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Other ____________________________________________ University Microfilm s International THE ECOLOGY OF A SINGLE-SPECIES-DOMINANT FOREST AND OF A MIXED FOREST IN ZAIRE, AFRICA By Terese Bulter Hart A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Botany and Plant Pathology ABSTRACT THE ECOLOGY OF A SINGLE-SPECIES-DOMINANT FOREST AND OF A MIXED FOREST IN ZAIRE, AFRICA. By Terese Butler Hart Low species diversity and pronounced dominance have been considered atypical for tropical humid forests. In the Ituri Forest of Zaire, however, a principal forest type is dominated by a single species. In mbau forest, Gilbertiodendron dewevrei accounts for approximately 90* of the canopy-level trees. A second forest type, the more species-rich mixed forest, is found at similar altitudes and on similar topography as mbau forest. The co-occurrence of these two communities suggests that some factor other than climate determines the diversity difference between the two forest types. Four general hypotheses to account for the differences in diversity were tested. Each hypothesis offered contrasting predictions for the two forest types and was tested as follows: 1. Monodominance is symptomatic of early secondary growth. Forest maturity was inferred from the representation of shade-tolerant species in the canopy and understory. 2. Low tree species diversity is found on very poor soils. Soils types were compared for differences in texture and chemical composition. 3. Frequent and/or large natural disturbances interfere with the ability of any single species to attain dominance. Treefall size and frequency was determined. 4. Heavy seed predation under parent trees prevents dominance by any single species. Seeds of canopy trees were monitored for survival both near and distant from adult conspecifics. The results failed to show that any of the proposed hypotheses account in a major way for the maintenance of the two patterns of diversity and dominance in the Ituri Forest. Both forest types have the structure and composition of mature communities and the mbau forest is not restricted to soils too poor for mixed forest. As predicted, there were more frequent treefalls in mixed forest than mbau forest; however, there was great variance in size and frequency of gaps in both forest types. Finally, the dominance of G. dewevrei cannot be attributed to a larger proportion of its seeds escaping predators. It is suggested that natural disturbance of a larger scale and more ancient than measured by this study may been a determinant of present forest distributions. This dissertation is dedicated to the memory of CHUKIZA, son of ANJIANI, nephew of KENGE, who enthusiastically shared his knowledge of the forest and to the memory of PH. GERARD whose enjoyment of mbau forest is alive in his writing. AKNOWLEDGMENTS The research project in the Ituri Forest of Zaire was made financially possible by a grant from the United States Man and the Biosphere Program (federal grant number 4789-6). The project was facilitated by the cooperation of the Institut Zairois pour la Conservation de la Nature (IZCN). Financial support during the period of data analysis and redaction was provided by the African Studies Center and the Department of Botany and Plant Pathology at Michigan State University. Dr. Peter Murphy's confidence and support were the catalyst for the research proposal and essential to its realization. John A. Hart has shared in, and contributed substantially to, all phases of this project. His field assistance, stimulating discussions, and companionship provided direction and the ability to persevere, as needed. Work in the forest would have been many times more difficult without the enthusiastic assistance of the Mbuti Pygmies of the Epulu area, notably, Chukiza, Kenge, Atoka, Mayalimingi, Avion, and Amanjau. Dr. H. Breyne, curator of the national herbarium in Kinshasa, deserves a special thanks for his logistical support and the identifications of plant specimens. Many additional identifications were provided by L. Liben of the National Botanical gardens in Meise, Belgium. The dissertation has been greatly improved by the editorial 10 comments and constructive criticism of Dr. Peter Murphy and my committee members, Dr. John Beaman, Dr. George Petrides, and Dr. Jean Stout. The questions and observations of colleagues have been incorporated into the conception, realization, and analysis of this study. In particular I wish to thank Dr. N. Brokaw, Dr. J. Connell, Dr. D. Kibbs, Dr. D. Livingstone, the ecology group at Kellogg Biological Station, especially, M. Leibold, Dr. T. Miller, and Dr. A. Winn, and my fellow students in the laboratory, especially, V. Dunevitz and Dr. G. Donnelly. In completing this study, as has been the case for all my projects, large and small, I feel deep gratitude toward my family. The encouragment of my parents and parents-in-law has been a constant source of strength. I owe a very special thanks to my husband, John, for his unfaltering good humour and his lavish praise, and, also, to our daughters, Sarah and Rebekah, who never doubted and always provided welcome distraction. 11 TABLE OF CONTENTS List of Tables ................................................... vii List of Figures .................................................. x Introduction ..................................................... 1 Chapter one: The Mbau Forest in Zaire: Edaphic and Successional Gradients in Relation to Single-Species-Dominance in an African Tropical Forest ..................................... 7 Introduction ................................................ 7 Background .................................................. 12 Variation of tropical forest composition with soil factors and successional stages ....................... 12 Single-species dominant forests in tropical Africa and other tropical areas .............................. 16 Objectives .................................................. 18 Study site .................................................. 21 Methods .................................................... 27 Results ..................................................... 30 Forest description .................................... 30 Major species ......................................... 37 Distribution of shade-tolerant species ................ 40 Soils ................................................. 42 Forest islands ........................................ 45 Discussion .................................................. 49 The mbau forest in the Ituri study area ............... 49 Mbau forest in other parts of Gilbertiodendron dewevrei's range ..................... 51 A comparison of other single-dominant tropical forests with the mbau forest of