Regeneration in Tropical Eucalypt Forest on Melville Island
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Regeneration in tropical eucalypt forest on Melville Island by ,0 V ```' f- R J. Fensham B. Sc. (Hons.) submitted in fulfillment of the requirements for the degree of Doctor of Philosophy UNIVERSITY OF TASMANIA HOBART SEPTEMBER 1990 DECLARATION This thesis contains no material which has been accepted for the award of any other degree or diploma in any tertiary institution and, to the best of my knowledge and belief, contains no material previously published or written by another person, except when due reference is made in this thesis. Roderick Fensham TABLE OF CONTENTS ACKNOWLEDGEMENTS ABSTRACT i ii LIST OF FIGURES vi LIST OF TABLES viii LIST OF PLATES CHAPTER 1 INTRODUCTION 1 1.1 Forest regeneration 1 1.2 Regeneration in tropical savanna 3 1.3 Thesis aims and structure 5 1.4 Geographical background 9 1.4.1 Study locality 9 1.4.2 Climate 9 1.4.3 Geology, geomorphology and soils 11 1.4.4 Vegetation 13 1.4.5 Fire 14 CHAPTER 2 ENVIRONMENTAL CONTROL OF OPEN VEGETATION GRADIENT ON WESTERN MELVILLE ISLAND 17 2.1 Introduction 17 2.2 Methods 19 2.2.1 Melville Island survey 19 2.2.2 Root excavation 23 2.3 Results 24 2.3.1 Indirect gradient analysis of Melville Island vegetation patterns 24 2.3.2 Direct gradient analysis of Melville Island vegetation patterns 30 2.3.3 Root excavation 43 2.4 Discussion 44 CHAPTER 3 PATTERNS AND DETERMINANTS OF WOODY GROWTH IN THE TROPICAL EUCALYPT FOREST 55 3.1 Introduction 55 3.2 Methods 56 3.2.1 Woody sprout growth 56 3.2.2 Xylem pressure potential 58 3.2.3 Response to fertilizer 58 3.2.4 Clipping versus burning 58 3.2.5 Mature tree growth 59 3.2.6 Analytical methods 59 3.3 Results 61 3.3.1 Generalized patterns of woody sprout growth 61 3.3.2 Species growth patterns 66 3.3.3 Comparisons between sites 67 3.3.4 Xylem pressure potential 68 3.3.5 Response to fertilizer 70 3.3.6 Clipping versus burning 70 3.3.7 Mature tree growth 70 4.3 Discussion 74 CHAPTER 4 NTERACTIVE EFFECTS OF FIRE AND SITE FACTORS IN TROPICAL EUCALYPT FOREST 82 4.1 Introduction 82 4.1.1 Background 82 4.1.2 Site history 84 4.1.3 Physical environment 86 4.2 Methods 86 4.2.1 Sampling procedure 86 4.2.2 Analytical methods 87 4.3 Results 88 4.3.1 Environmental determinants of vegetation gradients 88 4.3.2 Fire and aspect effects on species composition 91 4.3.3 Fire and aspect effects on vegetation structure 95 4.3.4 Fire effects on woody sprout density 97 4.4 Discussion 97 CHAPTER 5 STAND STRUCTURE AND COMPETITION INFLUENCES ON REGENERATION IN THE TROPICAL EUCALYPT FOREST 101 5.1 Introduction 101 5.2 Methods 103 5.2.1 Stand structure 103 5.2.2 Association of saplings in natural canopy gaps 104 5.2.3 Competition experiment 105 5.2.4 Old experimental sites without overstorey 106 5.2.5 Investigations of the lignotuber 107 5.3 Results 108 5.3.1 Stand structure 108 5.3.2 Association of saplings in natural canopy gaps 111 5.3.3 Competition experiment 111 5.3.4 Old experimental sites without overstorey 115 5.3.5 Investigations of the lignotuber 118 5.4 Discussion 123 5.4.1 Competitive influences 123 5.4.2 Can forest structure be maintained under present conditions? 131 CHAPTER 6 PHYTOPHAGOUS INSECT/VEGETATION INTERACTIONS IN THE TROPICAL EUCALYPT FOREST 134 6.1 Introduction 134 6.2 Methods 137 6.2.1 Insect damage 137 6.2.2 Insecticide experiment 141 6.2.3 Phytophagous insect community 142 6.3 Results 145 6.3.1 Insect damage 145 6.3.2 Insecticide experiment 158 6.3.3 Phytophagous insect community 160 6.4 Discussion 176 6.4.1 Comparison with other studies 176 6.4.2 Insect effects on vegetation 180 CHAPTER 7 ECOLOGICAL DETERMINANTS OF SEEDLING SUCCESS IN TROPICAL EUCALYPT FOREST 186 7.1 Introduction 186 7.2 Methods 187 7.2.1 Progress of young seedlings 187 7.2.2 Seedling garden experiment 187 7.2.3 Association of woody species with vegetation pattern 189 7.2.4 Effects of leaf litter on seedlings 189 7.2.5 Allelopathic effects of Eucalyptus tetrodonta leaves on tree seedlings 190 7.3 Results 191 7.3.1 Progress of young seedlings 191 7.3.2 Seedling garden experiment 195 7.3.3 Association of woody species with vegetation pattern 197 7.3.4 Effects of leaf litter on seedlings 201 7.3.5 Allelopathic effects of Eucalyptus tetrodonta leaves on tree seedlings 201 7.4 Discussion 203 CHAPTER 8 GENERAL DISCUSSION 206 8.1 Summary of findings and conclusion 206 8.2 The regeneration process 213 REFERENCES 218 APPENDICES 248 Appendix 1 Correlation of vegetation features and environmental variables for the entire Melville Island data set 248 Appendix 2 Correlation of vegetation features and environmental variables for forest plots only 250 Appendix 3 Soil chemical attributes 252 Appendix 4 Insect damage according to damage type 253 Appendix 5 Insect taxa in samples 259 ACKNOWLEDGEMENTS During the field work phase of my project, Dave Bowman, Bruce Wilson, Jeremy Russell-Smith and Clyde Dunlop provided much stimulus and encouragement. Clyde and his Herbarium staff made it possible to learn the unpublished flora of Melville Island. The implementation and perpetuation of this project was a hair- raising bureaucratic balancing act that was only possible through the support and enthusiasm of several people. Without Dave Bowman's deft administration and financial support, the project could never have got off the ground. Dave's enthusiasm for ecology and learning in general was an important inspiration. The Conservation Commission of the Northern Territory provided the infrastructure during the first year, through their forestry operation on Melville Island. For this period, Bill Freeland, Mery Haines, Ron Hooper and Tim McGuffog were instrumental. The Tiwi Land Council took over the forestry operation during the second year and John Hicks, Greg Sutton and the members of the Council allowed the project to continue, despite the withdrawal of the Conservation Commission from the island. Many visiting friends helped with field work. Dave Bowman, Judith Gallen, Leonie McDough and Bruce Wilson kindly organised the ferrying of gear between Melville and the mainland. Melville Island is home for the Tiwi and they allowed me to live and work on their island for two years. Sharing life with these people had a profound influence on my life and gave me a fresh perception of the bush. The Land Conservation Unit of the Conservation Commission conducted some of the soil analyses. Alan Andersen from CSIRO, Darwin, Gordon Gross, from the S. A. Museum, and Catriona McPhee, from the Museum of Victoria examined insects. Neil Gibson and Bruce Wilson got me started with computing. Zhenjie Lin wrote the computer programs for part of the analysis in Chapter 6. ii Jamie Kirkpatrick has been a great inspiration and friend since I first started studying the bush in 1983. He gave encouragement and feedback throughout the project and particularly during the 'writing up' phase in Hobart. Jamie and Dave Bowman were my excellent formal supervisors for the project. Many colleagues and friends participated in proof reading and the examiners of this thesis made many useful crticisms. Lisa put in the late nights that always seem necessary during the dying stages of episodes such as this one. I thank all of these people immensely. ABSTRACT The eucalypt species of the evergreen savanna forest in monsoonal Australia have a population structure characterized by mature trees, an abundance of short stems in the ground layer and low densities of saplings. Other common trees of these forests, such as the broadleaved species, are almost exclusively represented by these short woody . sprouts. The distinctive structure of the eucalypt forests begs the question as to how these forests are perpetuated. This study seeks to examine tree regeneration in the eucalypt forests of Melville Island in the wet-dry tropics of Australia. The first part of the thesis adresses the influence of the physical environment on regeneration by examining edpahic influences on vegetation structure, growth rates and fire response. The second part of the thesis concentrates on the evergreen eucalypts forests and examines possible causes for the supression of woody sprouts. An attempt was made to define the soil moisture conditions across the range of savanna types. However, elucidating the relationship between edaphic conditions and vegetation patterns is difficult because of the effect of rockiness and soil texture on root penetration. It is reasonably certain that evergreen eucalypts are excluded from one intensively studied site because a clay layer impedes root growth. This low forest is dominated by semi-deciduous eucalypts and the growth of all woody sprouts is slower than in the tall forest. Despite less vigorous growth, broadleaved tree species, such as Term inalia ferdinandiana and Planchonia careya, occupy the mid-canopy at this site. It is suggested that the tall evergreen eucalypts, Eucalyptus miniata, Eucalyptus nesophila and Eucalyptus tetrodonta, suppress the growth of other tree species. Overwood suppression is evident within the tall forest. Tree seedlings grew more rapidly under killed trees than live overwood. Furthermore, clearfelled sites develop saplings although the replacement of the tree canopy occurs sporadically. Few eucalypt saplings developed in an extensive experiment designed to examine the influence of eucalypt and wattle trees and herbage on tree iv regeneration. The saplings of two tree species were weakly associated with natural canopy gaps.