Padonou Elie (2015). Knowing bowalization, its impact on biodiversity, soil and human livelihoods in Benin (West Africa). PhD Thesis, Faculty of Agronomic Sciences, University of Abomey-Calavi, Republic of Benin, 172 pages.
Thesis Promotor: Prof. Ir. Brice SINSIN.
Summary: Desertification and land degradation are worldwide problems affecting soils, vegetation and especially the life of the rural population. Bowal (plural bowé) is a particular form of degraded land that occurs in tropical regions. It is the result of ferricrete exposure due to soil surface erosion. Few research have adressed this specific form of degraded land. The present work aimed at increasing the understanding of how bowalization affect biodiversity, soil and human livelihoods. The main objectives were to (i) analyze the spatial indicators of bowé; (ii) analyze the cause and effects relationships to assess past and future changes in distribution pattern of selected species and plant diversity on bowé and (iii) assess the risk on the consequences of different land use on bowalization. Nine chapters composed this work.
Chapter 1 was an introduction and established the rationale and objectives of the thesis.
Chapter 2 described the study area which covered the whole country of Benin Republic.
Chapter 3 focused on the farmers’ perception and coping strategies on bowé. This was done using semi-structured interviews and questionnaires with 279 households of nine ethnic groups in the semiarid (Peulh, Bariba, Dendi, Nagots and Mocolé) and the sub-humid zone (Fon, Mahi, Holli and Adja) of Benin. Pearson Chi-square Test and simple correspondence analysis were performed to analyze the perceptions on the causes, consequences and coping strategies with bowé in the two climate zones and according the ethnic groups. Bowalization was reported to be induced by non-adapted land use and soil erosion. Bowalizationleads to loss of biodiversity in the two climates zones and reduced water retention capacity in soils. It also induced rooting difficulties for crops and increase soil temperature. Farmers in the semiarid zone have adopted planting of cowpea and groundnut on bowé with hoe instead of animal traction. Ethnic groups of both climate zones that depend mainly on livestock herding have to practice transhumance and use food supply for the animals.
Chapter 4 determined the spatial distribution of bowal and the differences in physicochemical characteristics between bowal and woodland soils. Bowé sites were mapped and soil samples were taken on bowé and nearby woodland. The results show that bowé are directly related to ferruginous soils and rainfall regime. Bowé soils are characterized by significantly lower values of electrical conductivity, organic matter, extractable phosphorus, silt and total nitrogen than woodland soils, while potassium exchangeability of bowé soils was higher. Bowé can be expected wherever ferruginous and/or ferricretes soils are observed in unimodal rainfall regime condition.
Chapter 5 focused on the impact of bowalization on phytodiversity, life forms and plant morphology in the sub-humid zone of Benin using Combretum nigricans as a case study. Morphological variables of C. nigricans (height, number of stems, number of branches, diameter at breast height, and crown diameter) and the plant communities were determined on sand-clay soil, concretion soils and bowal. The results showed that plant communities were more diversified on sand-clay and concretion soils compared with those described on bowal. C. nigricans developed more stems (3.6 ± 1.4 stems vs. 1.3 ± 0.4 stems), more branches (5.9 ± 2.4 branches vs. 3.2 ± 0.6 branches) and large crown diameter (5 ± 1.48 m vs. 3.4 ± 1.2 m) on bowal than on sand-clay soil. The best adapted life forms on bowal were therophytes.
In Chapter 6, the following questions were addressed: what are the vegetation characteristics on bowé? How vegetation pattern varies on bowé? Which species, life forms, chorological types and plant families are characteristics of bowé? Which species, life forms, chorological types and plant families are impacted by bowé in their extension? The study revealed that bowé were characterized by grassland and savanna. The species composition on bowé varies according to the climate zone. Woody species were more frequent on bowé in sub-humid than in semiarid. Geophytes, hemicryptophytes and phanerophytes were more frequent on bowé in the sub-humid than in the semiarid climate zone. The difference between the two climate zones on the occurrence of therophytes on bowé was not significant. The frequency of chamaephytes was higher on bowé sites in the semiarid zone. Afro-tropical, Afro-Malgache, Pluri regional African and Pantropical chorological types were more frequent on bowé than in woodland in each climate zone while the opposite was found for Guineo-Congolian and Sudano-Zambesian chorotypes. Plant families such as Amaranthaceae, Zingiberaceae, Chrysobalanaceae, Connaraceae, Loganiaceae, Moraceae and Ochnaceae were only found on bowé in the sub-humid climate zone, while Convolvulaceae, Loganiaceae, Rhamnaceae, Araceae, Colchicaceae, Cucurbitaceae, Olacaceae, Pedaliaceae, Amaranthaceae, and Cyperaceae were only found on bowé in the semiarid zone.
Chapter 7 identified the resistant species towards climate change for ecological restoration of bowé by submitting the most common bowé species together with significant environmental variables (elevation, current bioclimatic variables, and soil types) to the ecological niche modeling program (Maxent, Domain and GARP). For future prediction (2050) IPCC4/CIAT and IPCC5/CMIP5 climate data were applied. Asparagus africanus, Andropogon pseudapricus and Combretum nigricans were identified as the most resistant species for ecological restoration of bowé in the semiarid climate zone, while Asparagus africanus,Detarium microcarpum and Lannea microcarpa were the most resistant for ecological restoration of bowé in the sub-humid climate zone.
The following questions were addressed in chapter 8: How land cover changes with bowalization? What factors govern land cover change and bowalization? Are there proofs of bowé occurrence due to agricultural practices? How many year of land use make occurrence of bowé? What would be the extension of bowé in the future? Land use land cover change analysis was used to addresse these questions based on the land cover maps of 1975, 1990 and 2010 of the municipality of Banikoara. Markovian chain was used to predict occurrence of bowé towards 2050.The results showed considerable change in land use land cover maps of the three periods (1975, 1990 and 2010). The land cover on which bowé occur (farmland and degraded savanna) have persisted, and increased at the rate of 0.0542 ha/year, 0.0952 ha/year during the periods 1975-1990, 1990-2010 respectively; while the natural vegetation (forest, woodland and tree savanna) have decreased at the same rate. The future scenarios also predict the same trend. A total of 26% (1286346 ha) and 31% (1293693 ha) of the area cover with natural vegetation would be converted to farmland and degraded savanna towards 2050 if we assume the dynamic recorded respectively from 1975-1990 and 1990-2010. Thus bowalization would persiste and increase towards 2050.
Chapter 9 synthesized the overall findings of this reseach project, and identified areas for further reseach and makes management recommendations. Bowalization is the result of ferricrete exposure due to non-adapted land use and soil surface erosion on ferruginous soil in unimodal rainfall regime condition. The important drivers of bowalization in the study were assessed based on soil, geomorphology, climate, and land use practices. However, other spatial indicators like soil parent rock could also be used. Bowé are also covered by a number of termitaria. Up to now, the type of termitaria on bowé and their role are not yet clearly established. Bowé showed significantly lower values of electrical conductivity, organic matter, extractable phosphorus, silt and total nitrogen than woodland soils, while its potassium exchangeability was higher. Knowing the dependence of the physical and chemical characteristics of the soils developed on bowé and the different type of parent rocks would help to increase the understanding of the drivers of bowalization. The identification of the most important drivers of bowalization at regional scale would also help to combat bowalization.
Bowalization has induced loss of species and modified the morphological structure of C. nigricans. The vegetation characteristics on bowé are grassland and savanna with dominance of Afro-Tropical, Afro-Malgache, Pluri regional African and Pan Tropical chorological types and therophytes life forms. Some species are suitable for ecological restoration of bowé. They are Asparagus africanus, Andropogon pseudapricus and Combretum nigricans in the semiarid climate zone, and Asparagus africanus, Detarium microcarpum and Lannea microcarpa in the sub-humid climate zone. Additional plant functional traits (vegetative, leaf, stem and regenerative traits) would help to increase our understanding of the effect of bowalization on plant diversity. Moreover, the quantification of the impacts of bowalization on crops production is needed. Coping strategies were developed by some farmers to reduce the impact of bowalization. These strategies need to be tested in order to identify the best to be promoted. It will also be relevant to consider environmental indicators combined with an extensive soil and botanical datasets to spatially predict and analyze the distribution ranges of plant species and pattern of plant diversity in relation to the main drivers of bowalization at regional scale.
The markovian model used in this study showed that bowalization has persisted and increased during the period considered (1975, 1990 and 2010), while the natural vegetation (forest, woodland and tree savanna) has decreased. The same trend would also pravail towards 2050 if we assume the dynamics recorded for the periods considered. However, the model is not spatial-explicit and assumes that transition probabilities are time homogenous. More detail spatial explicit model on land use land cover change may be used in future analysis to improve the understanding of the locations and pathways of land use land cover change dynamics which induce bowalization. In addition, this model has not taken into account the relation between the occurrence of bowé and surface erosion processes. Using the universal soil loss equation linked to GIS may improve the understanding of the relation between soil erosion and bowalization. Bowalization was indirectly assessed in this study base on the land cover with farmland and degraded savanna where they are common. However all the farmland and degraded savanna are not covered by bowé thus it was not possible to quantify and predict the exact cover of bowé. The limit observed with the method considered for this study should be taken into account for other similar study. Future researchs are needed in order to increase the understanding of bowalization and its impacts.