Population viability, ecological and genetic diversity assessment of the multipurpose baobab tree (Adansonia digitata) in Benin

ir. Achille Assogbadjo

Table of Contents

  1. General introduction on the baobab tree
  2. Problem statement
  3. Justification of research
  4. Objectives
  5. Methods
  6. Expected Output
  7. Relevance for Development

Generality on the baobab tree

The African baobab tree and its related species belong to the family Bombacaceae and the genus Adansonia. The tribe, which is pantropical, includes Bombax and Ceiba with species producing fruits fibres used as kapok. The family includes about 30 genera, six tribes and about 250 species. A number of these species are used locally for wood, fruits, seeds or gum. The distribution of the African baobab (Adansonia digitata) is limited in Africa. Adansonia possesses a unique fruit type with a woody pericarp surrounding a spongy pulp with reniform seeds. African baobab is a very long-lived tree with multipurpose uses. It is thought that some trees are over 1000 years old. Since it is not grown agronomically nor properly domesticated, there are no known varieties. Adansonia digitata is related to 7 other species that have not been well investigated, except for descriptions in floras. Most scientific references to these species date from 1960s.

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Problem statement

Adansonia digitata is a key economic tree used daily in the diet of rural communities in West Africa (Codjia et al. , 2001). Economic pressure, poor investment in designing and implementing forest management, inadequate legislation concerning sustainable non-timber resource exploitation, together with a lack of sufficient knowledge on the resources themselves are the most important depletion causes of these resources. Moreover, our investigations in several agro-ecological zones in Benin showed not only an absence of seedlings and saplings of baobab but also some morphological variability related to the species according to the climatic zones. These remarks also apply to Mali where farmers that use their own system to distinguish several varieties of baobab taking into account bark colour, pulp and leaf taste, or height and width of the tree are frequently observed (Sibidé et al., 1996). Surprisingly, quantitative information related to population viability and genetic aspects of baobab tree are poorly documented (Sidibe & Williams, 2002). The present investigation aims to fill these gaps.

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Over the world, abundance of literature on baobab is a proof of its importance. In fact, baobab is a multi-purpose species, used daily by populations in Africa. Concerning the economical aspects, a three months market investigation showed that commercialisation of baobab pulp generated US$ 30,000 per year for the rural population of Malanville (north of Benin) that is involved in this business (Codjia et al., 2001). Moreover, chemical analysis of organs showed presence of proteins, amino acids, iron, vitamin C, A, E and F (abundant compared to the daily need) in the leaves, seeds and pulp (Codjia et al., 2001, Sidibe & Williams, 2002). Thus, the economic importance of baobab and human pressure on the species expressed by the scarcity of seedlings in the wild, warrants studies on population viability, genetic diversity, ecology, natural regeneration and reproduction of the species for its better conservation and management within parklands and agro-sylvo-pastoral systems.

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The main objective of the project is to elaborate an efficient strategy for the conservation, valorisation and rational management of baobab in its natural environments. The specific objectives are to:

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The study will be performed in all different climatic zones of Benin (between 5°N and 12°N and 1,5°E et 3°E). These comprise the Guinean zone (6°25’N and 7°30’N), Sudano-guinean zone (7°30’N and 9°45’N) and Sudanian zone (9°45’N et 12° N). Within each climatic zone, 3 localities will be sampled; in each locality, 5 populations represented by 30 individuals each will be sampled for genetic studies. Selection criteria will be based on indigenous farmers’ knowledge related to traditional characterization, management method and socio-economical importance of the species.

For the genetic characterisation, fresh leaf material from 15 populations (450 individuals in total) will be analysed using molecular markers (microsatellite). Thus, allelic richness (A) for a fixed sample size, polymorphism rate (P), observed heterozygosity rate (Ho), expected heterozygosity (He) and the Wright’s inbreeding coefficient (Fis) used by many authors for assessing genetic variability of forest trees (Chevallier, 1999; Zongo, 1999) will be evaluated. Furthermore, the degree of genetic resemblance and dissemblance of the populations will be estimated by genetic distances like Nei (1978) distance and Prevosti distance.

For the ecological and demography studies, 8 plots (50 m x 30 m) will be established centred around baobab trees in each climatic zone. Within each plot, phyto-sociological surveys as described by Blaun-Blanquet (1932), dendrometical data of each baobab tree (diameter at breast height, total height, total number of branches, etc.) and seedlings will be recorded for population dynamics studies. In order to better understand natural regeneration, some plots will be established in particular stations (protected areas, traditional agroforestry system, familial concession, hill’s zones, fallows, etc.) colonised by baobab tree. Moreover, for a total of 15 populations, 15 soil samples (20cm deep cores) will be collected in the immediate neighbourhood of the baobab tree in order to measure pH, K, N, P, OM, texture, etc..

To analyse ecological and demographic data, Principal Component Analysis (PCA) will be done with SAS software in order to determine the associated variables and how they vary according the climatic zones. To examine relationships between genetic variation (A, Ho, He and Fis) and demography structure, analyses of covariance (ANCOVA) will be used. The ANCOVA will be performed with SAS for window software. Regarding natural regeneration data, size-specific growth, annual mortality rates (Mx) of seedlings, annual survival rate (Gx) of seedlings during the period (Tx) of data collection will be estimated. Thus, populations will be grouped into a number of life cycle stages. A transition matrix model on the form initially described by Lefkovitch (1965) and subsequently modified by Caswell (1989) (Nt+1 = A x Nt with A being a square matrix containing transition probabilities, N a vector containing the number of individuals in each category at time t and t+1) will be used to analyse baobab population dynamics.

Finally, various methods of seed germination will be tested based on different parameters as duration of seeds preservation, depth of seeding, physical and thermal treatment of seeds and seed provenance. Results will be analysed by using some parameters such as: germination rate, dormancy ending and growth rate of seedling. Analysis of variances will allow comparison of means (95%CI) of the several tested variables.

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Expected output

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Information on the relevance of the proposal in relation to development issues

The proposal is strongly relevant to the issue of development. It has an immediate and applied bearing on development work in general and sustainable development taking into account natural resources more specifically. It is the first time in Benin, that this kind of study which concerns one of the most important forest food resources for the rural population (80 % of total population of Benin) will be undertaken. Also, the research, which will be undertaken, is very important to Benin and other West African countries in the context of valorisation and sustainable utilisation of plant genetic resources in agriculture and forestry. It becomes urgent to create extensive reference data on baobab in order to integrate it in governmental forestry management programmes, targeting promotion of non-timber forest products. With genetic diversity assessment, it is possible to produce plant material geared for specific leaf or fruit production in order to improve the human diet, especially in dry areas. In fact, the general situation in the rural areas in Africa point to major lack of protein, calcium and vitamin C. Because of the richness of baobab organs in these nutrients, its selection based on genetic diversity studies would be important in filling these gaps. Moreover, with population viability analysis of the baobab tree, it will be possible to project future population size or structure of the species in parkland and agro-forestry system and then provide opportunity to evaluate the efficacy of various management options, the objectives of which are to build populations up to adequate sizes and to reduce the risks of extinction of this multipurpose tree for poor rural populations.

Finally, this study will generate a number of research questions and solutions that have direct impact on development. It will help to evaluate and promote development interventions in situations where most of the people are dependent on non-timber forest products like baobab tree for their livelihood and survival.

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Bibliography cited

Caswell, H. (1989). Matrix Population Models: Construction, Analysis and Interpretation. Sinauer Associates, Sunderland, MA, USA.

Codjia, J.T.C., Fonton-Kiki, B., Assogbadjo, A.E. & Ekue, M.R.M. (2001). Le baobab (Adansonia digitata) Une espèce à usage multiple au Bénin, 47 p., ISBN 99919-953-0-7.

Chevallier, M.H. (1999). Diversité génétique des arbres forestiers. Vers une approche régionale des ressources génétiques forestières en Afrique sub-saharienne. Act. du 1er atel. rég. de formation sur la conservation et l'utilisation durable des ressources génétiques forestières en Afrique, 16-27 mars 1998,(ed.). CNSF, Ouag., B-Faso. INRP, Rome, Italie, 130-137.

Lefkovitch, L.P. (1965). The study of population growth in organisms grouped by stages. Biometrics 21, 1-18.

Nei, M. (1978). Estimation of average heterozygoty and genetic distance from a small timber of individuals. Genetics 89, 7723-760.

Sidibe, M., Scheuring, J.F., Tembely, D., Sidibe, M.M., Hofman, P. & Frigg, M. (1996). Baobab - homegrown vitamin C for Africa. Agroforestry Today, 8(2), 13-15

Sidibe, M. & Williams J.T. (2002). Baobab. Adansonia digitata. International Centre for Underutilised Crops, Southampton, UK, 100 p, ISBN 08543-277-6-2.

Zongo, J.D. (1999). Organisation de la diversité génétique dans les populations naturelles. Act. du 1er atel. rég. de formation sur la conservation et l'utilisation durable des ressources génétiques forestières en Afrique, 16-27 mars 1998,(ed.). CNSF, Ouag., B-Faso, INRP, Rome, Italie, 114-123.

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