IMPROVEMENT OF TRADITIONAL ACACIA SENEGAL AGROFORESTRY: Ecophysiological characteristics as indicators for tree-crop interaction on sandy soil in western Sudan Abdalla Gaafar Mohamed Academic dissertation To be presented, with the permission of the Faculty of Agriculture and Forestry of the University of Helsinki, for public discussion in Auditorium XIII of the University of Helsinki Main Building, Unioninkatu 34, on 4 March 2005 at 12 o’clock noon Helsinki 2005 Supervisor: Professor Olavi Luukkanen Viikki Tropical Resources Institute (VITRI) Department of Forest Ecology University of Helsinki Finland Reviewers: Professor Juha Helenius Department of Applied Biology University of Helsinki Finland Associate Professor Suree Bhumibhamon Faculty of Forestry Kasetsart University, Bangkok Thailand Opponent: Professor (Emer.) Peter Tigerstedt University of Helsinki Finland 2 ABSTRACT The aim of the present study was to investigate the suitability ofAcacia senegal stands for agroforestry with regard to soil moisture depletion and physiological traits. In the first set of experiments, the effect of tree size on soil water depletion and on such tree characteristics as photosynthesis, stomatal conductance, leaf water potential, relative humidity and inter-cellular CO concentration was examined. The 2 physiological behavior ofA. senegal was assessed to elucidate its drought adaptation mechanisms. In the second set of experiments the effect of the density of a planted A. senegal stand on two traditional food and cash crops, sorghum and karkadeh, was evaluated to determine the interaction between trees and field crops, using gum and agricultural crop yields and physiological characteristics as criteria. The study was conducted during two rainy seasons in the Domokeya reserve forest near El Obeid town in western Sudan. Soil moisture was measured initially with a theta probe and subsequently with a neutron probe from different soil strata to the depth of 250 cm. A portable photosynthesis system was used for measuring, in trees as well as in field crops, the photosynthesis rate, stomatal conductance, relative humidity and inter-cellular CO 2 concentration. The leaf water potential was measured with a pressure chamber. The results indicated that as the tree size increases the amount of water depleted from the soil profile also increases. Significant positive correlation was found between the amount of water in the profile and the tree photosynthetic rate. The data indicated that water uptake by trees of different sizes came mostly from the 0-150 cm soil layer, with less uptake from deeper layers. The morning leaf water potential and stomatal conductance in trees were significantly affected by tree size. Gum production and tree physiological traits were found to be highly responsive to changes in soil water. It was concluded that A. senegal is capable of physiological adjustment in response to soil moisture as a form of ecological adaptation. All measured traits in trees were significantly affected by tree density and by the presence of agricultural crops. There was little evidence of complementarity in resource sharing between trees and crops, since both trees and field crops competed for soil water from the same soil depth. This was the most important interaction noticed between trees and crops. Gum yield increased when sorghum was inter- cropped with trees, and the per-tree and per-area gum yields were higher when the density of trees was higher. With intercropping of karkadeh, the gum yield showed an increasing trend at a low tree density and a decreasing one at a high tree density, but these effects were not statistically significant. This, however, supported the finding that gum production depends on the soil water status. In both field crops, when grown in an agroforestry system, the yield was higher with a lower density of trees but did not reach the yield level obtained in pure-culture. This effect seemed to depend on soil water availability. Overall, A. senegal, when planted at 266 trees ha-1, reduced the karkadeh flower yield by about 26% and its biomass by 37%, and the sorghum grain yield by 19% and biomass by 9%. Planting of 433 trees ha-1 reduced the karkadeh yield by 55% and biomass by 57%, and the sorghum grain yield by 44% and biomass by 45%, relative to sole crops. This variation seemed to be 3 caused by the influence of intercropping design on soil water. A. senegal agroforestry system seemed to have a higher rain use efficiency as compared to pure tree stands or crops. Intercropping design significantly affected the soil water status, photosynthesis, stomatal conductance and leaf water potential both in trees and in crops. Karkadeh appears to be more appropriate for intercropping with A. senegal than sorghum and particularly recommendable in combination with a low tree density. Overall, it was concluded that, in A. senegal agroforestry, tree density affects the competition for soil water between agroforestry system components. Modification of tree density can be used as management tool to mitigate competitive interaction in the intercropping system. Key words: Acacia senegal, agroforestry, gum arabic, soil water, tree-crop interaction, sorghum, karkadeh, Sudan. 4 PREFACE The present study is a part of research project initiated in 1998 under the name “Improvement of traditionalAcacia senegal agroforestry in the Sudan” by the Viikki Tropical Resources Institute (VITRI), Department of Forest Ecology, University of Helsinki. The project (now “Trees, agroforestry and land use in dryland Africa”, TALDA) is funded by the Academy of Finland. I would like to express my sincerest thanks and appreciation to my supervisor Prof. Olavi Luukkanen who has offered me the opportunity to join VITRI. His understanding, guidance, continuos encouragement and unlimited support were significantly crucial for completion of this work. I am also deeply indebted to Professor Ahmed Ali Salih, Director of the Forestry Research Centre, Khartoum, from whom I have learnt a lot, for his sincere technical and moral support in the field and continuos following up of my work till the end. I gratefully acknowledge the generous support provided by Dr. Mohamed El Fadl who provided unlimited backstopping during the period of this study, and moreover by his family who created a sincere warm, home-like atmosphere that helped me to overcome the severe effects of home-sick feelings during my stay in Finland. Special thanks go to Professor Pertti Hari from whom I have benefited in the form of discussion and unfailing help. Professor Juha Helenius and Associate Professor Suree Bhumibhamon (Kasetsart University, Bangkok) have reviewed the manuscript, and I am very grateful for their valuable comments and suggestions for improving this work. I also express my sincere gratitude to Professor Hassan Osman Abel Nour for his continuous encouragement, help and guidance for being a good forester. I also acknowledge the Gum Arabic Company for providing funds during the first stage to start the research work. Special thanks and appreciation are due to my friend Mr. Gerd Weusteny, and also to the Mikko Kaloinen Foundation for financial support. My thanks also goes to Jonas Andrö, Lund University, Sweden. I am gratefully acknowledging the support provided by the Forests National Corporation (FNC) to me during my study and fieldwork. I have been lucky to know and to work with a nice group at VITRI with which I have had many useful discussions. Special thanks are delivered to Elamin Raddad, Ping Zhou, Dr. Vesa Kaarakka, Dr. Mark Appiah, Dr. Riikka Otsamo, Dr. Eshetu Yirdaw, Sakina Elshibli, Anu Eskonheimo, Eddie Glover, Minna Hares, Jörn Laxén, Teija Reyes, and Olli Taskinen. During field experiments, I got unlimited assistance from my colleagues and friends at FNC and the Agricultural Research Corporation/ Forestry Research Center. From FNC, I wish to mention Dr. Abdel Azim Mirghani Ibrahim, Ali Korak, Abdel Hamid Adam, Mamoun Gasim Musa, and Osama Gasmel Seed. Very special and sincere thanks go to Hassan Amin, Mona Mahmoud, Mohamed Idrees, Sawsan Abdalla, Mohamed Osman, Fadlalla Sirag, Mustafa Yousif and Mohamed Harin for their very valuable assistance and help in fieldwork. From ARC, I wish to mention Dr. Mukhtar 5 Ballal, Hiba Mahmoud, Hatim Abdalla, Bashir Awad and Dr. Abdel Rahman Al Khidir. My thanks also go to the ARC staff in the Domokeya experimental forest. My sincere thanks are extended to Dr. Ayoub Khalil, University of Juba, for his valuable technical advice; to Dr. Abdel Azim Yassin at University of Khartoum, who introduced me patiently to statistical and experimental analysis; and to Dr. El Nour El Siddig at the same university for his help and support. My thanks are also extended to Dr. Balgis Osman and Nagmeldin Gutbi, Higher Council for Environment and Natural Resources, Sudan. Finally, I would like to express my deepest cordial thanks to my family, especially to my mother for her unlimited spiritual moral support, my wife Samira for her understanding and kind support by all means, and to my kids Alaa, Mohamed and Abu Bakr for always being so patiently waiting during my long absence. Helsinki, January 2005 Abdalla Gaafar 6 List of acronyms and abbreviations A Photosynthesis rate (m mol CO m-2s-1) 2 ABA Abscisic acid ANOVA Analysis of variance ARC Agricultural Research Corporation C Intercellular carbon dioxide concentration i DBH Diameter at breast height FNC Forests National Corporation g Stomatal conductance (mol CO m-2s-1) s 2 ha Hectare H Harvest index I IWUE Intrinsic water use efficiency MPa Megapascal N Nitrogen OC Organic carbon PAR Photosynthetically active radiation ppm Parts per million RH Relative humidity VPD Vapor pressure deficit m mol Micro mole Y Morning leaf water potential (MPa) Lm 7 CONTENTS Abstract Preface List of the main acronyms and abbreviations 1. INTRODUCTION......................................................................................................................................9 1.1BACKGROUND.........................................................................................................................................9 1.2FORESTRESOURCES................................................................................................................................9 1.3LAND USE, SHIFTING CULTIVATION AND AGROFORESTRY......................................................................10 1.4THE AIM OF THE STUDY.........................................................................................................................13 2.THEORETICAL FRAMEWORK AND SETTING OF HYPOTHESES.............................................14 2.1AGROFORESTRY POTENTIAL..................................................................................................................14 2.2TREE-CROP INTERACTIONS....................................................................................................................15 2.2.1 Competition and facilitation.......................................................................................................15 2.2.2 Physiological adjustment in response to soil water....................................................................19 2.2.3 Water use efficiency....................................................................................................................20 2.3HYPOTHESES.........................................................................................................................................22 3. MATERIAL AND METHODS................................................................................................................23 3.1AGRICULTURAL CROPS INKORDOFAN,SUDAN......................................................................................23 3.2STUDY AREA.........................................................................................................................................24 3.3FIELD EXPERIMENTS..............................................................................................................................25 3.4STATISTICAL ANALYSIS.........................................................................................................................28 4. RESULTS..................................................................................................................................................29 4.1SOIL WATER AND PHYSIOLOGICAL CHARACTERISTICS OFA. SENEGAL IN THE NATURAL FOREST.............29 4.1.1 Water in the soil profile..............................................................................................................29 4.1.2 Effect of tree size on soil moisture..............................................................................................29 4.1.3 Effect of soil water on physiological characteristics..................................................................32 4.2TREE SIZE AND PHYSIOLOGICAL CHARACTERISTICS IN THE NATURAL FOREST........................................36 4.3CORRELATIONS BETWEEN PHYSIOLOGICAL CHARACTERISTICS IN THE NATURAL FOREST.......................36 4.4GUM PRODUCTION.................................................................................................................................39 4.5NITROGEN AND CARBON UNDER NATURALA. SENEGAL..........................................................................41 4.6AGROFORESTRY(INTERCROPPING)........................................................................................................43 4.6.1 Correlation with soil water content............................................................................................43 4.6.2 Gum production..........................................................................................................................47 4.6.3 Soil water and agricultural crop physiology..............................................................................49 4.6.4 Agricultural crop production......................................................................................................52 4.6.5 Effect of agroforestry on cropping-system rain use efficiency (RUE)......................................56 5. DISCUSSION............................................................................................................................................58 5.1INTERACTION OF TREE SIZE, SOIL WATER AND TREE PHYSIOLOGICAL TRAITS.........................................58 5.1.1 Effect of tree size on soil moisture..............................................................................................58 5.1.2 Effect of tree size on physiological characteristics....................................................................59 5.1.3 Effect of soil water on tree physiological behavior....................................................................59 5.1.4 Interaction between physiological characteristics.....................................................................61 5.1.5 Gum production..........................................................................................................................64 5.1.6 Soil nitrogen status underA. senegal.........................................................................................65 5.2AGROFORESTRY....................................................................................................................................66 5.2.1 Effect of tree density on soil moisture........................................................................................66 5.2.2 Effects of intercropping design and soil water on gum production...........................................67 3.2.3 Effects of soil water and intercropping on tree physiology........................................................67 5.2.4 Effects of intercropping design on tree intrinsic water use and rain use efficiency.................68 5.2.5 Effects of intercropping design on crop yield and physiology...................................................68 6. CONCLUSIONS.......................................................................................................................................75 7. REFERENCES.........................................................................................................................................77 APPENDIX 1. COMPLEMENTARY FIGURES......................................................................................97 8 1. Introduction 1.1Background Sudan is a vast country with an area of approximately 2.5 million square kilometers. The most prominent feature of the country is the Nile valley. The Nile River is constituted by the Blue Nile and its tributaries which originate in the Ethiopian highlands, and the White Nile and its tributaries which originate from the Equatorial lakes. The soil in about 60% of the country in the northeast, north and northwest is predominantly sand. Heavy cracking clay soils form a triangular central and eastern plain, which makes up to 30% of the country. Red soils of different types are characteristics of the remaining southwestern portion. Most of the country is flat, except for some few mountain massifs such as Imatong Mountains, Jebel Marra and Erkawit, and other rocky hills. The country is characterized by its long latitudinal extension from approximately 3°N to 22°N, and as such it extends from the region of tropical mixed deciduous forests in the south with an annual rainfall exceeding 1500 mm to the northern desert with a negligible amount of rainfall. Sudan’s total population in 1999 was 29.9 million, of which 75% are rural. The total population growth rate is estimated at 2.9% per year. The economy is mainly agriculture (including livestock production, forestry and fishing), which contributes about 48% to the Gross Domestic Product (GDP). The forestry sector currently contributes 12% of the Sudanese GDP, a ratio expected to decrease with the increase of the oil revenue (Abdel Nour 1999). 1.2Forest Resources Forests in the Sudan render direct and indirect benefits. The indirect benefits include the protection of the environment, watershed protection, soil improvement, work opportunities, browsing and grazing for domestic and wild-life animals, biological diversity conservation, recreation, etc. The most important direct benefits are fuelwood and industrial and building timber. The Forest Products Consumption Survey (FNC, 1995) showed that 70% of the energy consumption in the Sudan is in the form of wood fuel and other biomass. This is equivalent to eight million tons of oil annually. In addition, Sudan is the world's largest producer and exporter of gum arabic, contributing 80 to 90%of the world supply of this commodity. In the mid-60s, gum was exported in great volumes, around 50,000 tons per annum, but the export has decreased steadily to 18,800 tons during the 90s (El-Dukheri 1997). The coverage of the natural forests was estimated by FAO (1990) as being 19% of the total land area (Mohamed and Bulgies 1997). The recent Global Forests Resources Assessment (FRA) undertaken by FAO (FAO 2000) gave a country forest cover estimate of 17% and an area for other wooded land as 10%, giving a total area of 27% of the country under forest or wooded vegetation cover. The annual deforestation rate was estimated as 1.4%. Taking the FRA (2000) figures, the deforestation rate in the Sudan is still higher than world average (0.24%) or the African average (0.78%). The rate of deforestation in the country has been estimated as being almost 29 times higher than the rate of the reforestation (Mohamed 1997). 9 The survey of forest product consumption (FNC 1995) and the National Forest Inventory (FNC 1998) gave a very grim picture of deforestation in Central Sudan (Mohamed and Bulgies 1997). The demand and supply study results projected to 1998 gave a total annual consumption of wood of 18.2 million m3, compared to an annual allowable removal estimate of 11.7 million m3. The same study also estimated that the deforestation rate was about 4.5%, a rate that can be considered very alarming. As a result, national attention has been given to agroforestry and community forest management. The government has realized that it lacks the capacity of managing these vast areas and coping with this situation. Gum Arabic Belt The most important forest in the Sudan may be the gum arabic belt, which lies within the low-rain savanna zone. The term gum arabic belt is used to indicate a zone of mainly 520,000 km2 in area that extends across Central Sudan between latitudes 10(cid:219) and 14(cid:176) N, accounting for one fifth of the country's total area (IIED and IES 1990). The belt is considered as an important area because it accommodates around one fifth of the population of the Sudan and two thirds of its livestock population. The belt acts as a natural barrier to protect more than 40% of the total area of Sudan from desert encroachment. The belt is also represents a site of intense and diverse human activities where most of the agriculture and animal production are practiced. This includes irrigated agriculture, mechanized rainfed agriculture, traditional rainfed agriculture and forestry (Ballal 2002). The commercial gum arabic is an ancient ingredient that has been used since 4000 BC or before. It exudes fromAcacia senegal (L.) Wild. trees in the form of large (5 cm diameter) nodules or “tears”. Mature trees, 4.5-6 m tall and 5-25 years old, are tapped by making incisions in the branches and stripping away the bark to accelerate exudation. A. senegal, the tree commonly known as “hashab”, grows naturally in this belt. The tree has an important role in fulfilling household wood energy and fodder demands, besides enriching the soil fertility, possibly also through biological nitrogen fixation (Ballal 1991). Based on a classification by Harrison and Jackson (1958), A. senegal occurs in a number of vegetation types ranging from the semi-deserts and grassland zone in the north of the Sudan to the Terminalia-Sclerocarya-Anogeissus- Prosopis savanna woodland in the south of the country. 1.3 Land use, shifting cultivation and agroforestry A typical land use practice that prevailed in Kordofan, western Sudan, in the past was shifting cultivation with or without bush-fallow (Seif El Din 1984). The A. senegal agroforestry system is practiced as a means of restoring the soil fertility and promoting gum arabic production (FAO 1978, DANIDA 1989). Gum production is a pillar of family economy and considered as an income-generating source that requires only a low input of work after the rainy season. The bush-fallow system was built in a way that achieves ecological balance using traditional shifting cultivation, where a long fallow period maintains the soil fertility. Until recently, the traditionalA. senegal-based agroforestry system was recognized and considered one of the most successful forms of natural forest management in the tropical drylands (Fries 1990), and regarded as sustainable in terms of its environmental, social and economic benefits (Ballal 1991). 10