future climate change scenarios for southern Africa are accurately captured in the climate simulation models.

AMS RADIOCARBON DATING OF VERY LARGE

AFRICAN BAOBAB TREES FROM SAVÉ VALLEY, ZIMBABWE

Authors. Adrian Patrut¹*, Roxana T. Patrut², Laszlo Rakosy²,  Karl F. von Reden³, Daniel A. Lowy⁴, Dragos Margineanu¹

Affiliations.

¹Babeş-Bolyai University, Faculty of Chemistry, Cluj-Napoca, Romania.

²Babeş-Bolyai University, Faculty of Biology and Geology, Cluj-Napoca, Romania.

³NOSAMS Facility, Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, U.S.A.

⁴Nova University, Alexandria Campus, Alexandria, VA, U.S.A.

Abstract. The Savé Valley Conservancy is a large wildlife area (3442 km²), located in the semi-arid South East Lowveld of Zimbabwe. The Conservancy hosts thousands of African baobabs, out of which 4 specimens located all to the north of Turgwe river, have very large dimensions (circumference over 23 m) and ages older than 1000 years.

Several tiny wood samples were collected from the inner cavities and/or from different areas of the trunk of the 4 baobabs. The samples were investigated by AMS radiocarbon dating for determining the architecture and the age of the oldest part of the baobab specimens.

Somewhat surprisingly, the two oldest baobabs are basically unknown in the literature and are not included in the registers of the Tree Society of Zimbabwe.

In 2011, a very old baobab was discovered in the Bedford Block of the Humani Ranch. The Humani Bedford baobab (circumference 23.65 m; wood volume 240 m³) has a closed ring-shaped structure, which consists of three fused stems that close partially a false cavity. A fourth stem toppled more than one century ago, thus opening the false cavity. The oldest dated sample has a radiocarbon date of 1655 ± 14 BP, which corresponds to a calibrated age of 1575 ± 30 yr. According to this value, the Humani Bedford baobab is around 1800 years old and becomes the oldest living African baobab and angiosperm.  

The second oldest baobab of Savé Valley is located in the Matendere Ranch. The Matendere Big baobab (26.30 m; 300 m³) has a closed ring-shaped structure, with 5 stems that incorporate a false cavity. The false cavity has only a very tall opening, at a height over 6 m. The oldest sample has a radiocarbon date of 1529 ± 14 BP, which corresponds to a calibrated age of 1430 ± 45 yr. This value suggests an age of 1600 years for the Matendere baobab.

The two biggest and best known baobabs of Savé Valley are located in the Mokore and Chishakwe Ranches. According to our investigation, the Mokore Giant baobab (28.11 m; 320 m³) possesses 7 stems of different sizes and ages and a large false stem which acts as a structural support/anchor. It has a closed ring-shaped structure, with a ring composed of 4 stems that close a false cavity. The cavity has an opening at the height of 5 m.

The Chishakwe baobab (26.56 m; 375 m³) consists of 7 fused stems. It has a closed ring-shaped structure, with a ring composed of 5 or 6 stems that close completely a false cavity inside. The cavity has only a tall opening at the height of 7-8 m.

The two biggest baobabs of Savé Valley are younger, having ages of only 1100–1200 years. These values confirm that the largest baobabs are not necessarily the oldest.

The AMS radiocarbon investigations were performed at the NOSAMS Facility of the Woods Hole Oceanographic Institution.

The research was funded by the Romanian Ministry of Research and Education CNCS-UEFISCDI under grant PN-III-P4-ID-PCE-2016-0776, No. 90/2017.

MAIN RESULTS OF THIRTEEN YEARS OF

RADIOCARBON INVESTIGATION OF LARGE AND OLD AFRICAN BAOBAB TREES

Authors. Roxana T. Patrut¹*, Adrian Patrut², Stephan Woodborne³, Laszlo Rakosy¹,  Karl F. von Reden⁴, Daniel A. Lowy⁵, Grant Hall⁶, Ileana-Andreea Ratiu¹

Affiliations.

¹Babeş-Bolyai University, Faculty of Chemistry, Cluj-Napoca, Romania.

²Babeş-Bolyai University, Faculty of Biology and Geology, Cluj-Napoca, Romania.

³NOSAMS Facility, Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, U.S.A.

⁴Nova University, Alexandria Campus, Alexandria, VA, U.S.A.

Abstract. In 2005, we started an in-depth research project to elucidate several controversial aspects concerning the architecture, growth and age of the African baobab (Adansonia digitata). This research is based on our new approach which enables the investigation of standing live specimens. Our approach consists of AMS radiocarbon dating of small wood samples collected especially from inner cavities, but also from deep incisions/entrances in the stems, fractured/broken stems and from the outer part/exterior of large baobabs.

The obtained results were unexpected and surprising, showing that the African baobab has several unique features. Here we disclose the main findings of our research.

Radiocarbon results demonstrate that big baobabs are always multi-stemmed, having up to 18 stems. This is a consequence of the baobabs’ ability to generate new stems periodically, such as other tree species produce branches. Hence, baobabs develop over time architectures of increasing complexity. Therefore, we focused on the investigation of very large and potentially old specimens.

We identified the open and closed ring-shaped structures, which are the most significant architectures that enable African baobabs to reach old ages and large sizes. According to dating results, open and closed ring-shaped structures form progressively and close over time, as they usually consist of 3-8 stems belonging to several generations.

Many old baobabs have large hollow parts in the central area of their trunk/stems. In most cases we found that the age sequence of samples extracted from the cavitiy shows a continuous increase from the cavity walls up to a certain distance into the wood, after which it decreases toward the outer part. The only reasonable explanation for this age anomaly is that such cavities are in fact false cavities, i.e., natural empty spaces between fused stems disposed in a closed ring-shaped structure. The oldest part of the fused stems is located between the false cavity walls and the outer part/exterior of each stem.

For certain baobabs the outermost rings were found to have ages of several hundreds of years, instead of being very young. Such results show that baobab stems can stop growing due especially to old age or stress factors.

Radiocarbon results indicate that several reported stems, which are triangular in horizontal section, are in fact false stems which act as an anchor. The oldest age can be found toward the upper contact with the larger adjacent stem, while the age decreases toward the opposite sharp extremity.

In several cases, we found an anomalous ring frequency. For these cases, the number of rings between two dated segments of a sample was found to be significantly lower or significantly higher than the calendar age determined by radiocarbon dating.

The oldest dated sample had a radiocarbon date of 2429 ± 14 BP. By this value, the African baobab becomes the angiosperm with the longest life span.

The AMS radiocarbon investigations were performed at the NOSAMS Facility of the Woods Hole Oceanographic Institution.

The research was funded by the Romanian Ministry of Research and Education CNCS-UEFISCDI under grant PN-III-P4-ID-PCE-2016-0776, No. 90/2017.