This book provides a quantitative analysis of the role of woody plants in semi-arid regions, for the aSSessment of their benefits in agrosylvopastoralland-use systems with productive and sus­ tainability objectives. The insights presented and conclusions drawn allow the additional benefits of woody plants for specific climatic and physical site conditions and land-use systems to be estimated. The Sahel and Sudan zones in West Africa, on which the book focusses, represent resource-poor conditions, whose ecological dynamics have been relatively well studied. The role of woody plants in this region, as assessed in this book, is extrapolated to other semi-arid regions, leading to general conclusions on agroforestry's potential as an option for sustainable land use in semi-arid regions. The origins of this book go back to 1982, when the Club du Sahel requested that available data on woody plants in the Sahel region be synthesised, to provide basic information to enable better attention to be given to woody plants in rural development programmes. We are grateful to the Club du Sahel for this challenge. Various people contributed to studies used in this book. The preliminary inventory of the data available was made by Frits Ohler; later his work was continued by Franciska Dekker.

Inhalt

1 Introduction.- 2 The Distribution and Canopy Cover of Woody Species.- 2.1 Introduction and Methodology.- 2.1.1 Introduction.- 2.1.2 Climatic Zones.- 2.1.3 Landscapes, Geomorphological Units and Groundwater.- 2.1.4 Land-Use Types.- 2.1.5 Canopy Cover.- 2.2 Distribution and Canopy Cover Data.- 2.2.1 1975-1982.- 2.2.2 Climatic Influences.- 2.3 An Ecological Interpretation of Woody Vegetation in the SSWA.- 2.3.1 Introduction: Water Availability and Soil Texture.- 2.3.2 African Savannas.- 2.3.3 The Sahel Zones.- 2.3.4 The Sudan Zones.- 2.3.5 Conclusions.- 2.4 Human Influences on Woody Plants.- 2.4.1 Introduction.- 2.4.2 Fire.- 2.4.3 Livestock Grazing and Browsing.- 2.4.4 Agriculture.- 2.4.5 Wood Exploitation.- 2.5 Conclusions.- 3 Production of Woody Plants.- 3.1 Introduction.- 3.2 Productivity and Biomass of Woody Plant Communities.- 3.2.1 Gross and Net Productivity.- 3.2.2 Biomass and Net Productivity.- 3.2.3 Conclusions.- 3.3 Production of Woody Plant Components.- 3.3.1 Wood Production.- 3.3.2 Foliage Production.- 3.3.3 Flower and Fruit Production.- 3.3.4 Conclusions.- 3.4 Estimating Woody Plant Production.- 3.4.1 Introduction.- 3.4.2 Foliage Production and Canopy Cover.- 3.4.3 Methods to Estimate Woody Plant Productivity.- 3.4.4 Conclusions.- 3.5 Phenology.- 3.5.1 Introduction.- 3.5.2 Phenological Data from the SSWA.- 3.5.3 Conclusions.- 3.6 Nutrient Concentrations, Digestibility and Palatability.- 3.6.1 Introduction.- 3.6.2 Nutrient Concentrations of Woody Plant Components.- 3.6.3 Digestibility of Forage from Woody Plants.- 3.6.4 A Comparison with Herbaceous Foliage.- 3.6.5 Palatability of Forage from Woody Plants.- 3.6.6 Conclusions.- 4 The Influence of Woody Plants on Plant Production Factors.- 4.1 Introduction.- 4.2 Interactions Between the Woody and Herbaceous Layer.- 4.2.1 Introduction.- 4.2.2 Relationships Between the Woody and Herbage Layer.- 4.2.3 Effects on Herbage Production by Bush Clearing or Tree Felling.- 4.2.4 Herbage Species Composition in Relation to Canopy Cover.- 4.2.5 Conclusions.- 4.3 Root Systems of Woody Plants.- 4.3.1 Root Systems of Woody Plants in Semi-Arid Regions.- 4.3.2 Root System Dynamics.- 4.3.3 Comparison with Herbaceous Root Systems.- 4.4 Influences of Woody Plants on Water Availability.- 4.4.1 Introduction.- 4.4.2 Processes Influencing Water Availability.- 4.4.3 Water Availability and Water Use.- 4.4.4 Conclusions.- 4.5 Influences of Woody Plants on Nutrient Availability.- 4.5.1 Introduction.- 4.5.2 Patterns of Soil Fertility in Relation to Woody Plants.- 4.5.3 Soil Fertility Changes During Fallowing.- 4.5.4 Woody Plant Processes Influencing Nutrient Availability.- 4.6 Soil Organic Matter and its Functions in Relation to Woody Plants.- 4.6.1 Introduction.- 4.6.2 Effect of SOM on Soil Physical and Chemical Properties.- 4.6.3 Decomposition Processes.- 4.6.4 Mineralisation Processes.- 4.6.5 SOM Contents in Semi-Arid Regions.- 4.6.6 Decomposition of Woody Litter in Semi-Arid Regions.- 4.6.7 A Quantified Example from the SSWA.- 4.6.8 Conclusions.- 4.7 Influences of Woody Plants on Light Availability.- 4.7.1 Introduction.- 4.7.2 Light Intensity and Plant Production.- 4.7.3 Distribution and Quality of Light Under Woody Canopies.- 4.7.4 Light Availability Under a Closed Canopy.- 4.7.5 Light Availability Under Isolated Woody Plants.- 4.7.6 Conclusions.- 4.8 Potential Influences of Woody Plants on Plant Production.- 4.8.1 Introduction.- 4.8.2 Net Production Limited by Light.- 4.8.3 Net Production Limited by Water.- 4.8.4 Actual Net Production, Limited by Water and Nutrients.- 4.8.5 Woody Plants and Plant Production Factors.- 5 Synthesis, Generalisation and Recommendations.- 5.1 Introduction.- 5.2 Woody Plants and Plant Production Factors.- 5.3 Productive Benefits by Woody Plants.- 5.3.1 Introduction.- 5.3.2 Woody Plant Characteristics.- 5.3.3 Management of Woody Plants.- 5.3.4 Maximum Productive Benefits by Woody Plants.- 5.3.5 Woody Plants and Spatial Concentration of Resources.- 5.4 The Role of Woody Plants in Land-Use-Systems.- 5.4.1 Introduction.- 5.4.2 Woody Plant Influences on Crops and Pastures.- 5.4.3 Production by Woody Plants.- 5.4.4 Woody Plants and Production Security and Sustainability.- 5.4.5 Sylvopastoralism.- 5.4.6 Agrosylviculture.- 5.4.7 Woody Plants and Socio-Economic Conditions.- 5.4.8 Conclusions and Recommendations.- References.