Applied Tree Biology E-Book

Andrew Hirons

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 912

Veröffentlichungsjahr: 2017

Beschreibung

Many arborists learn tree work practices without fully understanding the biological and physiological principles behind them. However, outcomes for the health and longevity of trees are greatly improved when an arborist understands the science behind the care of tree root systems and crowns. In Applied Tree Biology, Drs. Hirons and Thomas draw upon their decades of experience in the laboratory, classroom, and the field – as well as the expertise of distinguished contributors to this volume – to provide those responsible for tree care with the scientific information that informs best practices for planting, pruning, soil decompaction, irrigation, and much more.

• Takes a multidisciplinary approach, integrating knowledge from plant biology, physiology, arboriculture, ecology, and more
• Provides a systematic presentation of fundamental tree biology and the scientific principles informing high quality tree care
• Presents accessible scientific information and best practices that help promote the health and longevity of trees
• Reflects the authors’ decades of experience as tree biology researchers and educators, as well as their years of professional experience across the globe

Applied Tree Biology is an indispensable source of practical, succinct information on tree biology, physiology, and ecology for professionals and interested amateurs involved with the care of trees. Arborists, foresters, and horticulturists at all stages of their careers will find this text particularly useful. 

Leseprobe

Table of Contents

Cover

Title Page

List of Contributors

Foreword

Preface

A Note on the Text

1 Introduction

Value of Trees Globally

Value of Urban Trees

Managing Trees

References

2 The Woody Skeleton

What is a Tree?

How Does a Tree Grow?

Tree Design

How Shoots Grow

Apical Dominance and Apical Control

Epicormic Shoots and Sprouting

Practical Considerations of Sprouting

Secondary Growth

Bark and Secondary Phloem

Secondary Xylem – Wood

Different Cell Types Found in Wood

Variation in Wood Structure

Sapwood and Heartwood

Trade‐offs in Wood Design

Moving Water Around a Tree – Vascular Sectorality

References

3 Leaves and Crowns

Angiosperm Leaves

Gymnosperm Leaves

Juvenile Leaves

Sun and Shade Leaves

Leaf Arrangement

Compound Leaves

Evergreen and Deciduous Leaves

Leaf Phenology

Tree Crowns

Branch Shedding as a Natural Process

Tree Pruning

Tree Crown Support

References

4 Tree Roots

Root Growth and Development

Root Systems

Secondary Root Growth

Root Architecture

Tree Anchorage

Extent of Root Systems

When Do Roots Grow?

Soil Compaction

Estimating Appropriate Soil Volumes for Tree Roots

Improving Soil Volumes in Urban Environments

References

5 The Next Generation of Trees

Flowers, Seeds and Fruits

Variation in Flowers and Pollination

Not All Seeds Require Pollination

Cost of Reproduction

Numbers Involved

Flowering and Fruiting in Urban Landscapes

Tree Crops

Vegetative Reproduction

Growing Trees

Tree Establishment – From Production to the Landscape

Momentum of Tree Establishment

Tree Species Selection

Tree Quality

Rooting Environment

Arboricultural Practices

References

6 Tree Water Relations

Water is Fundamental to Tree Development

Importance of Water Potential

Trees Experience Soil Water Potential, Not Soil Water Content

Managing Soil Water Availability

Fine Roots are Critical for Water Absorption

Hydraulic Redistribution

Ascent of Sap from Roots to Shoots

Transpiration

Resistance to Water Loss

References

7 Tree Carbon Relations

Carbon Moves from Source to Sink via the Phloem

Light and Other Environmental Variables That Influence Photosynthesis

Other Key Factors Influencing Photosynthesis – Temperature, Nutrition and Water

Species Differ Widely in Their Leaf Photosynthetic Capacity

The Big Picture – Carbon Gain Over the Years

Carbon Dynamics in Trees: Production, Use and Storage

How Do Trees Die?

Improving the Carbon Balance in Landscape Trees

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

References

8 Tree Nutrition

Essential Nutrients

Nutrient Uptake

Nutrient Cycling

Managing Tree Nutrition

References

9 Interactions With Other Organisms

Trees as Habitats and Hosts

Plants and Epiphytes

Microorganisms

Defence of Stems

Insects

Mammals and Birds

Managing Trees as Habitats

Deadwood

References

10 Environmental Challenges for Trees

Avoidance and Tolerance of Plant Stress

Acclimation and Adaptation

Cold‐Hardiness

High Temperatures

Drought and Water Deficits

Flooding and Waterlogging Tolerance

Salt Tolerance

References

Index

End User License Agreement

List of Tables

Chapter 02

Table 2.1 Xylem cell types and principal functions.

Chapter 03

Table 3.1 Variations in the evergreen and deciduous habits of trees.

Table 3.2 Summary of important pruning practices in amenity trees.

Table 3.3 Important examples of national standards or work codes for arboricultural practice (adapted from Johnston and Hirons 2014).

Chapter 04

Table 4.1 Soil volumes required to deliver a determined quantity of water based, on an available soil water content ranging from 10% to 20%. This assumes no rain for 14 days and that roots occupied the entire soil volume.

Chapter 05

Table 5.1 Species that have different ease of storage (orthodox, intermediate and recalcitrant) and different degrees of dormancy. All ‘hard’ seeds (see text) are orthodox.

Table 5.2 Key elements and criteria that may be used to generate robust planting specifications for work contracts and method statements. Adapted from Johnston and Hirons (2014).

Chapter 06

Table 6.1 Important principles when managing soil water availability for trees.

Table 6.2 The water potential of the air (Ψ

air

) at different levels of relative humidity (%) and temperature (°C). Relative humidity describes the degree of saturation in the air as a percentage of the maximum possible saturation at a given temperature. The more negative the water potential, the greater the forces leading to evaporation of water from inside the leaves.

Chapter 08

Table 8.1 Essential nutrients, the available forms and their functions within the tree. When dissolved in water, these include nutrients that have a positive charge (cations) and those with a negative charge (anions). Forms and functions are included that are not discussed elsewhere in the text: they are included for completeness and the interested reader can readily find information on these elsewhere. Concentrations found in parentheses under each element indicate approximate concentrations per gram of dry plant shoot matter sufficient for adequate growth, based on Epstein and Bloom (2005). Other data are from Marschner (2012) and Jones (2012).

Table 8.2 Macronutrients in leaf litter of some temperate tree species. Note that litter of the nitrogen‐fixing grey alder

Alnus incana

has significantly higher levels of nitrogen.

Table 8.3 Mean (± standard deviation) nitrogen (N) and phosphorous (P) concentration in mature leaves (in milligrams per gram of leaf), and resorption efficiency. Different letters within columns indicate statistical difference.

Table 8.4 Nutrient content of the litter layer in temperate deciduous forests in kilograms per hectare (kg ha

–1

). This is based on data from 20 sites across Europe and North America. Values given are the mean and range, and are also translated to an area of 100 m

2

which is taken to represent the canopy area of a single mature tree.

Table 8.5 Annual nutrient uptake rates (kg ha

−1

year

–1

) in temperate deciduous forests. Values given are as in Table 8.4.

Chapter 09

Table 9.1 Summary of the major components of stem defence.

Chapter 10

Table 10.1 Known effects of high temperatures on major cellular, leaf and whole tree processes. An increase, decrease or no change in a process in response to high temperatures is indicated by +, – and 0, respectively. More than one symbol associated with a process indicates between‐ or within‐species variation. ‘Yes’ indicates that acclimation in response to high temperatures has been reported in the literature; ‘No’ indicates that no acclimation has been reported; ‘?’ indicates that acclimation may exist but evidence is limited. PSII is photosystem II; VOCs are volatile organic compounds; thylakoid membranes surround the green chloroplasts in plant cells; rubisco is a protein used in photosynthesis. Many of the cellular and leaf processes are described in Chapter 3.

Table 10.2 Scale used by Niinemets and Valladares (2006) to rank 806 temperate tree species according to their drought tolerance. Trees were allocated a ranking based on their ability to survive on a site, with <50% foliage damage and dieback. P: PET is the ratio of precipitation to potential evapotranspiration.

List of Illustrations

Chapter 01

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.

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.

Figure 1.3 (a) An ash tree

Fraxinus

sp. conflicting with overhead wires in northern Japan. This tree now requires intensive management if it is to persist on this site. (b) A mature oak

Quercus

sp. in Atlanta, USA that has had to endure decades of pruning because it was planted in an unsuitable location.

Figure EB1.1 Infrared image showing people resting in the cool shade provided by park trees. The red in the background shows an area of tarmac, while the yellow shows grass in the sun.

Figure EB1.2 The relative cooling performance of five small street trees in Manchester, UK.

Figure EB1.3 The relative cooling performance of Callery pears

Pyrus calleryana

growing in conventional soil pits within pavements; in grass verges; and in compaction‐resistant Amsterdam structural soil in Manchester, UK.

Chapter 02

Figure B2.1 Cross‐section of a mature tree trunk.

Figure 2.1 (a) Global distribution of tree species known to reach 70 m in height. Most of the tallest species are either conifers from the west coast of North America (represented by blue stars for the top five species and light blue dots for the remainder) or eucalypts in Tasmania (red stars for the three tallest species and light red dots for the remainder), although one dipterocarp species from Borneo (yellow star) and one conifer from New Guinea (blue star) rank among the top 10. Other angiosperm species that can exceed 70 m (pale yellow dots) are found in Southeast Asia, especially Borneo. One tall conifer (pale blue dot) occurs in Eurasia.

Figure 2.2 Tree design is a compromise between carbon gain, stress resilience and biomechanical stability; these primary factors are informed by a number of other factors identified by the subheadings.

Figure 2.3 (a) A woody shoot showing 3 years’ growth. Bud scale scars can be seen as rings at the base of the current and preceding years’ extension growth. Apical meristems can be found at the apex of the main shoot and its side branches. (b) An apical meristem viewed in cross‐section showing the meristem, young leaves (leaf primordia), surrounded by the protective bud scales. The new growth will be linked to the internal plumbing (the vascular tissue) shown developing at the base of the bud.

Figure 2.4 Cross‐section of a young dicotyledonous stem showing the primary structure including the vascular tissue made up of ‘vascular bundles’ or rods, each containing xylem and phloem.

Figure 2.5 An expanding mixed bud of horse chestnut

Aesculus hippocastanum

showing the immature flower structure in the centre of the bud that is surrounded by the characteristic palmate leaves of the species. The bud scales can be seen just hanging on at the base of the developing shoot.

Figure 2.6 Naked buds of (a) frangipani

Plumeria rubra

and (b) candlenut

Aleurites moluccana

.

Figure 2.7 Three patterns of shoot growth (elongation) of temperate tree species. Fixed growth or determinate species show very rapid elongation and reach their maximum elongation with a few weeks. Free or indeterminate species display some shoot elongation throughout most of the growing season. Some species will also occupy intermediate positions.

Figure 2.8 Mature leaves of various maples. (A)

Acer rubrum

(one leaf per node). Early leaves in upper row, youngest (node 1) on the left. (B) Heterophyllous leaves in (a)

Acer tataricum

[nodes 2, 5]; (b)

A. rufinerve

[1, 3]; (c)

A. campestre

var.

leiocarpum

[2, 5]; (d)

A. buergerianum

[2, 7]; (e)

A. monspessulanum

[1, 8] and (f)

A. sempervirens

[1, 4].

Figure 2.9 Black poplar

Populus nigra

often develops sylleptic shoots (arrows) under the favourable growth conditions such as those found at this field trial site in northern Italy.

Figure 2.10 (a) The excurrent tree form of a young giant sequoia

Sequoiadendron giganteum

determined by weak apical dominance and strong apical control. (b) The decurrent form of a mature sycamore

Acer pseudoplatanus

determined by strong apical dominance but weak apical control.

Figure 2.11 Names given to types of sprouts in standing trees. Not all sprouting forms will be found in every species.

Figure 2.12 Cross‐section of an oak log showing branch and epicormic structures: (a) a knot of a regular branch; (b) living epicormic bud with only minor trace expansion, signifying that it has not sprouted; (c) sprouted epicormic buds showing distinct bud traces and expansion of the bud trace at the time of sprouting, as well as development across the cambium; (d) primary epicormic sprout that sprouted 7 years after initiation and subsequently died.

Figure 2.13 As the original crown declines, stem epicormic shoots can often help to develop a new crown that can help the tree survive for many more years, as seen in this sweet chestnut

Castanea sativa

in the Royal Botanic Gardens, Kew. (a) Remnants of the original tree crown can be seen just above the new crown, formed by epicormic development. (b) A closer view of the epicormic branches formed on the lower portion of the trunk.

Figure 2.14 Basal sprouting on narrow leaved ash

Fraxinus angustifolia

, Royal Botanic Gardens, Kew. (a) Early signs of decline can be seen in the crown. (b) A closer view of the basal sprouts.

Figure 2.15 (a) Branch epicormic sprouts growing within the crown on common lime

Tilia

 × 

europea

. (b) Branch epicormic sprouts growing in response to storm damage on sycamore

Acer pseudoplatanus

.

Figure 2.16 Layering occurs when a branch comes into contact with the soil. This produces adventitious roots and allows epicormic sprouts to develop from the point of soil contact. (a) A weeping beech

Fagus sylvatica

‘pendula’ has layered and subsequently produced a number of stems around the periphery of the original crown that have the effect of substantially increasing the radial dimensions of this tree. (b) Layered branches on a horse chestnut

Aesculus hippocastanum

. Both examples are from the Royal Botanic Gardens, Kew.

Figure 2.17 A large lignotuber formed at the base of a eucalyptus tree, growing to the west of Sydney, Australia.

Figure 2.18 Root suckers on: (a) Japanese zelkova

Zelkova serrata

; and (b) Caucasian wingnut

Pterocarya fraxinifolia

, Royal Botanic Gardens, Kew.

Figure 2.19 Vascular cambium development in typical trees. The cambium initially forms between the primary xylem and phloem of the vascular bundle, before expanding around the circumference between the vascular bundles to form a continuous cylinder of meristematic tissues.

Figure 2.20 The cambial zone of a tree. The cambial initials (the original meristem cells) divide in two distinct ways. Anticlinal divisions (perpendicular to the axis of the tree) produce new cambium cells increasing the circumference of the cylinder, and periclinal divisions (parallel to the axis) produce new xylem and phloem.

Figure 2.21 Layers of a cell wall of a woody cell: middle lamella (ML), primary wall (PW) and secondary walls (S

1

, S

2

, S

3

) – see text for a description of these.

Figure 2.22 Cross‐section of a cork oak

Quercus suber

stem showing the thick, corky bark that offers protection from forest fires.

Figure 2.23 Simplified diagram of initial bark or periderm formation in the cortex. Together the cork (phellem), cork cambium (phellogen) and phelloderm make up the periderm.

Figure 2.24 (a) Continuous internal periderms formed immediately underneath the initial periderm gives the smooth bark of trees such as beech

Fagus sylvatica

and hornbeam

Carpinus betulus

. (b) Lenticular periderms formed immediately beneath the initial periderm produces a more textured bark, typical of many temperate trees.

Figure 2.25 (a) Lenticels can often be seen in the form of light flecks on the younger (green) portions of shoots, as in field maple

Acer campestre

. (b) However, lenticels can also be seen in many older stems, as seen in white poplar

Populus alba

. In more deeply fissured bark, the lenticels cannot be easily seen but they will be present.

Figure B2.2 Three‐dimensional structure of wood: (a,b) transverse or cross‐section (×100); (c,d) tangential section (×200); and (e,f) radial sections (×400).

Figure 2.26 (a) Tangential view of

Salix alba

with uniseriate rays (one cell thick) containing heterocellular cells (upright cells on upper and lower margins); and (b)

Acer pseudoplatanus

with multiseriate rays (multiple cells thick) with homocellular cells (all procumbent cells). Further details on the definitions of these terms can be found in the text.

Figure 2.27 Scanning electron micrographs of perforation plates between vessel elements. (a) Simple perforation plates with a single large opening from

Pelargonium

; (b) the parallel bars of a scalariform perforation plate from

Rhododendron

; (c) foraminate perforation plate with circular perforations from

Ephedra

; (d) contiguous scalariform and reticulate perforation plates from

Knema furfuracea

. From Ohthani

et al

. (1992).

Figure 2.28 Structure of pits in dicotyledonous and gymnosperm wood. (a) Transverse section of dicotyledonous xylem tissue showing vessels connected through pitted walls. (b) Each vessel is made up of multiple vessel elements joined end‐on‐end through a perforation plate, but vessels are connected sideways through bordered pit‐pairs with a pit membrane consisting of two primary cell walls and a middle lamella. (c) Electron microscope scan showing ‘homogeneous’ pit membrane of angiosperms, with a uniform deposition of microfibrils across the surface of the membrane. (d) Transverse section of typical gymnosperm xylem tissue made up of tracheids with bordered pits located in radial walls. (e) The architecture of bordered pits is similar to that of vessels although the pit membrane structure is different, (f) because it has a central thickening (torus) and very porous outer region (margo).

Figure 2.29 A block of gymnosperm wood, American arbor‐vitae

Thuja occidentalis

. The centre of the tree is to the bottom right. The block shows one complete growth ring of early‐wood and late‐wood, and the cambium just lifted off the surface of the xylem. The axial system is composed of tracheids and some parenchyma cells. The rays only contain parenchyma cells.

Figure 2.30 A block of wood from a dicotyledon, tulip tree

Liriodendron tulipifera

. The axial system consists of vessel members joined by scalariform perforation plates, fibre‐tracheids and parenchyma strands. The centre of the tree is to the bottom left and, as in Figure 2.26, the cambium is shown lifted off the xylem.

Figure 2.31 Cross‐section of wood showing: (a) ring porous vessel arrangement in pedunculate oak Q

uercus robur

; and (b) diffuse porous vessel wood in sugar maple

Acer saccharum

. In each case, the centre of the tree is below the diagram, the vertical dark lines are rays, and the horizontal lines are the breaks in growth caused by winter.

Figure 2.32 Dark heartwood surrounded by lighter sapwood in common laburnum

Laburnum anagyroides

.

Figure 2.33 The width of sapwood in different species of gymnosperms and angiosperms determined by the proportion of rays that contain living cells across a radial transect through the sapwood. There can be much variation among individuals in the same species. Each line represents an individual (i.e. four individuals are shown for each gymnosperm). Gymnosperms shown are eastern white pine

Pinus strobus

, eastern hemlock

Tsuga canadensis

and European larch

Larix decidua

. Angiosperms shown are red oak

Quercus rubra

, bigtooth aspen

Populus grandidentata

, white ash

Fraxinus americana

, paper birch

Betula papyrifera

and red maple

Acer rubrum

(each with a designated line pattern, e.g. dotted for

Fraxinus

and dash‐dotted for

Acer

).

Figure 2.34 Sap movement (officially described as a flux density in grams of water conducted per square metre of cross‐section, per second) measured at different radial depths in two angiosperms with wide sapwood: red maple

Acer rubrum

(11 cm sapwood width, 35 years old) and white ash

Fraxinus americana

(9 cm sapwood width, 30 years). The

inner

depth in each species represents sapwood within 1 cm of the heartwood–sapwood boundary;

outer

depth represents the outermost 1 cm of sapwood and

middle

is equidistance between the two.

Figure 2.35 Scanning electron micrographs of

Quercus

xylem. (a) Tyloses are visable as balloon‐like outgrowths from parenchyma cells that extend into the vessels via pits. (b) After tylosis formation is complete, the cell lumen is fully blocked.

Figure 2.36 The trade‐offs between the major ecological driving forces (competitive ability, resistance to stress and disturbance). Important wood properties to cope with these are reported around the triangle and the outer circle identifies key wood traits associated with these wood properties. MOR (modulus of rupture) and MOE (modulus of elasticity) are widely used measures of wood mechanical strength.

Figure 2.37 The effect of the Hagen–Poiseuille equation on the hydraulic conductivity of cylindrical channels. Water flow is much faster through a wider tube so each of these three groups of tubes would conduct the same amount of water per unit of time.

Figure 2.38 Percentage loss of conductivity for frozen and thawed (•) and control () stems plotted against mean conduit diameter. Stems were spun in a centrifuge to simulate a moderately negative pressure of –0.5 MPa (Ag,

Acer grandidentatum

; Ai,

Alnus incana

; AI,

Abies lasiocarpa

; An,

Acer negundo

; Bo,

Betula occidentalis

; Cs,

Cornus sericea

; Ea,

Elaeagnus angustifolia

; Ek,

Euonymus kiautschovicus

; Hh,

Hedera helix

; Pv,

Prunus virginiana

; Qg,

Quercus gambelii;

Ra,

Rhus aromatica

).

Figure 2.39 Adaptations within the pits between xylem cells that help regulate gas‐seeding and embolism. (a) The top row shows the bordered pits of gymnosperms with their torus‐margo pit membranes. The middle and bottom rows show the more homogeneous pit membranes in angiosperms. The pit membranes are in a relaxed state facing no hydraulic stress as the suction (shown as –1 MPa) is equal on both sides. (b) A prolonged period of drought increases the pressure difference between the water‐filled conduit on the left and the air‐filled, embolised conduit on the right, causing the pit membranes to deflect. At a critical pressure difference, gas‐seeding occurs with air being pulled through into the water‐filled conduit. (c) However, various adaptations in pit design can prevent gas‐seeding at the same pressure difference as in (b): (top) an increase in the size of the central torus in comparison to the pit aperture diameters in gymnosperms, creating a better seal; (middle) the growth of

vestures

inside the pit which limit how far the membrane can be stretched; and (bottom) thicker pit membranes with reduced pores in angiosperms.

Figure 2.40 Vulnerability to drought‐induced cavitation in several European oak (

Quercus

) species from different habitats. (a) Loss of conductivity is plotted against water potential (Ψ); and (b) the relationship between the length of the dry (aridity) period and the water potential that gives a 50% loss of hydraulic conductivity (PLC

50

). (a) Redrawn from Tognetti

et al

. (1998) for

Q. pubescens

; Tyree and Cochard (1996) for

Q. robur

and

Q. suber

; Vilagrosa

et al

. (2003) for

Q. coccifera

, Corcuera

et al

. (2005) for

Q. ilex

subsp.

ballota

; Esteso‐Martínez

et al

. (2006) for

Q. faginea

; and Corcuera

et al

. (2006) for

Q. pyrenaica

.

Chapter 03

Figure 3.1 Simple leaves of (a) gutta‐percha

Eucommia ulmoides;

(b) mountain camellia

Stewartia ovata;

(c) moose‐bark maple

Acer pensylvanicum;

and (d) tulip tree

Liriodendron tulipifera

.

Figure 3.2 Compound leaves. (a) Palmate leaves of Ohio buckeye

Aesculus glabra

and (b) pinnate leaves of tree of heaven

Ailanthus altissima

. The pinnate leaf has a petiole (leaf stalk) that continues up between the leaflets where it is referred to as the rachis.

Figure 3.3 (a) Mountain beech

Fuscospora cliffortioides

, a native of New Zealand, seen here at the timberline. (b) A close‐up of a lower branch showing the small leaves of about 7–10 mm in length.

Figure 3.4 (a)

Beccariophoenix madagascariensis

, a rare forest palm found in forest around Sainte Luce in southeast Madagascar, has massive leaves. (b) A single leaf about 6–7 m long; note the gecko on the rachis towards the bottom of the picture. Other palms can have even bigger leaves.

Figure 3.5 The traveller’s palm

Ravenala madagascariensis

in southeast Madagascar. Younger leaves in the centre of the crown (or on younger plants in the foreground) are less torn, whereas older leaves are progressively torn by the action of the wind. This process of tattering provides an effective way of reducing drag without any substantive reduction in leaf area.

Figure 3.6 (a) She‐oak

Casuarina equisetifolia

, Noosa Heads National Park, Queensland, Australia, showing photosynthetic stems. (b) The diminutive leaves of this species can be seen as yellow bands that are formed at the nodes of these modified stems.

Figure 3.7 Dicotyledonous leaf seen in cross‐section.

Figure 3.8 Gymnosperm leaves come in various forms. (a) Bunya pine

Araucaria bidwillii

and (b)

Podocarpus macrophyllus

have relatively broad, flat leaves. Note the new younger leaves of the

P. macrophyllus

are a much lighter green than the older leaves. (c) Scots pine

Pinus sylvestris

and (d) eastern hemlock

Tsuga canadensis

have needle‐leaves. (e) Giant sequoia

Sequoiadendron giganteum

and (f) hiba

Thujopsis dolabrata

have scale‐leaves.

Figure 3.9 Cross‐section of a typical pine needle from eastern white pine

Pinus strobus

. In this five‐needled pine, the triangular cross‐section represents a segment of the original needle cylinder to emerge from the bud, as indicated in the lower left of the figure.

Figure 3.10 (a) Juvenile and (b) adult leaves of

Eucalyptus

species.

Figure 3.11 Sun and shade leaves can markedly differ in size. Here, the relative size of sun (left) and shade (right) leaves are shown for holm oak

Quercus ilex

.

Figure 3.12 Partial cross‐section of a sun (upper) and shade (lower) leaf from European beech

Fagus sylvatica

. The sun leaf has thicker epidermal layers, two layers of densely packed palisade mesophyll cells, a deeper spongy mesophyll layer and a larger mid‐rib (leaf vein) hosting the xylem and the phloem. The shade leaf has thinner epidermal layers, only a single layer of palisade mesophyll, a thinner layer of spongy mesophyll and a much smaller mid‐rib. Shown at relative size.

Figure 3.13

Cecropia peltata

, a monolayered tree growing in a gap in tropical cloud forest in Honduras. Growing in the shade, all the leaves are aligned in a single layer to reduce self‐shading and maximise the amount of light they receive.

Figure 3.14 Flat, non‐overlapping layers of leaves on a branch (planar arrays) can often be observed in trees growing in shaded environments. This sugar maple

Acer saccharum

in the understorey demonstrates how precise arrangement of leaves can minimise self‐shading and maximise light interception.

Figure 3.15 Leaves are arranged in spiral patterns around shoots to help reduce self‐shading: (a) white oak

Quercus alba

; (b) sweetgum

Liquidambar styraciflua

; (c) old man banksia

Banksia serrata;

and (d) monkey puzzle

Araucaria araucana

.

Figure 3.16 Some eucalypt forests are sometimes known as ‘shadeless forests’ because they hang their leaves vertically to reduce the radiation load. This means that the environment underneath these trees can seem quite light, as shown here in a karri

Eucalyptus diversicolor

forest of Leeuwin‐Naturaliste National Park, Western Australia.

Figure 3.17 Staghorn sumac

Rhus typhina

colonising a gap on the edge of woodland in Ithaca, New York. This is a good example of a compound‐leaved tree being suited to invading gaps by investing less in a compound leaf than in a branch with simple leaves.

Figure 3.18 Holm oak

Quercus ilex

, an evergreen broadleaved species from the warm‐temperate regions of Europe and Central Asia. This species is now widely planted across Europe.

Figure 3.19 Cork oak

Quercus suber

growing in Andalucia, Spain, is a leaf exchanger (see Table 3.1) that drops one set of leaves and immediately grows the next set. The commercially valuable corky bark on the main trunk has been removed.

Figure 3.20 Early leaf emergence benefits understorey species. (a) Photosynthesis is much easier in spring for understorey species if they expand their leaves before the overstorey. The understorey is predominantly hawthorn

Crateagus monogyna

, in this shelterbelt. (b) By summer the canopy has closed, shading the understorey and reducing the ability to photosynthesise effectively.

Figure 3.21 The thermal time to budburst (see text for definition) decreases for five elm species (

Ulmus

spp.) as they are exposed to more days with mean temperatures below 5 °C. With increased chilling, buds need less spring heat to trigger their opening, but despite the fact that these elms are related (members of the same genus), the exact relationships between thermal time, chill days and budburst are quite varied.

Figure 3.22 Most of the leaves of this Norway maple

Acer platanoides

have been shed in response to shortening day length in the autumn, but the leaves immediately around the streetlight on the left are still experiencing long days and are fooled into keeping going for longer.

Figure 3.23 An abscission zone forming at the base of a leaf.

Figure 3.24 A gap in the forest canopy allows direct sunlight to reach the ground in a ‘light triangle’ (upper left). The number of sunlight hours a tree experiences within a single triangle increases from the ground to the canopy. When the sunlight passing through more than one gap is considered, however, a more complex pattern is found (upper right), with understorey areas affected by one, two, three or more neighbour gaps (indicated by numbers). Where the cones of several gaps intersect, a uniform or homogeneous light field is produced. Both the distance between the trees and the shape of the crown of these trees determine the duration of direct sunlight in the understorey (lower figures). Pyramidal crowns allow little sunlight to reach the understory, whereas the reverse is true for flat and broad crowns. Adapted from Terborgh (1992).

Figure 3.25 Understorey trees that grow on a slope are faced with the choice of growing vertically, which is mechanically optimal (top left), or with their trunks inclined downward according to the light gradient that occurs from the ground to upper canopy, the photosynthetically optimal angle of growth (top right). Depending on their light requirement or shade tolerance, species are expected to exhibit two ranges of trunk angle, as shown in the lower figure.

Figure 3.26 The conical shape of boreal conifers assists these trees in intercepting light when the sun is low in the sky, and foliage properties help to scatter light deep within the crowns to maximise light interception.

Figure 3.27 (a) The aerofoil form of a savannah

Vachellia sp

. (formally members of

Acacia

) in the Maasai Mara national reserve, Kenya. This crown shape helps reduce the impact of drying winds; helps to shade the stem; and keeps valuable foliage out of reach from most browsers. (b) The same tree is also seen at distance showing the scarcity of other trees. Giraffes can obviously still reach the foliage but because they help pollinate the trees by pollen sticking to their faces, some loss of foliage is a low price to pay.

Figure 3.28 An open‐grown chestnut‐leaved oak

Quercus castaneifolia

in the Royal Botanic Gardens, Kew. This great dome of foliage captures diffuse light from a cloudy sky very efficiently.

Figure 3.29 Tree ferns, such as this

Cyathea

sp. in Sherbrooke Forest, Victoria, Australia, have very simple, unbranched crowns that are held on a single stem. Despite this rudimentary form, they remain an important part of the understorey in many forest ecosystems.

Figure 3.30 Leaves exposed to turbulent wind at 20 m s

−1

(45 miles per hour or 72 kilometres per hour): (a) tulip tree

Liriodendron tulipfera

; (b) white poplar

Populus alba

; (c) false acacia

Robinia pseudoacacia

; and (d) American holly

Ilex opaca

.

Figure 3.31 A wind‐altered crown. This reduces drag on the tree as shown in (a) hawthorn

Crateagus monogyna

on a coastal site overlooking Morecambe Bay, UK and (b) beech

Fagus sylvatica

found in an exposed mountain location in Corsica, France.

Figure 3.32 Diagrammatic representation of the interlocking grain found at the apex of stem unions in trees: (a,b) show the development of interlocking grain within a fork (see also Figure 3.35); (c,d) represent the interlocking grain being formed at a branch‐to‐stem attachment.

Figure 3.33 Whorled grain at the apex of the branch junction has been shown to have an important biomechanical role in branch attachment. The bark has been removed from this sample so that the grain can easily be seen.

Figure 3.34 Indian banyan

Ficus benghalensis

has aerial roots that grow down from lateral branches and act as natural props, facilitating very wide spreading crowns. This is a relatively small specimen growing in Brisbane City Botanical Gardens, Australia but a champion tree growing in the Royal Botanic Gardens of Calcutta, India, however, covers 1.3 hectares and has 2800 subsidiary trunks formed by these aerial roots.

Figure EB3.1 A key anatomical feature of branch junctions is the branch bark ridge (BBR). (a) Co‐dominant branch junction in silver birch

Betula pendula

, with a white arrow identifying the apex of the BBR. (b) Branch‐to‐stem junction formed in the same tree, with a white arrow identifying the presence of a less prominent BBR. (c) Dissection of branch junction A with a white arrow identifying the apex of the BBR and a fine white line running centrally down the denser wood tissues of mixed grain orientation formed centrally under the BBR. (d) Dissection of branch‐to‐stem junction B with a white arrow identifying the apex of the BBR and a fine white line running centrally down the denser interlocking wood tissues evident in the axil of the branch and stem. This junction also has the base of the smaller branch embedded as a knot into the larger branch, which provides some mechanical support to this junction, in contrast to the co‐dominant junction.

Figure EB3.2 (a) A bark‐included junction formed in southern beech

Lothozonia alpina

. The seam of bark (indicated by the white arrows) lies exactly where the dense wood with interlocking grain patterns would normally form under a BBR. In this case, the BBR and its associated woody tissues are wholly absent at the junction, weakening it greatly. (b) Failure of a bark‐included junction in common ash

Fraxinus excelsior

. The presence of bark within a branch junction is a common cause of branch and limb failure in a wide range of tree species.

Figure EB3.3 From static pulling tests and analysis of the fracture surfaces of branch junctions in hazel

Corylus avellana

, it was found that weaker junctions are those where a larger area of bark lies at the top of the branch junction (bottom right), and other forms (embedded bark and cup unions) were significantly stronger.

Figure EB3.4 A bark‐included junction (lower white arrow) formed in a semi‐mature grey alder

Alnus incana

as a result of the natural brace formed higher up in the tree (upper white arrow).

Figure EB3.5 Three common forms of natural brace and their association with the formation of bark‐included branch junctions below them: (a) crossing branches; (b) entwining branches; and (c) fused branches.

Figure EB3.6 A bulging bark‐included branch junction in a semi‐mature beech

Fagus sylvatica

(arrow). The junction is bulged because it has been released to movement after the death of a branch that formed a natural brace above this junction for many years.

Figure EB3.7 (a) A mature common alder

Alnus glutinosa

where a lateral branch straddles across the junction between the two main stems of the tree. The lower white arrow identifies the bark‐included junction with some minor bulging present, and the upper white arrow identifies the natural brace. (b) The swelling of the branch rubbing against the other stem identifies that this natural brace has been

in situ

for many years. It would be very foolish to cut away this rubbing branch, despite current industry guidance that encourages this action, for that would make it much more likely that this bark‐included junction would fail and half of the tree would fall down in the next strong wind event.

Figure 3.35 Reaction wood. Here, compression wood in a gymnosperm is seen as the darker portion on the lower side of this stem. If this was an angiosperm stem, the tension wood would be in the upper part of the stem. The inset illustrates how it is sometimes necessary to reorientate the crown to achieve vertical growth.

Figure 3.36 Cladoptosis in pedunculate oak

Quercus robur

. (a) Small twigs with green leaves can be seen littered around the base of this veteran tree. (b) Evidence of cladoptosis is confirmed by the clean abscission zone at the original point of attachment.

Figure 3.37 The biological consequences of pruning. The inner circle represents some important main outcomes whilst the outer circle gives likely secondary outcomes. The lower portion of the figure gives characteristics that are likely to be decreased as a consequence of pruning; the upper portion of the figure gives characteristics that are likely to be increased.

Figure 3.38 Poor pruning practices, such as flush cuts and stub cuts, should be avoided. (a) Flush cuts remove the branch collar and branch bark ridge to leave a large wound relative to the size of branch being removed. This results in considerable damage to the stem that is being kept. (b) Stub cuts leave an excessive branch stub that is easily colonised by microorganisms. They are also likely to grow epicormic sprouts that could lead to future management problems.

Figure 3.39 Locations of correct removal cuts for branches with and without branch collars. Where a branch collar is present (left), the final pruning cut should be completed just outside of the collar. If there is no visible branch collar and only a branch bark ridge is present (right), the finishing cut should be between cuts 1 and 2. Cut 1 has the advantage of minimising the surface area of the cut and reducing subsequent decay, but cut 2 reduces the likelihood of some stub dieback. No cut should be made closer to the main stem than cut 2.

Figure 3.40 Natural target pruning can help restrict the development of wood discoloration and decay associated with the wound. Here, a correctly pruned apple

Malus

sp. branch shows only a very discrete area of discoloration 3 years after the original pruning cut was made. Note, also, that the growth of new wood to cover over (occlude) the wound has begun around the wound margins.

Figure 3.41 When removing a branch that is (a) the same size (co‐dominance) or (b) bigger that the branch being left (reduction cut), cut 2 is preferred. Making a cut nearly parallel to the branch bark ridge (cut 3) leaves the union weak because supporting wood is too thin and the surface area of the cut larger than needed. Often, less decay occurs in response to cut 1 if the cambium does not die back under the cut, so it provides a safe alternative cut to cut 2. Retained lateral branches should be at least one‐third the diameter of the pruning cut.

Figure 3.42 Bracing in trees. (a) Non‐invasive, flexible bracing should be installed at approximately two‐thirds of the branch length from the defect to branch tip: these may be static, as shown, or include an energy absorber to allow movement of stems within the bracing system. (b) Further support can be installed at the branch base using a tethering system. If this type of support system is installed, it must be carried out by a professional arboriculturist.

Figure 3.43 Potential configurations of non‐invasive bracing in trees. Braces may have an integrated energy absorber, as indicated by the top right image.

Figure 3.44 Bespoke support system used for a 400‐year‐old Chinese banyan

Ficus microcarpa

in Kowloon Park, Hong Kong. Cables fixed to vertical steel pillars support the crown. In turn, a ridged steel framework mounted on piles prevents the heavy structure from damaging the tree roots. Such intensive (and expensive) approaches are only really viable for trees of high cultural or historical importance.

Chapter 04

Figure 4.1 The apical region of a root.

Figure 4.2 Root development after injury or deflection around an object.

Figure 4.3 Tree root development in red maple

Acer rubrum

. (a) Root fans growing from the younger portions of the developing coarse root system. (b) Reoccupation of a soil area near the base of the tree: I, root fans growing from the younger portions of the woody roots have extended a distance of several metres from the tree; II, root fans on adventitious roots have only recently emerged from the zone of rapid taper or root collar, now occupying the area near the base of the tree; III, sinker roots penetrating down into the soil.

Figure 4.4 Examples of the branching fine root systems of nine North American tree species. These are intact segments of the fine root systems that were washed free of soil, their images digitised on a flatbed scanner and converted to black and white. The species are: sugar maple

Acer saccharum

, tulip tree

Liriodendron tulipifera

, balsam poplar

Populus balsamifera

, white oak

Quercus alba

, one‐seed juniper

Juniperus monosperma

, white spruce

Picea glauca

, pinyon pine

Pinus edulis

, slash pine

P. elliottii

and red pine

P. resinosa

.

Figure 4.5 Root cross‐sections of Norway maple

Acer platanoides

, showing a typical pattern of increasing root diameter and secondary (woody) development with increasing root order. Notice that first‐ and second‐order roots have little or no secondary development, and first‐ to third‐order roots still possess intact root cortical cells; fourth‐ and fifth‐order roots have lost all cortex and instead have secondary xylem. Triangles depict simplified patterns of root function (absorptive and transport capacity) and root traits (respiration rate per gram of root; lifespan; total non‐structural carbohydrates (TNC); and other aspects of tissue chemistry) with increasing root order. Root function may not change linearly, depending upon the trait and species. It is also worth noting that despite their recognised importance to root function, many aspects of tissue chemistry (including cellulose, suberin and phenolic content) are not well studied, and patterns of root function with root order may vary across species.

Figure 4.6 Survivorship curves of fine roots of 12 temperate tree species grown together in central Pennsylvania, USA. Survivorship as a proportion of all the fine roots is shown over 800 days (over 2 years). The median root lifespan (in days) is given for each species. The species are: manitoba maple

Acer negundo

, red maple

A. rubrum

, sugar maple

A. saccharum

, pignut hickory

Carya glabra

, black walnut

Juglans nigra

, tulip tree

Liriodendron tulipifera

, eastern white pine

Pinus strobus

, Virginia pine

P. virginiana

, quaking aspen

Populus tremuloides

, white oak,

Quercus alba

, red oak

Q. rubra

and sassafras

Sassafras albidum

.

Figure 4.7 Secondary growth of a woody root, showing development of vascular cambium and production of secondary xylem and phloem.

Figure 4.8 Cross‐sections of structural roots of Sitka spruce

Picea sitchensis

. (a) ‘I‐beam’ shape that was well developed on the prevailing wind side of the tree; (b) ‘T‐beam shape’ that was more characteristic on the leeward side of the tree.

Figure 4.9 Cross‐section of a tree root system with names that are used in several European schools of classification. Note that not all features will be present in every root system.

Figure 4.10 Four principle tree root types found in temperate trees: (a) plate root system; (b) sinker root system; (c) heart root system; (d) tap root system. Details of each are given in the text.

Figure 4.11 Kauri

Agathis australis

has an example of a sinker root system, the remains of which can be seen in this old kauri root plate on display outside the Kauri Museum in Matakohe, Northland, New Zealand.

Figure 4.12 (a) Large buttress roots formed on an emergent species in Khao Sok National Park, Thailand. (b) Special stilt roots formed on the lower stem of a young

Uapaca

sp. growing in the littoral forest near Sainte Luce, Madagascar.

Figure 4.13 The cumulative proportion of root biomass with depth into the soil. The data is an average across trees from temperate deciduous, temperate coniferous, tropical deciduous, tropical evergreen and tropical savannah biomes.

Figure 4.14 Maximum rooting depth of trees from different types of forest.

Figure 4.15 Scale diagram of an excavated lateral root of red maple

Acer rubrum

, at the top, and red oak

Quercus rubra

, at the bottom growing in the same area of Harvard Forest, Massachusetts. Both trees were around 60 years of age.

Acer rubrum

roots were found in the top 10 cm of soil, while

Quercus rubra

roots were found at a depth range of 5–50 cm with an average depth of 30 cm, thus demonstrating a species preference for rooting depth that will limit competition between these species. Arrows indicate that the root tips were not found and therefore these roots continue somewhat further than is shown. (Top)

Figure 4.16 Seasonal root development (brown) tends to start in advance of shoot development (green) in a range of temperate tree species. Here, variations in seasonal shoot and root growth are shown for seven species of forest tree. Seasonal starting and stopping of growth are indicated by arrows. The species are: northern red oak

Quercus borealis

, Scots pine

Pinus sylvestris

, Norway spruce

Picea abies

, black cottonwood

Populus trichocarpa

, false acacia

Robinia pseudoacacia

, silver birch

Betula pendula

and Japanese larch

Larix kaempferi

.

Figure 4.17 Patterns of root growth in temperate regions of the northern hemisphere.

Figure 4.18 Patterns of root growth during 2008–2010 for 12 temperate tree species in central Pennsylvania, USA. Vertical grey bars are included for reference to show 1 July. Common names are given in Figure 4.6.

Figure 4.19 Soil temperature at 20 cm depth (dark grey line) and air temperature (light grey line) recorded at the Russell E. Larson Agricultural Research Center in central Pennsylvania, USA, with corresponding above‐ and below‐ground growth periods in 2012. Growth periods are reported for six temperate tree species and include: the period from first root production to peak root production (brown bars); timing of bud break (light green bars); and duration of active leaf expansion (dark green bars). Full scientific names are given in Figure 4.18 and common names in Figure 4.6.

Figure 4.20 (Top) Gas‐diffusion coefficient (shown as Ds/Do) of several typical urban soil covers. On the right side, the soil cover types are grouped into four classes. Means with the same letters are not significantly different, with 95% probability. The bottom and top of the box are the 25th and 75th percentiles, respectively, and the band near the middle of the box is the 50th median. The ends of the whiskers represent 1.5 times the interquartile range from the box, and outliers are indicated by points. (Bottom) The relationship between fine root density and relative gas diffusivity at three different depths in the soil. The black lines and shaded areas show key trends in the data.

Figure 4.21 Increase in soil bulk density after a series of passes by a pick‐up truck. Most of the increase in bulk density occurs as a result of the first four passes, after which the increases become more modest until a maximum soil bulk density is reached. Based on data courtesy of Glynn Percival.

Figure 4.22 (a) Elongation rate of roots is reduced as root penetration resistance increases in peas (black circles) and maize (open circles). It should be noted that penetrometer resistance is 2–8 times greater than the root penetration resistance. (b) Penetrometer resistance plotted against soil bulk density. Penetrometer resistance becomes increasingly sensitive to bulk density at higher values of bulk density (>1.4 g cm

−3

in this example). (c) The penetrometer resistance of soil at two matric potentials (moisture levels) of −0.01 and −0.1 MPa, plotted against soil bulk density.

Figure 4.23 (a) Soil compaction caused by construction traffic running alongside a woodland. (b) Soil characteristics modified by soil compaction. Increasing or decreasing band width indicates the impact of soil compaction on the named soil characteristic. Dashed lines indicate that trends are likely to be non‐linear. Hirons & Percival (2012). Licensed Open Government Licence v3.0, http://www.nationalarchives.gov.uk/doc/open‐government‐licence/version/3/.

Figure 4.24 The impact of soil compaction on the crown of a copper beech

Fagus sylvatica

‘Purpurea’. Both trees are on the same site: (a) was protected from soil compaction during the expansion of a car park, whilst (b) was subjected to significant soil compaction and has suffered serious decline within the crown as a result of damage to the root system.

Figure 4.25 The use of boardwalks in Waipoua Forest, New Zealand prevents soil compaction whilst allowing people to experience the forest.

Figure 4.26 (a) Soil aeration can be improved by using specialist equipment, such as the VOGT® GeoInjector, which fractures compacted soil by injecting compressed air into the soil, potentially in combination with granular material or a liquid. (b) Air cultivation around the root‐zone using an air tool, such as the Air Spade® (shown), can break up compacted soil and integrate organic matter into the root‐zone.

Figure 4.27 (a) Radial mulching where a series of trenches are dug with air tools and backfilled with soil mixed with mulch or other organic matter. (b) Air cultivation decompacts a larger area, typically a region around the whole stem with several segments that extend to the dripline.

Figure 4.28 Water use (in litres) per day of red maple

Acer rubrum

, Chinese elm

Ulmus parvifolia

, southern magnolia

Magnolia grandiflora

and slash pine

Pinus elliottii

grown in tree lysimeters (see text for an explanation) as part of a project on water use of landscape plants, based at the University of Florida. Note the different scales on the axes.

Dia

. indicates the diameter at ground level in centimetres. See Expert Box 4.1 for more details on this research.

Figure EB4.1 Large suspension lysimeter used to measure the loss of water from the containerised tree.

Figure 4.29 The estimated soil volume needed for trees, based on the potential mature tree crown projection – the area inside the edge of the crown (or the dripline). This should be used as a guide only, and used with particular caution if estimating soil volume requirements for more upright, fastigiate cultivars.

Figure 4.30 The impact of rooting environment on tree growth in a car park in Gelsenkirchen, Germany. Trees planted in central areas have minimal soil volume; trees around the edge of the car park share a more expansive soil volume and so have grown larger.

Figure 4.31 Trees growing in very small amounts of soil: (a) Corsican pine

Pinus nigra

subsp.

laricio

growing on the slopes of mountains in Corsica, France; (b)

Eucalyptus

spp. growing in incredibly tough conditions in the Watarrka National Park, Northern Territory, Australia.

Figure 4.32 Section of large stone skeleton substrate installation for a new planting.

Figure 4.33 A crate system, in this case a RootSpace

TM

system, designed to provide the trees with the maximum amount of uncompacted soil to root into. The crate itself bears the load so the soil inside remains uncompacted. (a) An artist’s impression of the crates being used underneath a paved surface; (b) the RootSpace

TM

crate.

Figure 4.34 The Urban Tree Plaza experiment based at the Bartlett Tree Experts research laboratories in Charlotte, North Carolina, USA. Trees growing in the suspended pavement system that mimics the crate‐based systems discussed in the text, out‐performed those growing in structural soils (Stalite and Gravel) or compacted soil (Compacted). In lower image, ‘New’ represents replacement trees where the first planted tree has previously died.

Chapter 05

Figure 5.1 (a) Typical angiosperm flower of the deciduous camellia

Stewartia pseudocamellia

, composed of white petals (collectively called the corolla) surrounding a ring of male stamens (each made of the pollen producing anther borne on a filament). In the centre is the female carpel, composed of the ovary at the bottom (containing the ovules which will become the seeds) from which grows the style surmounted by the stigma, the part of the carpel that receives the pollen. (b) Female cones of western hemlock

Tsuga heterophylla

. Gymnosperms do not have flowers but produce seeds using male and female cones (called strobili).

Figure 5.2 (a) The individual small flowers of the staghorn sumac

Rhus typhina

are fairly insignificant, but when grouped into a tight inflorescence (flower head) they are more visually attractive to pollinating insects from a distance. (b) Pink trumpet tree

Tabebuia heptaphylla

, native to tropical South and Central America and used as a street tree in sub‐tropical areas, flowers before the leaves are opened to increase the visibility of the flowers: the large number of flowers on a single tree act as a very prominent visual signal to draw pollinating bees from a very wide area.

Figure 5.3 (a) Wind pollinated male flowers of European ash

Fraxinus excelsior

. The petals are reduced to a lobed fringe around the base of the flower, and the visible parts of the flowers are masses of unopened male anthers. (b) Male catkins of hop hornbeam

Ostrya carpinifolia

. Each little scale in the catkin is really a short branch ending in a brown bract, behind which are sheltered a group of male flowers consisting of nothing more than stamens. A female catkin looks similar, but the stigmas and styles from the female flowers poke out around the bracts in order to catch the pollen.

Figure 5.4 A branch of horse chestnut

Aesculus hippocastanum

. The terminal growing points die once they have flowered and fruited, so next year’s growth will be from buds behind (a) leaving forks (b) where the flowers and fruits have previously been. This gives the branches of older trees a distinctive forked appearance.

Figure 5.5 Flowering cherries can have an attractive impact on a landscape. In this case

Prunus

‘Kanzan’ largely hides from view a rather untidy street market at Hirosaki Park, Aomori, Japan.

Figure 5.6 Seedlings of European ash

Fraxinus excelsior

and suckers of wych elm

Ulmus glabra

growing profusely on top of an embankment near the village of Keele, UK. The local council has to repeatedly cut down the seedlings each summer to avoid obscuring sight lines for drivers – an expensive process.

Figure 5.7 Systems for training branches of fruit trees: (a) fan; and (b) espalier. These have the advantage of keeping the trees small, making maximum use of space (by keeping them tight to a wall or frame) and encouraging maximum fruit production which is easy to reach.

Figure 5.8 (a) Hazel

Corylus avellana

growing out of a garden hedge. (b) The base of the tree has a number of stems of different ages even though the crown of the tree is undamaged and healthy. This is a common occurrence in hazel and is referred to as ‘self‐coppicing’.

Figure 5.9 A mature sweet chestnut

Castanea sativa

growing in a lawn with a lower branch (on the right) that has touched the ground and produced adventitious roots. The branch end has now become a tree in its own right. Both may be connected by a communal root system, but there is potential for the two trees to completely separate and become independent trees.

Figure 5.10 (a) An American lime

Tilia americana

that has fallen but maintained a connection with living roots. Side shoots that had buds in high light have taken over as the new leaders, and branches that were pushed against or into the ground have now rooted and are growing up as new trees. In this way, a tangled clone of new trees, all genetically identical with the parent, is growing up. (b) It is clear that the branch starting on the left of the picture has rooted, because the branch itself is much fatter near to the first new stem, and much fatter between the two new stems.

Figure 5.11 Climate change may have a profound impact on parts of Europe over the next century. Here, projected changes are for 2071–2100, compared with 1971–2000, based on the average of a multimodel ensemble. All changes marked with a colour (i.e. not white) are statistically significant. Individual models from the EURO‐CORDEX ensemble or high‐resolution models for smaller regions may show different results.

Figure 5.12 The main factors regulating the breaking of dormancy and the germination of seeds. Seeds can be non‐dormant and ready to germinate straight away, or they may need a suitable combination of temperature and moisture to break the dormancy. For germination, temperature and moisture are also key factors, although the temperature requirements may be quite different from that needed to break dormancy.

Figure 5.13 Germination can be either (a) epigeal or (b) hypogeal. In epigeal germination the cotyledons are raised above the soil surface and, when they emerge from the seedcase will turn green and photosynthesise. In hypogeal germination, the cotyledons remain inside the seed case at or below the soil surface.

Figure 5.14 Common walnut

Juglans regia

, like many tree species, produces a taproot after germination. In both cases, the seedling is being held at the soil level. (a) Showing the root development of a 1‐year‐old seedling, the arrow indicates the remnants of the seed (walnut) which was buried by squirrels. Notice that in this case, the epicotyl has had to grow vertically about 10 cm before the true leaves could emerge above the soil. (b) Showing a 2‐year‐old walnut seedling with a slightly more developed taproot.

Figure 5.15 Two Père David’s maple

Acer davidii

saplings both grown from seed and transplanted from 0.5‐L pots into 3‐L Air Pots. The sapling on the left started the growth season as the much shorter tree (approximately half the size shown in the photo) but had a well‐developed root system at time of transplanting. The sapling on the right started as the taller sapling of the two but had a very poor root system at transplanting. Three months into the growth season, the sapling with the well‐developed root system (left) has been able to fully expand its leaves and grow about 30 cm in height. The leaves are large and well‐formed, ensuring effective photosynthesis that will foster further tree growth: it has developmental momentum. Conversely, the sapling with the poor root system (right) has failed to fully expand its leaves, leading to a severely reduce leaf area and negligible shoot elongation. Both saplings have been watered regularly and have access to the same volume of soil. This serves to illustrate the impact that a poor quality root system can have on subsequent tree development.

Figure 5.16 The momentum of tree establishment can last for decades. Here, a line of penduculate oaks

Quercus robur

were planted in 1991 along a roadside next to the Swedish University of Agricultural Sciences (SLU), Alnarp. Half of the trees came under the jurisdiction of the university and half came under the jurisdiction of the local authority. (a) The trees on university land were well‐watered during the establishment phase and are making a significant contribution to the landscape. (b) The trees on the local authority land were neglected during establishment and, although still surviving, are much smaller, making only a minor contribution to the landscape. They are very unlikely ever to achieve the stature of the trees that were cared for in their early years.

Figure 5.17 The process of filtering out unsuitable tree species for urban environments using three main filters (see text for details). These help to refine a potential list of trees that are suitable for any particular planting site.

Figure EB5.1 When analysing a tree’s capacity to grow in urban environments, its natural background or ecological heritage can give valuable guidance. Trees growing on steep, south‐facing mountain slopes with shallow, rocky soil layers have developed traits that makes them tolerant of these types of conditions. The growing environment is similar to paved urban sites, so these species/genotypes are a better choice than those originating from moist river valleys, which have more in common with park environments.

Figure 5.18 Root pruning or regular transplantation in the nursery is vital if a compact root system is to be created. Without root pruning, the majority of the root system can be left in the field at time of purchase (top and middle left [a]). Regular root pruning can create a compact root system (bottom left [a]) that can then be wrapped with hessian and a wire cage – rootballed (UK) balled and burlapped (USA) – prior to transportation to the planting site (right [b]).

Figure 5.19 (a–c) Once the roots hit the inside wall of a container, their roots get deflected and begin circling. (d) Serious root defects can occur as a consequence. These will compromise the future stability of the tree and impede root development.

Figure 5.20 Air‐Pots®. Trees grown in these do not suffer from girdling roots that grow around the inside of a normal pot. (a) Manchurian alder

Alnus hirsuta

growing in an Air‐Pot®; (b) a close‐up of a lateral root that will be ‘air‐pruned’.

Figure 5.21 (a) Containerised production of dawn redwood

Metasequoia glyptostroboides

using an Air‐Pot® (the black pipes are irrigation lines). (b) These create a fibrous root system free from significant defects. Photographed at Stairway Trees, Stair, UK.

Figure 5.22 Young root systems of the Japanese snowbell tree

Styrax japonicus

grown in (a) an Air‐Pot® propagation tray and (b) a Jiffy® paper propagation plug. In both images, the root systems have been washed to reveal a roots system free from defects that are ideal for growing on in larger containers or a nursery bed.

Figure 5.23 Likely impacts of supplying good quality mulch to a tree root‐zone. Triangles indicate whether the general trend of each factor increases or decreases with the addition of organic mulch. Dashed margins suggest that the relationships are not necessarily linear but represent a ‘direction of travel’.

Figure 5.24 Planting pit design for a standard tree. Circular or square‐sided holes are acceptable; cultivated soil should be in proportion to the size of the root‐ball and should allow the tree to sit in the hole at the correct depth. If support is necessary, a number of different staking formats may be used for tree stability, including staking less than one‐third the height of the tree) and underground guying. Mulch should be applied over the planting pit to a depth of 5–10 cm.

Figure 5.25 Tree shelters, known as Tuley tubes in the UK, protecting young oak trees from deer (hence their height). As well as keeping deer and smaller browsing animals away they also act as greenhouses, keeping the saplings warmer and helping them grow faster.

Chapter 06

Figure 6.1 (a) Soil water release curves for a sand and a loam soil, showing the typical relationship between the soil water content and the soil water potential (the ease with which a plant can extract water from the soil). The turgor loss point (below which plants cannot grow) for many agricultural crops is taken as –1.5 MPa, and for many temperate trees it ranges between –2 and –4 MPa. At soil water potential below –5 MPa, water is hygroscopically bound to the soil so tightly that it is completely unavailable to plants. (b) A general relationship between soil water content and soil type.

Figure 6.2 (a) An irrigation tube being installed in a tree planting pit. (b) A watering bag placed around a recently planted tree in Copenhagen, Denmark. The bag has a porous base that slowly releases water to the root ball and surrounding soil over a number of hours. This helps reduce surface run‐off and ensures deeper soil water recharge. Here, a Treegator® bag is being used, but a number of different brands and designs are available.

Figure 6.3 A permeable geotextile barrier reduces soil evaporation and, importantly, weed competition in a field trial based at the Swedish University of Agriculture (SLU), Alnarp, Sweden.