PhD Dissertation

Population ecology, uses and conservation of Sclerocarya birrea (a. Rich.) Hochst. (anacardiaceae) in Benin, West Africa

There is a growing interest in food tree species in general, and particularly indigenous fruit tree species in developing countries. This is because they are inherent to most tropical landscapes and serve the dual function of local livelihood support and biodiversity conservation. The main objective of this study was to provide information for the sustainable management of Sclerocarya birrea (A. Rich.) Hochst. subsp. birrea (Anacardiaceae) under present and future climate in Benin. The specific objectives were to assess (i) the use values, pattern and diversity of use and different knowledge associated to the species, (ii) the impacts of land use on the population structure and abundance of the species, (iii) the sex ratio, the relative spatial repartition of males and females individuals and their implication for the population dynamics of the species , (iv) the potential for domestication through study of intra-and-inter-population variability in fruit traits, and (v) how far climate change can affect the habitat suitability and conservation status of the species.

Chapter 1 of this thesis describes the general characteristics of the study species and the geographical features of the study area. Sclerocarya birrea (is widely distributed in the semi arid areas from Southern Africa through Eastern Africa and the semi-arid zones of West Africa. That species has three different subspecies, viz. S. birrea subsp. caffra growing from Southern Africa to Central Kenya, while S. birrea subsp. birrea is found mainly in Western, Central and Eastern Africa, and S. birrea subsp. multifoliolata is endemic to Tanzania. This work has been carried out on Sclerocarya birrea (A. Rich.) Hochst. subsp. birrea . Owing to its multiple potential, the species has been considered as one of the most important tree to be domesticated for rural livelihood improvement in semi-arid Africa but baseline information relevant to its sound use and management is still scanty.

In chapter 2 (published in 2011) we combined quantitative and qualitative ethnobotanical approaches to investigate uses and factors affecting the use value of S. birrea subsp. birrea. Nine group surveys as well as 161 individual interviews were held in the dry and typical Sudanian zones. Seven different ethnic groups were involved and the survey focused on local uses and perception of factors affecting the dynamics of the species. S. birrea had a multitude of uses; all organs were used for more than 20 different purposes. The results highlighted how far gender, local availability, ethnicity and community location interact to influence the utilization value of that species. People living in drier areas with high occurrence of S. birrea use it more than those living in wetter areas with low occurrence. While domestic and subsistence uses do not appear to threaten the species, carving, clearing and drought were the major causes of its decline. Many factors and their interactions influence the pattern of use of the species within and between communities. When compared to the level of exploitation of S. birrea subsp. caffra in southern Africa, the subspecies birrea is at this point relatively underutilized.

In chapter 3 (published in 2009), the population structure and abundance in two contrasting land use systems (Agroforestry systems versus protected areas) were assessed. Adult tree density was about nine times higher in the protected area (P < 0.001) compared to agroforestry systems (agro-systems). Seedling occurrence was similar in both land use types even though seed germination was best favoured in agroforestry systems. Saplings and adult trees with 5 to 20 cm dbh were almost absent in agroforestry systems. The mean diameter in agroforestry systems was about twice higher than in the protected area. Although a log-linear analysis showed a difference in the size class distributions between land use types (P < 0.0001), they were all positively skewed. Green’s index showed a clumped distribution in the protected area (0.48) compared to agroforestry systems (0.05). Population structure variation could mainly be explained by agricultural pressure. It was concluded that saplings conservation is required in agroforestry systems to ensure sustainable use.

In chapter 4 (published in 2011), we assessed sex ratio, spatial distribution among male and female adult trees using second order spatial statistics and assessed folk perception of dioecism among the natural populations in protected areas and surrounding agroforestry systems. A field survey showed that 55% of respondents were aware of sex separation in S. birrea. Some local people used bark appearance to make distinction between sexes, but this morphological criterion was not consistent with statistical results. The sex ratio did not deviate significantly from 0.5 in any of the districts or land use types. Bivariate spatial analysis with pair correlation function revealed no spatial association between male and female individuals. Moreover, a strict spatial segregation of sexes was not observed even though some individuals of the same sex could sometimes be found together. Results supported the functional dioecy of S. birrea and showed that it did not display any apparent sex specific dimorphism outside the reproduction period or any apparent sex specific requirement for environment conditions.

In chapter 5 (published in 2011), we assessed phenotypic variations in fruits and components of Sclerocarya birrea subsp. birrea indigenous to West African semi-arid areas following an aridity gradient. Fruits were collected from 42 trees of various diameters in agroforestry parklands from wet and dry Sudanian climates. They were partitioned into peel, juice/flesh and pit (shell + kernel). Each fruit was labelled and its components were measured and weighed keeping the identity through the series of assessment. The overall mean fruit mass was 18.58 ± 0.24 g (mean ± SE) but fruits from drier zone population were significantly bigger (19.90 ± 0.37g vs 17.02 ± 0.24g; P < 0.001). Results showed high correlations between fruit mass and components masses in general (p < 0.05). Tree diameter was very weakly correlated with fruits and components’ traits. There was a high intra and inter-population variation in fruits and components traits. The intra-population variation represented the most important part (67 to 100%) of the total variation in traits. Selected trees, mainly from the drier zones showed superior phenotypical traits. Five groups of trees capable of providing different fruits morphotypes were identified for various prospective exploitations. Results stood as mainstay for preliminaries practical actions towards domestication and conservation of S. birrea in West Africa.

In the two last chapters, we contributed in the ongoing debate of the impact of climate change on the natural ecosystems and how conservation should be designed to mitigate the adverse effects. Therefore, in chapter 6 (under review), a discriminant analysis was used to assess variation in ecological characteristics of Sclerocarya birrea subsp. birrea throughout its distribution range in Africa and its impact on the predictive power of regional and general habitat suitability models built within Maxent modelling framework. Species’ locality data were obtained through field work, published accounts and herbarium records. Results showed variation in ecological conditions of S. birrea. Western and central records were much closer and could be regarded as homogenous entity compared to the eastern records which formed a distinct group. Model built in one region performed poorly when projected in other region and performed better in its own region when compared to the general model which under-estimated or overestimated the proportion of the predicted area. We discussed some implications of the regional variations in abiotic conditions on the genetic structuring of S. birrea. The results suggested that spatial partitioning of locality records was important to improve the accuracy of any prediction of S. birrea or any other wide-ranging tree species distribution under present or future climate condition in African savannas. In the chapter 7 (to be submitted) , we used the climate envelope modeling techniques implemented in MaxEnt combined with GIS to forecast the current and future distribution (2050) of Sclerocarya birrea, under present and three future climate models in Benin. Environmental variables were derived from monthly temperature and rainfall obtained from Wordclim database. The most characteristics and least correlated were selected after a cluster analysis. Results showed that the suitable current range of S. birrea remained mainly restricted to the dry Sudanian zone of Benin, encompassing the two main protected areas. Under future climate, results differed with the climate model with two of them predicting a reduction in the suitable habitat of S. birrea. The range was predicted to shift partially from the protected areas suggesting a reduction in the effectiveness of reserves to conserve S. birrea in the future. The conservation in agricultural systems stands as a most plausible mitigation action under change in climate.

In the last section of this work (General conclusion), we provided a synthesis of the results and their implications for the sustainable management of S. birrea. We recommend a predominant assisted seed based propagation in agroforestry systems to enhance its performance in terms of useful service for local people and for ecosystem. Enhanced conservation effort involving enriching planting of seed or seedling in the protected areas located southwards of the two main reserves and currently suitable but with low presence density for the species could be a useful present in-situ conservation planning for the future. Furthers research recommendations have been made to enhance the level of scientific knowledge on the species in order to enable its sustainable use.