Grazing Management and Plant Community Composition on Bodmin Moor by Gavin Stewart A thesis submitted to the University of Plymouth in partial fulfilment for the degree of Doctor of Philosophy School of Geographical and Geological Sciences and School of Biological Sciences, Faculty of Science November 2002 iv UNIVERSITY OF PLYMOUTH Item No. ~ 0 \ <Sb~ Daie - 3 t-EB 2003 ~ Class No. J -r £ Table of contents List of figures X Lists of tables xiv Acknowledgements xvii Summary xviii 1. General Introduction 1 1.2 Introduction 1 1.2.1 Project rationale ............... ... .......... ................................. ! 1.2.2 The upland resource .............. ....... ................................. ! 1.3 Grazing in upland systems 3 1.4 Environmental heterogeneity within the British uplands 5 1.5 Bodmin Moor 5 1.6 Aims 6 1. 7 Thesis outline 7 2. The ecological character of Bodmin Moor 10 2.1 Introduction 10 2.2 Physical features 10 2.2.1 Geology and Geomorphology .. .. .............. ...................... 10 2.2.2 Landform, drainage and settlement. ............................. 13 2.3 Climate 15 2.3.1 General description ....... .... .... ...... ........................... ...... 15 2.3.2 Quantifying rainfall and oceanicity ............................... 16 2.4 Soils 18 2.4.1 Introduction. ........ ........................................................ IS 2.4.2 The Soils of Bodmin Moor ..... ................. ...................... 19 2.4.3 Vegetation and soils .......................... ......... .................. 22 2.5 Land use on Bodmin Moor 23 2.5.1 Past management and vegetation history ..................... 23 2.5.2 Vegetation management and land use during the 20th century .......................... ............................................................ ...... 31 2.6 Vegetation and conservation 37 2.6.1 The vegetation of Bodmin Moor and its conservation value ............................................................................................ 37 2.6.2 Conservation legislation and objectives. The future management of Bodmin Moor? .............................................. 45 2. 7 Conclusions 51 3. Grazing management and vegetation in upland habitats 53 3.1 Introduction 53 3.2 The direct and indirect mechanisms by which grazing affects vegetation 53 V Table of contents 3.2.1 Grazing and species diversity .......... ... .......... ................ 53 3 .2 .2 Trampling effects ........... ....... ... ... ... ............... .... .. ... .... .. 54 3.2.3 Effects of defecation .. .................... ............................... 55 3.3 The effect of different species and breeds of grazer within and between plant communities 56 3 .3 .1 Introduction ................................... ............. ................. 56 3.3.2 The effects of different stocking intensities ................... 64 3 .3.3 The effects of timing .. ............ ......... ..... ......... ................ 66 3 .3.4 The effect of supplementary feed .. .. ............. .. ....... ........ 67 3 .3.5 The effect of stock control (shepherding) .. ... .... .............. 68 3 .3.6 Interactions between grazing and other environmental variables ............................ ....... ................ ... ....... .. ................ 68 3.4 Conclusions 70 4. Succession in relation to grazing 72 4.1 Introduction 72 4 .1.1 The need for a coherent su ccessional paradigm .. .. ........ 72 4 .1.2 Definitions of succession .................... .. .. ...................... 72 4.2 Models of succession 74 4.2.1 Introduction ......................... ............... ............ ........... .. 74 4.2.2 Discrepancies between the models ....... .. ... ................... 74 4.3 Reductionism and competition theory 77 4.4 Multiple pathways 82 4.5 The effect of burning and grazing on upland plant community succession 83 4 .5 .1 Introduction ............... .................. .. ..... .............. ........... 83 4.5.2 Mire and wet heath communities ....... .......................... 88 4.5.3 Heathland communities ........... ... .. ...... .. .............. ......... 90 4.5.4 Grassland communities .... ... ...... ... .. .................... ......... 93 4.5.5 Overall implications for Countryside Stewardship prescriptions .. .. ... .......... ................................. .... .................. 95 4.6 Predictive models 95 4.7 The problem of scale 96 4.8 Models like methods are dependent upon objectives 98 4.9 Conclusions 98 5. The plant communities and environmental gradients of Bodmin Moor 102 5.1 INTRODUCTION 102 5.1.1 The history of vegetation description in Britain ... ....... 102 5 .1.2 Vegetation description in the South-west uplands of Britain ......... ................................. ......... .......... ................ .. .. ......... 103 5.1.3. Aims .. ..... ... ............. .............. .............. ................ ...... 104 5.2 Methods 104 5 .2.1 Sampling strategy ... .......... .. .............. ......................... 104 5 .2.2 Vegetation description ..... ...................... ...... .. .. .... ....... 105 5.2.3 Collection of environmental data ................................ 106 5 .2.4 Analysis of vegetation and environmental data ........... l lO 5.3 Results 115 vi Table of contents 5.3.1 Classification ............................................................. 115 5.3 .2 Ordination ........................... ...................................... 120 5.4 Discussion 131 5.4.1 Interpretation of species assemblages ........................ 131 5.4.2 The relationships between the different surveys and their classifications .... .................................. ........................ ....... 149 5.4.3 The use of vegetation classification for hypothesis generation concerning grazing mediated transitions ............................. 154 5.4.4 Interpretation of environmental gradients .................. 157 5 .5 Conclusions 160 6. The Soil Seed Banks of Bodmin Moor 162 6.1 Introduction 162 6 .1.1 Aims ............. .. .. ................... ....... ... .. .......................... 164 6.2 Methods 164 6.2.1 Collection of soil seed bank samples .......................... 164 6.2.2 Treatment of seed bank samples and seedling emergence counts ............................. ......................... ........ ...... .. .......... 165 6 .2.3 Data analysis ............................................................. 167 6 .3 Results 168 6 .3 .1 Surface Vegetation and seed bank composition .......... 168 6 .3 .2 Propagule availability .......... ... ...................... .. ............ 170 6 .3 .3 Germination timing .................................................... 173 6 .3.4 Variation in seed bank composition with depth .... ...... 174 6 .4 Discussion 188 6.4.1 Surface vegetation ........... .. ........ ................................ 188 6.4.2 Spatial variation in the composition of seed bank samples ..................... ...................... ......... ................ ... ................... 189 6.4.3 Propagule availability for dwarf-shrub regeneration .... 190 6.4.4 Competitive interactions between germinating species within safe-sites ................. ............... ........................................ .... 193 6.4.5 Heathland restoration: The spatial distribution of germinating propagules within assemblages on Bodmin Moor ................ 197 6.4.6 Heathland restoration: Variation in the timing of germination ................ ....... ...... .. .. ......................................................... 198 6.4.7 Does variation in seed bank composition with depth reflect seed bank persistence and temporal variability? ................. 199 6.4.8 Management implications .. ........................................ 202 6 .5 Conclusions 205 7 . Countryside Stewardship monitoring 207 7 .1 Introduction 207 7 .1.1 General Introduction .................................................. 207 7.1.2 Aims ................... ....................................................... 207 7 .2 Methods 208 7. 2.1 Data collection ....................................... .................... 208 7.2.2 Data analysis ..... ................. ....................................... 2 11 7.3 Results 2 13 vii Table of contents 7.3.1 Temporal variation in plant community composition assessed by frequency ................................. .. ....................... ............. 213 7 .3.2 Temporal variation in plant community composition assessed by biomass .................................. .. .... ................................. 221 7.3.3 Differences between patterns of frequency and biomass225 7 .3.4 Temporal variation in abundance of individual species225 7.4 Discussion 229 7 .4.1 Plant community responses ... ......................... ........... 229 7 .4.2 Individual species responses ...................................... 233 7 .4.3 Management implications .......................................... 236 7 .5 Conclusions 240 8. The response of Molinia caerulea to defoliation 243 8.1 Introduction 243 8.1.1 General Introduction .................................................. 243 8 .1.2 Aims ........................................................... ............... 246 8.2 Methods 246 8.2.1 Fieldwork .. .................... ........ ..................................... 246 8 .2.2 Data analysis ........ ..................................................... 249 8.3 Results 249 8.3.1 Species composition within the experimental plot.. ..... 249 8.3.2 Molinia response to experimental treatments ............. 253 8.4 Discussion 256