
Applied Tree Biology
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Content
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
1
Introduction
Value of Trees Globally
The three trillion trees around the world (Crowther et al. 2015) are hugely important to us and to the well-being of our planet (Figure 1.1). Their value is usually described in terms of ecosystem services - what trees and forests can do to help us humans. A detailed list of ecosystem services provided by trees and forests would fill this book (the UK National Ecosystem Assessment 2011 provides a very good summary) so, by way of illustration, here are just three major services.
Figure 1.1 Forests are globally important to mankind for storing carbon, helping to determine weather patterns and providing a habitat for a vast range of life. This scene is of the temperate forest in Robert H. Treman State Park, New York.
One of the major services is storing carbon. Forests hold around 45% of the carbon stored on land (i.e. not including the reserves held in oceans) which amounts to 2780 Gt of carbon (Giga has nine zeros; i.e. billions). This is about 3.3 times the amount already in the atmosphere (829 Gt). Carbon dioxide in the atmosphere has increased from 280 ppm in pre-industrial times to 404 ppm at the time of writing, an increase of 42%. If all the world's trees died and decomposed to release their carbon into the atmosphere, the atmospheric level of carbon dioxide would rise to 1700 ppm (>600% pre-industrial) with catastrophic effects on our world (UNEP 2008), so global carbon storage in trees and forests is a hugely important service.
Forests also help to determine weather patterns. This is partly by forests evaporating large amounts of water, producing clouds that release rain downwind. Furthermore, it has recently been discovered that a chemical released by trees, pinene (one of the monoterpenes), can help 'seed' clouds by acting as nuclei for water to condense around, and so help clouds to form and rain to fall (Kirby et al. 2016). It seems plausible that other volatile organic compounds (VOCs) emitted by trees have a similar effect. Trees and forests are also beneficial by acting as sponges, slowing the journey of rainfall to the ground and helping to improve soil structure, both of which encourage water to sink into the soil rather than run off the surface. This delays water discharge to streams and rivers, helping to reduce flooding and soil erosion.
Most of the world's biodiversity is held in forests. Tropical forests, which cover 7% of land surface, hold more than 60% of the world's species of terrestrial animals and plants (Bradshaw et al. 2009), and all the world's forests hold more than 80% of species (Balvanera et al. 2014).
Value of Urban Trees
On a smaller scale, urban trees and woodlands also have an important role in our well-being, but for slightly different reasons. Fundamentally, urban trees make our towns and cities better places to live. Quite apart from making urban areas look more appealing, trees can provide a sense of place and time. They help provide outdoor recreation opportunities and make the urban environment more pleasant. Economic benefits of urban trees include higher property values; reduced energy costs of buildings; and reduced expenditure on air pollution removal and storm water infrastructure (Roy et al. 2012; Mullaney et al. 2015). There are also many environmental benefits, the most important of which are summarised in Expert Box 1.1.
With more than half of the world's population now living in cities, one of the most important contributions that trees and green spaces make is to our health. There is a growing body of information that shows that exposure to trees and green spaces improves wellness and our sociability (Wolf and Robbins 2015). Studies have also shown that the positive health impact of trees is independent of access to green space in general. For example, in Sacramento, California, higher tree cover within 250 m of home was associated with better general health, partially mediated by lower levels of obesity and better neighbourhood social cohesion (Ulmer et al. 2016). There is also a body of information that shows that psychological benefits of trees can affect the physiology of our bodies by reducing pulse rate and levels of cortisol, a major stress hormone (Ochiai et al. 2015). This works even when looking at pictures of trees. There is also a physiological response because chemicals released by some trees affect us directly. For example, Ikei et al. (2015) found that oil from the Hinoki cypress Chamaecyparis obtusa, widely used in fragrances in soap, toothpaste and cosmetics in Japan, positively affects brain activity and induces a feeling of 'comfortableness'. This is the basis for shinrin-yoku (forest-air breathing or forest bathing), a popular form of relaxation in Japan, walking through wooded areas or standing beneath a tree and slowly breathing (Figure 1.2). The same monoterpenes that cause cloud formation are known to reduce tension and mental stress, reducing aggression and depression and increasing feelings of well-being. Even a short lunchtime walk of 1.8 km through green areas can improve sleep patterns that night (Gladwell et al. 2016). Moreover, the physiological effects stay with us. A study by Li (2010) found that a 3-day forest visit had positive effects on the immune system up to 30 days later.
Figure 1.2 A sign encouraging people to breathe in the air in a forest in northern Honshu Island, Japan. This shinrin-yoku (forest-air breathing) is a popular form of relaxation in Japan.
The loss of trees from urban environments has also been demonstrated to have negative outcomes for human health. Over 100 million ash Fraxinus spp. trees have been lost in the north-eastern USA since 2002 as a result of the emerald ash borer (EAB), an invasive beetle. This huge loss of trees has been linked to increased human mortality as a result of higher levels of cardiovascular and respiratory diseases (Donovan et al. 2013). Social costs, such as an increase in crime, have also been associated with the loss of trees caused by EAB (Kondo et al. 2017). Consequently, there is a growing body of evidence that the presence of trees in and around our urban environments provides major public health and societal benefits.
However, in some cases, the much-championed value of urban trees is perhaps not all that is claimed. Examples of this include oxygen production and carbon sequestration (the locking-up of carbon). It is true that trees produce an abundance of oxygen. For example, urban forests in the USA have been estimated to produce enough oxygen (61 Mt of it) annually to keep two-thirds of the US population breathing (Nowak et al. 2007). However, given the enormous reserves of oxygen in the atmosphere, this is a fairly minor benefit of urban trees. Another benefit of urban trees that is often over-played is their role in mitigating carbon emissions. Roland Ennos, Expert Box 1.1, points out that Greater London's 8.4 million trees are estimated to store 2.4 million tonnes of carbon (t C) and sequester about 77 200 t C each year (Rogers et al. 2015). This amounts to about 3% of the city's annual carbon emissions or, to put it another way, enough to cover the city's emissions for about 12 days. London's trees sequester only about 0.2% of annual carbon emissions. This is not to disparage carbon sequestration in urban trees, but just to put it into perspective; urban trees are very valuable to us but planting them will not be a solution for climate change or even offset the carbon emissions of our towns and cities to any great extent. In this regard, conservation of the world's forests is of much greater significance.
Although trees are overwhelmingly beneficial for our landscapes and for us, they can also create problems, particularly if they are inappropriately planted, the wrong species is selected for the site or the site is poorly designed with respect to tree development. Trees can get too big for their location; they can conflict with buildings, utilities and sightlines. At certain times of year, pollen from trees can contribute to discomfort amongst those with hay-fever; litter from flowers, fruit and leaves can create slip hazards or block drains. Tree roots sometimes cause damage to pavements, making them uneven, and they may exacerbate damage to pipes by exploiting them as a source of water and nutrition. Occasionally, in dry years, certain species growing on shrinkable clay soils can extract enough water to cause subsidence damage to built structures. Trees may also pose a risk to persons or property if they are structurally unstable or develop extensive decay. But should these potential problems prevent us keeping and planting urban trees? Emphatically not.
Even though many of the problems associated with trees in urban landscapes can be linked to poor planning, design and workmanship, the tree is invariably blamed. Despite the evidence for the benefits of trees, widespread loss of trees from our urban environments is often reported. In the USA it has been estimated that four million urban trees are lost per year (Nowak and Greenfield 2012) and a similar trend can be seen across Europe. More insidiously, even where the total number of trees is not appreciably declining, the size of the tree is changing. In the UK, the...
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