ANDREW D. HIRONS is a Senior Lecturer in Arboriculture at University Centre Myerscough, UK. He has international experience as a climbing arborist and a plant health care practitioner. As well as lecturing on a range of arboricultural courses he is also actively involved in research. His current research activity is motivated by the need to create resilience in our urban forests, and is focused on using plant traits to inform species selection for urban environments.

PETER A. THOMAS is a Reader in Plant Ecology at Keele University, UK. He has more than 30 years of experience in ecological aspects of trees and forests in the UK, Europe, North & Central America, Africa, Russia, Asia and Australasia.

List of Contributors xv

Foreword xvii

Preface xix

A Note on the Text xx

1 Introduction 1

Value of Trees Globally 1

Value of Urban Trees 2

Managing Trees 5

References 11

2 The Woody Skeleton: Trunk and Branches 15

What is a Tree? 15

How Does a Tree Grow? 15

Tree Design 18

How Shoots Grow 19

Building Blocks: Meristems and Buds 19

New Shoots From Buds 22

Apical Dominance and Apical Control 25

Epicormic Shoots and Sprouting 26

Epicormic Shoots (Sprouts) 27

Basal Sprouts 29

Branch and Trunk Sprouts 30

Opportunistic Sprouts 31

Sprouts Originating Underground 32

Practical Considerations of Sprouting 33

Sprouting in Response to Mechanical Injury 34

Sprouting in Response to Disturbance 34

Secondary Growth 34

The Vascular Cambium 35

Thickening of Woody Cell Walls 38

Programmed Cell Death 40

Bark and Secondary Phloem 40

How Bark Grows 41

Variation in Bark 43

Secondary Xylem - Wood 44

Different Cell Types Found in Wood 46

Living Cells in the Wood - Parenchyma 47

Non¿Living Cells in the Wood - Vessels, Tracheids and Fibres 49

Variation in Wood Structure 52

Gymnosperm Wood 52

Dicotyledonous Wood 53

Sapwood and Heartwood 56

Sapwood and Water Movement 57

Heartwood 59

Sapwood and Heartwood - Considerations for Pruning Operations 62

Trade¿offs in Wood Design 62

Trade¿offs and the Movement of Water 63

Freezing¿Induced Cavitation 66

Drought¿Induced Cavitation 67

Moving Water Around a Tree - Vascular Sectorality 71

References 72

3 Leaves and Crowns 77

Angiosperm Leaves 78

Angiosperm Leaf Anatomy 82

Gymnosperm Leaves 83

Gymnosperm Leaf Anatomy 85

Juvenile Leaves 86

Sun and Shade Leaves 87

Leaf Arrangement 90

Compound Leaves 91

Evergreen and Deciduous Leaves 93

Value of Evergreen and Deciduous Leaves 94

Leaf Phenology 97

Tree Crowns 101

Shape of Tree Crowns 104

Role of Branches in Tree Crowns 107

Biomechanical Design of Tree Crowns 108

Reaction Wood 122

Branch Shedding as a Natural Process 124

Tree Pruning 126

Pruning Practices 129

Tree Crown Support 133

References 135

4 Tree Roots 141

Root Growth and Development 141

Root Systems 146

Secondary Root Growth 148

Root Architecture 150

Tree Anchorage 153

Extent of Root Systems 154

When Do Roots Grow? 157

Soil Compaction 161

Soil Resistance to Root Development 163

Management of Soil Compaction 165

Mulching 168

Decompaction 168

Estimating Appropriate Soil Volumes for Tree Roots 171

Improving Soil Volumes in Urban Environments 179

References 183

5 The Next Generation of Trees: From Seeds to Planting 187

Flowers, Seeds and Fruits 187

Variation in Flowers and Pollination 189

Not All Seeds Require Pollination 191

Cost of Reproduction 193

Numbers Involved 195

Flowering and Fruiting in Urban Landscapes 196

Tree Crops 198

Vegetative Reproduction 199

Growing Trees 203

Seeds and Their Origins 203

Storing Seeds 205

Seed Dormancy 205

Germination 207

Seedlings 208

Tree Establishment - From Production to the Landscape 210

Momentum of Tree Establishment 211

Tree Species Selection 212

Tree Quality 219

Nursery Production 219

Rooting Environment 225

Arboricultural Practices 227

References 231

6 Tree Water Relations 239

Water is Fundamental to Tree Development 239

Importance of Water Potential 240

Trees Experience Soil Water Potential, Not Soil Water Content 241

Managing Soil Water Availability 243

Fine Roots are Critical for Water Absorption 249

Hydraulic Redistribution 251

Ascent of Sap from Roots to Shoots 253

Transpiration 255

Resistance to Water Loss 255

References 258

7 Tree Carbon Relations 261

Carbon Moves from Source to Sink via the Phloem 262

Light and Other Environmental Variables That Influence Photosynthesis 263

Coping With Low Light 266

Coping With Too Much Light 268

Practical Implications of the Light Environment and Shade Tolerance 269

Other Key Factors Influencing Photosynthesis - Temperature, Nutrition and Water 270

Species Differ Widely in Their Leaf Photosynthetic Capacity 271

The Big Picture - Carbon Gain Over the Years 273

Carbon Dynamics in Trees: Production, Use and Storage 275

How Do Trees Die? 278

Improving the Carbon Balance in Landscape Trees 280

Annual Carbon Dynamics of the Tree and the Timing of Arboricultural Work 280

References 281

8 Tree Nutrition 285

Essential Nutrients 287

Nutrient Uptake 287

Symbiotic Relationships That Help Nutrient Acquisition 289

Other Factors That Influence Nutrient Availability -pH, Moisture, Aeration, Temperature 292

Nutrient Cycling 294

Managing Tree Nutrition 298

References 301

9 Interactions With Other Organisms 303

Trees as Habitats and Hosts 303

Plants and Epiphytes 303

Microorganisms 306

Symbiotic Fungi 306

Commercial Inoculants 308

Pathogenic Fungi 309

Defence of Stems 315

Historical Context of Stem Defence 316

Stem Defence 316

Effect of Wounding to the Bark 318

Effect of Wounding to the Sapwood 320

Pruning and Wounding 323

Decay in Stems 324

Bacteria 326

Insects 328

Pollinators and Defenders 328

Sap Suckers and Defoliators 328

Wood and Bark Borers 330

Synergy of Pests, Diseases and Environmental Stress 332

Mammals and Birds 333

Seed Dispersers 333

Injury by Birds and Mammals 334

Managing Trees as Habitats 334

Deadwood 342

References 346

10 Environmental Challenges for Trees 351

Avoidance and Tolerance of Plant Stress 351

Acclimation and Adaptation 352

Cold¿Hardiness 353

Acquiring Cold¿Hardiness 353

Cold¿Hardiness Maps 354

Cold Injury to Trees 356

Avoiding Freezing in Below¿Zero Temperatures 357

Ice Formation Outside of the Cell Protoplast 357

Frost Injury 358

High Temperatures 361

Coping with High Temperatures 362

Drought and Water Deficits 364

Water Deficits and Tree Development 365

Resistance of Water Deficits Using Avoidance and Tolerance Strategies 369

Drought Tolerance for Difficult Urban Sites 372

Flooding and Waterlogging Tolerance 376

Flooding Injury 377

Flooding and Soils 378

Variation in Tolerance to Flooding 378

Structural Adaptations to Flooding 378

Physiological Adaptations to Flooding 381

Riparian Trees Adapted to Urban Environments 382

Salt Tolerance 382

Dehydration and Toxicity Injuries in Saline Soils 383

Managing Saline Soils in Amenity Tree Planting 384

References 385

Index 391