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The Role of Shrub Agroforestry Systems in Increasing Food Security for the West African Sahel

Bright, Matthew Burton Hall

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2017, Doctor of Philosophy, Ohio State University, Environment and Natural Resources.
The West African Sahel (WAS) faces environmental and socioeconomic constraints that routinely threaten regional food security. Subsistence farmers must contend with drought, low soil fertility, and land degradation while sustaining livelihoods from rainfed agriculture without the aid of synthetic fertilizers and pesticides. Rapid population growth and climate change only exacerbate the challenges. Consequently, the WAS urgently needs scientifically-validated, sustainable agricultural systems to guarantee landscape conservation, to provide sufficient nutrient and water supplies, and to optimize crop productivity. A truly effective system for combating food insecurity, however, requires the adoption of local, low-cost inputs that simultaneously increase yields and coexist within the framework of the socio-economic and environmental demands of the region. This dissertation, therefore, examined the use of a ubiquitous and increasing component of the natural savanna ecosystem in the WAS – shrubs – to enhance the growth and yields of the staple crops, pearl millet (Pennisetum glaucum) and groundnut (Arachis hypogeae). The first research chapter (Chapter 2) investigated the long-term (eleven years) effect of a dominant, semi-evergreen shrub, Piliostigma reticulatum, on yields, soil fertility, soil carbon, and nutrient cycling at an experimental site in southern Senegal. The density of P. reticulatum was increased to 1000 shrubs ha-1 in a split-plot factorial design with the presence or absence of shrubs as the main plot and different rates of fertilizer as the subplot. Millet and groundnut were planted in a yearly rotation. The results showed a strong positive effect of P. reticultaum on millet water use efficiency, yields, and soil nutrients. The results also clearly demonstrated that this cropping system promotes C sequestration. Furthermore, the study showed, surprisingly, that the shrubs use water from the previous year’s rainy season for the following year’s growth which helps explain why P. reticulatum does not compete with crops for limited water. Chapter 3 studied the long-term (twelve years) effect of another dominant shrub, Guiera senegalensis, to increase the soil quality, growth, and yields of millet and groundnut using the same experimental design as found in Chapter 2 (1500 shrubs ha-1) but at a site in central Senegal with extremely sandy, degraded soils. The results unequivocally demonstrated that G. senegalensis could maintain yields and improve soil quality through limited rainfall. The mechanisms behind the shrub effect were examined more closely in this chapter: its effect on soil water content, temperature, and root growth between plants. There was a temporal offset between growth of G. senegalensis and millet roots. G. senegalensis primarily grew during the dry season, whereas millet grew during the rainy season which again shows why shrubs and crops do not compete for limited nutrients. Chapter 4 reports the first-ever ecological survey of the beneficial symbiotic fungi, arbuscular mycorrhizae (AM), conducted in the rooting zone of G. senegalensis at four farmers’ fields in the Sahelian and Sudanian ecological zones of Senegal. These fungi simultaneously grow in the soil and infect roots, and the beneficial symbiosis is an exchange of nutrients for photosynthetically-fixed carbon from the plant. We hypothesized that this symbiosis is critical to enhanced millet growth in shrub intercropping systems of the WAS which receive little inputs of synthetic fertilizer. Therefore, we measured infection rates in the roots of millet growing adjacent to and far from the shrub as well as in the shrub itself. We also examined AM communities in the soil using fatty acid methyl esters (FAME) and spore morphology and found high levels of colonization at all four sites with the highest levels in the drier Sahelian zone. Likewise, FAME analysis showed greater AM soil communities in this northern drier region. Compared to previous studies, spore densities were larger both near and far from G. senegalensis. In addition, we found thirteen AM species previously-unrecognized in the WAS. Overall, this study lays the foundation for future AM mechanistic studies and shows the importance of this symbiosis in the region. Chapter 5 is a literature review of the ecology, management, benefits, socioeconomic implications, and agroforestry potential of shrubs in the WAS. In total, twenty-eight species are described in the review. Most of them were the dominant woody vegetation in the region, and the study showed that they have excellent potential for agroforestry systems that can increase food security. G. senegalensis and P. reticulatum in particular possess morphological and physiological traits that make them very well-suited for wide-scale agroforestry across the region. This review characterizes new research avenues for shrubs and is the first to examine this subset of the woody vegetation as distinct from trees. Overall, this dissertation adds to the scientific knowledge on shrubs in the Sahel. It can further be used to guide shrub-based agroforestry research in other dryland regions of the globe. The results clearly indicate the importance, efficacy, and potential of native shrubs for increasing food security in the WAS.
Richard Dick (Advisor)
301 p.

Recommended Citations

Citations

  • Bright, M. B. H. (2017). The Role of Shrub Agroforestry Systems in Increasing Food Security for the West African Sahel [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1503071499488486

    APA Style (7th edition)

  • Bright, Matthew. The Role of Shrub Agroforestry Systems in Increasing Food Security for the West African Sahel. 2017. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1503071499488486.

    MLA Style (8th edition)

  • Bright, Matthew. "The Role of Shrub Agroforestry Systems in Increasing Food Security for the West African Sahel." Doctoral dissertation, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1503071499488486

    Chicago Manual of Style (17th edition)