Durham E-Theses The impact of herbivores on the natural regeneration of temperate deciduous woodland Mallinson, Julian Robert How to cite: Mallinson, Julian Robert (1999) The impact of herbivores on the natural regeneration of temperate deciduous woodland, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/4591/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk 2 THE IMPACT OF HERBIVORES ON THE NATURAL REGENERATION OF TEMPERATE DECIDUOUS WOODLAND Julian Robert Mallinson Department of Biological Sciences University of Durham The copyright of this thesis rests with the author. No quotation from it should be published without the written consent of the author and information derived from it should be acknowledged. A thesis submitted for the degree of Doctor of Philosophy University of Durham, 1999 To my family and friends DECLARATION I declare that this thesis is original and entirely my own work, except where otherwise stated. No part of it has been submitted previously for a degree at any other university. COPYRIGHT The copyright of this thesis rests with the author. No quotation from it should be published without their prior written consent and information derived from it should be acknowledged. HI ABSTRACT Many studies have reported deficient or intermittent patterns of natural regeneration in temperate deciduous woodland. The present study aimed to assess the relative impact of herbivore-mediated plant mortality on the natural regeneration dynamics of representative tree species (Acer pseudoplatanus, Betula pendula, Fraxinus excelsior, Ilex aquifolium, Sorbus aucuparia, Taxus baccata and Ulmus glabra) in representative temperate deciduous woodland (Shipley Wood and Derwent Gorge, County Durham). Sapling density and the density, spatial association and size distribution of adult tree populations varied significantly between tree species and study sites, reflecting contrasting patterns of current and historical regeneration. Rates of post-dispersal seed predation and seedling herbivory were quantified using field-based 'cafeteria' trials. Rodents were the principal agents responsible for seed predation, whereas seedling herbivory was attributable to a mixed suite of herbivores including invertebrates, rodents and larger mammals. Rates of seed predation and seedling herbivory varied significantly between tree species, most likely reflecting individualistic, trade-off responses to chemical and physical attributes. Fine-scale spatial variation most likely reflected the preferential foraging of rodents beneath protective vegetation cover. There was no consistent evidence to support the hypothesis that dispersed seeds may escape disproportionately high offspring mortality beneath parent plants resulting from increased herbivore activity. Natural seedling density varied significantly between years and between tree species, according to the abundance of viable seeds produced by conspecific adult trees. Although seedling survivorship varied significantly between species, the survivorship of each species was similar between years and between cohorts of the same year. Canopy cover, field layer cover or correlated factors were significant determinants of seed germination and seedling emergence, establishment and survival, according to age- and species-specific tolerances. In relative terms, patterns of natural regeneration were primarily herbivore- limited (Acer and Taxus), microsite-limited (Betula, Fraxinus, Ilex and Ulmus) or limited by herbivores and microsites (Sorbus). The availabihty of viable seeds may have also limited the recruitment of Ilex, Sorbus, Taxus and Ulmus. Vegetative expansion, mast seeding, seed bank regeneration and repeated, prolonged reproduction may have reduced the actual impact of herbivory on natural regeneration, such that long-lived iteroparous tree species were unlikely to have been critically dependent on current recruitment. IV ACKNOWLEDGEMENTS I am particularly indebted to my supervisors, Phil Hulme and Brian Huntley, for their continued guidance, interest and support. I greatly appreciate the advice, time and encouragement they generously gave. I would like to thank Eric Henderson, Michael Bone, Gordon Bainbridge, Margaret Turner and Paul Simpson for all their technical assistance. I am also grateful to Phil Gates, Dave Hyde, Mike Stacey and Peter Evans for their kind support. I have had the privilege of working alongside fantastic lab-mates, including Dawn Scott, Steve Willis, Liam Simms, Yih-Yia Liao, Panos Giannakos, Laura Bonesi and Caroline Labum. Thanks for answers to questions, stimulating discussion, coffee breaks, inspiration, stress relief, nights out and friendship. These are precious memories. I am also grateful to fellow ecologists Mark O'Cormell, Sean Twiss, Kate Lewthwaite, Larry Griffin and Claire McSorley for their advice and encouragement. Special thanks go to Justin Slowey for his great company and assistance in the field. I would like to thank Lanibton Estates, Mr. P. Oughtred and Mr. W. Metcalf for kindly permitting access to study sites and Simon Walker of English Nature for providing further support in the field. Research was funded by a University of Durham Teaching Assistantship to Julian Mallinson. This thesis is dedicated to my family and friends. Thanks for everything. "The architect, by the relationships which he creates he wakes in us profound echoes, he gives us the measure of an order which we feel to be in accordance with that of our world, he determines the various movements of our heart and of our understanding. It is then that we experience the sense of beauty" Le Corbusier TABLE OF CONTENTS Page ABSTRACT iv ACKNOWLEDGEMENTS v TABLE OF CONTENTS vi LIST OF TABLES XI LIST OF FIGURES xiv LIST OF PLATES xvi CHAPTER 1 GENERAL INTRODUCTION 1 1.1 Global vegetation patterns 1 1.1.1 Characteristics of temperate deciduous forest 2 1.1.2 Status of temperate deciduous forest 5 1.2 Regulation of plant populations and communities 5 1.3 Impact of herbivores on plant performance 7 1.3.1 Impact on plant survival 7 1.3.2 Impact on plant growth 10 1.3.3 Impact on plant fecundity 10 1.3.4 Anti-herbivore defence 12 1.3.5 Plant compensation 13 1.3.6 Plant benefits from herbivore activity 14 1.4 Impact of herbivores on plant demography 15 1.4.1 Impact on natural regeneration 16 1.4.1.1 Pre-dispersal seed predation 17 1.4.1.2 Post-dispersal seed predation 17 1.4.1.3 Seedhng predation 18 1.5 Study aims 18 CHAPTER 2 STUDY SITES AND STUDY SPECIES 21 2.1 General description of study sites 21 2.1.1 Shipley Wood 21 VI 2.1.2 Derwent Gorge 24 2.2 General description of study species 27 2.3 Detailed description of tree populations 31 2.3.1 Materials and methods 31 2.3.2 Results 32 2.3.2.1 Composition, density and canopy basal area of adult tree populations 32 2.3.2.2 Spatial association of adult tree populations 34 2.3.2.3 Sapling recruitment and the size distribution of adult tree populations 35 2.3.2.4 Soil pH 44 CHAPTER 3 POST-DISPERSAL SEED PREDATION 46 3.1 Introduction 46 3.1.1 Studies of post-dispersal seed predation 46 3.1.2 Study aims..... 52 3.2 Materials and methods 53 3.2.1 Design of field trials.... 53 3.2.2 Analysis of seed characteristics 56 3.2.3 Statistical analysis 56 3.3 Results 58 3.3.1 Seed encounter 58 3.3.1.1 Variation between treatments 58 3.3.1.2 Spatial variation 59 3.3.1.3 Variation between species 60 3.3.2 Seed exploitation 61 3.3.2.1 Variation between treatments 61 3.3.2.2 Spatial variation 62 3.3.2.3 Variation between species 63 3.3.3 Variation with respect to seed characteristics 64 3.3.4 Impact of predator groups 65 3.4 Discussion 66 3.4.1 Impact of predator groups 66 Vll 3.4.2 Rodent foraging behaviour 66 3.4.3 Variation between species 68 3.4.4 Variation between microhabitats 72 3.4.5 Impact of seed predation on plant demography 76 CHAPTER 4 DISTANCE- AND DENSITY-DEPENDENT POST-DISPERSAL SEED PREDATION 77 4.1 Introduction 77 4.1.1 Studies of distance- and density-dependent post-dispersal seed predation 77 4.1.2 Study aims 83 4.2 Materials and methods 85 4.2.1 Design of field trials 85 4.2.2 Statistical analysis 86 4.3 Results 88 4.3.1 Seed encounter 88 4.3.1.1 Variation between treatments and species 88 4.3.1.2 Density-dependent seed encounter 89 4.3.1.3 Distance-dependent seed encounter 91 4.3.2 Seed exploitation 93 4.3.2.1 Variation between treatments and species 93 4.3.2.2 Density-dependent seed exploitation 94 4.3.2.3 Distance-dependent seed exploitation 95 4.3.3 Impact of predator groups 96 4.4 Discussion 97 4.4.1 Impact of predator groups 97 4.4.2 Variation between tree species 97 4.4.3 Variation with seed density 98 4.4.4 Variation with distance from the aduh 99 4.4.5 Variation within tree species 100 4.4.6 Variation between predator groups 101 4.4.7 Evaluation of the 'Herbivore Escape Hypothesis' 101 Vlll CHAPTER 5 SEEDLING HERBIVORY 104 5.1 Introduction 104 5.1.1 Studies of seedling herbivory 104 5.1.2 Study aims 109 5.2 Materials and methods 110 5.2.1 Nursery practice 110 5.2.2 Design of field trials Ill 5.2.3 Analysis of seedling characteristics 112 5.2.4 Statistical analysis 112 5.3 Results 114 5.3.1 Seedling encounter 114 5.3.2 Seedling damage 117 5.3.3 Variation with respect to seedling characteristics 119 5.4 Discussion 121 5.4.1 Impact of predator groups 121 5.4.2 Variation between species.