Advanced Series in Agricultural Sciences 23 Co-ordinating Editor: B. Yaron, Bet-Dagan Editors: B. L. McNeal, Gainesville F. Tardieu, Montpellier H. Van Keulen, Wageningen L. D. Van Vleck, Ithaca Advanced Series in Agricultural Sciences Volumes already published in the series Volume 10 Volume 20 Saline and Sodic Soils (1982) Water Flow and Solute Transport E. Bresler, B. L. McNeal, D. L. Carter in Soils (1993) D. Russo, G. Dagan (Eds.) Volume 11 A Theory of Feeding and Growth of Volume 21 Animals (1982) Production of Sugarcane: J. R. Parks Theory and Practice (1993) G. Hunsigi Volume 12 Fertilization of Dryland and Volume 22 Irrigated Soils (1982) Management of Water Use J. Hagin, B. Tucker in Agriculture (1994) K. K. Tanji, B. Yaron (Eds.) Volume 13 Agricultural Soil Mechanics (1983) Volume 23 A. J. Koolen, H. Kuipers Woody Plants in Agro-Ecosystems of Semi-Arid Regions (1995) Volume 14 H. Breman, J.-J. Kessler Energy and Agriculture (1984) G. Stanhill (Ed.) Volume 24 Soilless Culture Management (1995) Volume 15 M. Schwarz The Rhizosphere (1986) E. A. Curl, B. Truelove Volume 16 Population Genetics: Basic Principles (1987) D. P. Doolittle Volume 17 Irrigation with Treated Sewage Emuent (1991) A. Feigin, 1. Ravina, J. Shalhevet Volume 18 Advances in Statistical Methods for Genetic Improvement of Livestock (1990) D. Gianola, K. Hammond (Eds.) Volume 19 Electromagnetic Radiations in Food Science (1992) 1. Rosenthal Henk Breman J an-J oost Kessler Woody Plants in Agro-Ecosystems of Semi-Arid Regions with an Emphasis on the Sahelian Countries With 55 Figures and 75 Tables Springer-Verlag Berlin Heidelberg New York London Paris Tokyo Hong Kong Barcelona Budapest Dr. HENK BREMAN Ir. JAN-JOOST KESSLER Research Institute for Agrobiology and Soil Fertility Ministry of Agriculture, Nature Management and Fisheries P.O. Box 14, 6700 AA Wageningen The Netherlands ISBN-13: 978-3-642-79209-0 e-ISBN-13: 978-3-642-79207-6 DOl: 10.1007/978-3-642-79207-6 CIP-Data applied for This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and strorage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. © Springer-Verlag Berlin Heidelberg 1995 Softcover reprint of the hardcover 1s t edition 1995 The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Best-set Typesetter Ltd., Hong Kong SPIN: 10135483 31/3130/SPS - 5 4 3 2 1 0 - Printed on acid-free paper This book is dedicated to Paul Sciarone, a graduate in law and in physics, whose insight into the value of the studies on rangeland dynamics in the Sahel region led to the initiative of providing funds for subsequent studies on the role and potential of woody plants in this region. Preface 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. Useful contributions were made by Chris Geerling and Frits Penning de Vries. We thank Arnoud Budelman, Sjaak Conijn and Meine van Noordwijk for their constructive criticism of drafts of some chapters, Pieter Oostra for spending many hours in correcting the list of references, and we are very grateful to Peter Uithol for doing the simulation studies and the layout. The Wageningen Agricultural University, Forestry Department is acknowledged as having enabled the second author to finalise this book. The work involved in creating this book was supported by the Research Institute for Agrobiology and soil Fertility (AB-DLO) in Wageningen and, indirectly, by the Netherlands VIII Preface Ministry of Foreign Affairs, through their support to the AB-DLO for research in Sahelian countries. Wageningen, The Netherlands H. BREMAN December 1994 1.-1. KESSLER Contents 1 Introduction ............................... . 1 2 The Distribution and Canopy Cover of Woody Species .......................... . 4 2.1 Introduction and Methodology ............... . 4 2.1.1 Introduction ............................... . 4 2.1.2 Climatic Zones ............................ . 4 2.1.3 Landscapes, Geomorphological Units and Groundwater .......................... . 6 2.1.4 Land-Use Types ........................... . 8 2.1.5 Canopy Cover ............................. . 9 2.2 Distribution and Canopy Cover Data ......... . 12 2.2.1 1975-1982 ................................ . 12 2.2.2 Climatic Influences ......................... . 19 2.3 An Ecological Interpretation of Woody Vegetation in the SSW A ........... . 25 2.3.1 Introduction: Water Availability and Soil Texture .......... . 25 2.3.2 African Savannas .......................... . 27 2.3.3 The Sahel Zones ........................... . 28 2.3.4 The Sudan Zones .......................... . 32 2.3.5 Conclusions ............................... . 33 2.4 Human Influences on Woody Plants .......... . 34 2.4.1 Introduction ............................... . 34 2.4.2 Fire ...................................... . 35 2.4.3 Livestock Grazing and Browsing ............. . 39 2.4.4 Agriculture ................................ . 47 2.4.5 Wood Exploitation ......................... . 50 2.5 Conclusions ............................... . 51 3 Production of Woody Plants ................. . 54 3.1 Introduction ............................... . 54 3.2 Productivity and Biomass of Woody Plant Communities ................ . 54 3.2.1 Gross and Net Productivity .................. . 54 x Contents 3.2.2 Biomass and Net Productivity ................ . 55 3.2.3 Conclusions ............................... . 59 3.3 Production of Woody Plant Components ...... . 60 3.3.1 Wood Production .......................... . 60 3.3.2 Foliage Production ......................... . 63 3.3.3 Flower and Fruit Production ................. . 66 3.3.4 Conclusions ............................... . 68 3.4 Estimating Woody Plant Production .......... . 69 3.4.1 Introduction ............................... . 69 3.4.2 Foliage Production and Canopy Cover ........ . 69 3.4.3 Methods to Estimate Woody Plant Productivity ............................... . 71 3.4.4 Conclusions ............................... . 76 3.5 Phenology ................................ . 77 3.5.1 Introduction ............................... . 77 3.5.2 Phenological Data from the SSW A ........... . 77 3.5.3 Conclusions ............................... . 84 3.6 Nutrient Concentrations, Digestibility and Palatability ............................ . 84 3.6.1 Introduction ............................... . 84 3.6.2 Nutrient Concentrations of Woody Plant Components ................ . 88 3.6.3 Digestibility of Forage from Woody Plants ..... . 92 3.6.4 A Comparison with Herbaceous Foliage ....... . 94 3.6.5 Palatability of Forage from Woody Plants ..... . 94 3.6.6 Conclusions ............................... . 97 4 The Influence of Woody Plants on Plant Production Factors .................. 98 4.1 Introduction ................................ 98 4.2 Interactions Between the Woody and Herbaceous Layer ....................... 99 4.2.1 Introduction ................................ 99 4.2.2 Relationships Between the Woody and Herbage Layer .......................... 100 4.2.3 Effects on Herbage Production by Bush Clearing or Tree Felling .............. 107 4.2.4 Herbage Species Composition in Relation to Canopy Cover ................. 109 4.2.5 Conclusions ................................ 113 4.3 Root Systems of Woody Plants ................ 114 4.3.1 Root Systems of Woody Plants in Semi-Arid Regions ........................ 114 4.3.2 Root System Dynamics ...................... 121 Contents XI 4.3.3 Comparison with Herbaceous Root Systems .... 123 4.4 Influences of Woody Plants on Water Availability ....................... . 124 4.4.1 Introduction ............................... . 124 4.4.2 Processes Influencing Water Availability ...... . 125 4.4.3 Water Availability and Water Use ............ . 146 4.4.4 Conclusions ............................... . 152 4.5 Influences of Woody Plants on Nutrient Availability ..................... . 153 4.5.1 Introduction ............................... . 153 4.5.2 Patterns of Soil Fertility in Relation to Woody Plants ................. . 154 4.5.3 Soil Fertility Changes During Fallowing ....... . 163 4.5.4 Woody Plant Processes Influencing Nutrient Availability ............. . 167 4.6 Soil Organic Matter and its Functions in Relation to Woody Plants ................. . 199 4.6.1 Introduction ............................... . 199 4.6.2 Effect of SOM on Soil Physical and Chemical Properties .................... . 200 4.6.3 Decomposition Processes ................... . 201 4.6.4 Mineralisation Processes .................... . 204 4.6.5 SOM Contents in Semi-Arid Regions ......... . 205 4.6.6 Decomposition of Woody Litter in Semi-Arid Regions ....................... . 209 4.6.7 A Quantified Example from the SSW A ....... . 213 4.6.8 Conclusions ., ............................. . 217 4.7 Influences of Woody Plants on Light Availability ....................... . 219 4.7.1 Introduction ............................... . 219 4.7.2 Light Intensity and Plant Production .......... . 220 4.7.3 Distribution and Quality of Light Under Woody Canopies .................... . 223 4.7.4 Light Availability Under a Closed Canopy ..... . 225 4.7.5 Light Availability Under Isolated Woody Plants ................ . 227 4.7.6 Conclusions ............................... . 231 4.8 Potential Influences of Woody Plants on Plant Production ........................ . 232 4.8.1 Introduction ............................... . 232 4.8.2 Net Production Limited by Light ............. . 234 4.8.3 Net Production Limited by Water ............ . 237 4.8.4 Actual Net Production, Limited by Water and Nutrients .............................. . 248 4.8.5 Woody Plants and Plant Production Factors ... . 260