Trees and forests _ a colour guide _ biology, pathology, propagation, silviculture, surgery, biomes, ecology, conservation

TREES AND FORESTS A Colour Guide Biology, Pathology, Propagation, Silviculture, Surgery, Biomes, Ecology, Conservation Edited by BRYAN G BOWES Formerly Senior Lecturer at the Department of Botany University of Glasgow, UK MANSON PUBLISHING

General note All figures are supplied by the author(s) of each chapter unless otherwise acknowledged. Copyright © 2010 Manson Publishing Ltd ISBN 978-1-84076-085-9 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means without the written permission of the copyright holder or in accordance with the provisions of the Copyright Act 1956 (as amended), or under the terms of any licence permitting limited copying issued by the Copyright Licensing Agency, 33–34 Alfred Place, London WC1E 7DP. Any person who does any unauthorized act in relation to this publication may be liable to criminal prosecution and civil claims for damages. A CIP catalogue record for this book is available from the British Library. For full details of all Manson Publishing titles, please write to: Manson Publishing Ltd 73 Corringham Road London NW11 7DL, UK Tel: +44 (0)20 8905 5150 Fax: +44 (0)20 8201 9233 Website: www.mansonpublishing.com Commissioning editor: Jill Northcott Project manager: Paul Bennett Copy editor: Janet Tosh Layout: DiacriTech, Chennai, India Colour reproduction: Tenon & Polert Colour Scanning Ltd, Hong Kong Printed by: Grafos SA, Barcelona, Spain

Contents Preface 5 Temperate evergreen forests of 77 Contributors 6 South Africa 80 Abbreviations 7 82 Dedications 8 Temperate evergreen forests of northern and central Chile SECTION 1 INTRODUCTION TO WOODY HABITAT Temperate mixed evergreen forests of the Mediterranean Chapter 1 Survey of trees: their global significance, architecture, and early Chapter 5 Tropical and sub-tropical evolution 9 rain and dry forests 87 Bryan G Bowes Ghillean T Prance Introduction 9 Introduction 87 Types of trees 11 The forests of tropical America 90 Importance of trees for civilization 17 The forests of southeast Asia and Australia 97 Tree architecture 24 The forests of Africa 99 The evolution of land plants and trees 32 Old world dry semi-deciduous forests 101 Montane forests 101 SECTION 2 WORLD DISTRIBUTION OF FORESTS Secondary forests 102 Chapter 2 Northern boreal and montane coniferous forests 40 SECTION 3 TREE MORPHOLOGY, ANATOMY, 104 Aljos Farjon Introduction 40 AND HISTOLOGY 104 Conifers of the boreal forest belt 41 Montane conifers 44 Chapter 6 Woody thickening in trees 107 Coastal conifers 46 and shrubs 120 Concluding remarks 50 Bryan G Bowes Introduction Chapter 3 Temperate deciduous and Vascular activity in broadleaved and temperate rain forests Hugh Angus coniferous trees Temperate deciduous forests (TDFs) Structure and function of bark Temperate rain forests (TRFs) 51 SECTION 4 TREE PATHOLOGY 126 Chapter 7 The role of cell-wall polymers 51 in disease resistance in woody plants 126 61 Christopher T Brett Introduction 128 Chapter 4 Temperate mixed evergreen 68 The molecular structure of cell-wall forests 133 Stephen D Hopper, Erika Pignatti Wikus, 68 polymers and Sandro Pignatti 70 The cell wall as a physical barrier to 135 Introduction 76 137 Temperate evergreen forests of Australia infection Temperate evergreen forests of Tasmania Active defence mechanisms involving and New Zealand the cell wall Future developments

4 Chapter 8 Microbial and viral pathogens, 139 Classification of systems 212 and plant parasites of plantation and Even-aged systems 212 forest trees 139 Uneven-aged systems 218 Stephen Woodward 140 Silvicultural systems in different parts Introduction 149 220 Tree root diseases 155 of the tropics 224 Diseases of stems 160 Concluding remarks Diseases of foliage 165 Decay 169 Chapter 12 Tree pruning and surgery in 225 Rust diseases 173 arboriculture Wilt diseases 174 David Thorman 225 Virus diseases 177 Introduction 229 Parasitic plants Rationale and problems of tree surgery 230 Acknowledgements Tree pruning techniques 234 Phenology 234 Chapter 9 Insect pests of some important 178 Pollarding 235 forest trees Artificial support systems 236 Claire Ozanne 178 Tree felling 236 Introduction 179 Concluding remarks Sap feeders 183 Bark and stem borers 188 Chapter 13 Tree propagation for forestry 238 Shoot borers 191 and arboriculture Bark chewers 192 Brent McCown and Thomas Beuchel 238 Defoliators 195 Introduction 238 Termites 197 Overview of concepts 239 Concluding remarks Propagation by seed 242 Vegetative propagation 248 SECTION 5 FOREST ECOLOGY AND Handling of young propagules 251 MANAGEMENT, TREE SURGERY, Ethics in tree propagation PROPAGATION, AND CONSERVATION Chapter 10 General forest ecological 198 Chapter 14 Forest and woodland 252 processes conservation Peter A Thomas 198 Ghillean T Prance 252 Introduction 198 Introduction 253 Size and growth 199 Reserve size 255 Light 203 Population size 257 Water 203 Conservation genetics 257 Nutrients 206 The human component 261 Coarse woody debris 207 Maintaining biological interactions 262 Evergreen and deciduous leaves 209 Reserve management 262 Concluding remarks Education and capacity building 262 Concluding remarks Chapter 11 Silvicultural systems 210 References and general reading 264 Peter Savill and Nick Brown 210 Index 279 Introduction 210 Ecological principles in forest regeneration

5 Preface A grove of sequoias [Sequoiodendron Buddha is believed to have attained enlightenment giganteum] – occupy an area of perhaps less while sitting under the bodhi tree (Ficus religiosa), than a hundred acres – The perfect specimens while ginkgo (Ginkgo biloba) is almost extinct in the not burned or broken are singularly regular and wild of China but has survived as a sacred tree in symmetrical – showing infinite variety in general temple gardens. unity and harmony; the noble shafts with rich purplish brown fluted bark, free of limbs for one Sadly, for the last century or more, such respect hundred and fifty feet or so, ornamented here for trees has generally been forsaken. Today, and there with leafy rosettes; main branches of environmentally detrimental logging and forest the oldest trees very large, crooked and rugged, clearances still continue, either illegally or by official zigzagging stiffly outward seemingly lawless, yet connivance, driven by world population growth, unexpectedly stooping at the right distance from industrialization, market globalization, and the the trunk and dissolving in dense bossy masses pursuit of maximal (but patently unsustainable) of branchlets, thus making a regular though commercial profits. greatly varied outline, – a cylinder of leafy, outbulging spray masses, terminating in a noble The principal aim of this present volume is to dome, that may be recognised while yet far off bring together and review a number of features of upheaved against the sky – the king of all forests, trees, their development, natural environ- conifers, not only in size but in sublime majesty ments, and ecology, which are usually disparately of behaviour and port. considered. These topics are expertly treated by scientists from various countries and presented in a (John Muir, My First Summer in the format in which, as an integral part of each chapter, Sierra, September 17th 1869) numerous high-quality colour illustrations comple- ment the concise but clearly written text. A reverence and respect for trees, such as are exhibited in Muir’s scientific but beautiful prose This Guide will be of importance and interest to describing his experiences in the Californian Sierra anyone studying plant science, forestry, or ecology Nevada is deeply embedded in the human psyche. A and of practical and theoretical use to conserva- sacred oak (Quercus) was at the centre of the Greek tionists, foresters, tree propagators, and tree surgeons oracle at Dodona founded in about 1800 BP; while working in the field. The 16 contributing authors, Artemis, the goddess of woodlands, and Apollo (her based in Britain, the USA, Australia, and Italy, reflect twin brother) were said to be born under a date a wealth of expertise, which will help ensure that this palm. For the Pehuenche Amerindians of Chile, the volume is of value and use for students and interested monkey puzzle tree (Araucaria araucana) is holy, amateurs in countries throughout the world. Bryan G Bowes

6 Contributors Chapters 1 and 6 Chapter 8 BRYAN G BOWES STEPHEN WOODWARD Formerly at the Department of Botany, School of Biological Sciences, University of Glasgow, Glasgow, UK University of Aberdeen, Aberdeen, UK Chapter 2 Chapter 9 ALJOS FARJON CLAIRE OZANNE Royal Botanic Gardens Kew, Richmond, School of Human and Life Sciences, Surrey, UK Roehampton University, London, UK Chapter 3 Chapter 10 HUGH ANGUS PETER A THOMAS Head of Tree Collections, Westonbirt Arboretum, School of Life Sciences, Keele University, Keele, UK near Tetbury, Gloucestershire, UK Chapter 11 Chapter 4 PETER SAVILL STEPHEN D HOPPER Oxford Forestry Institute, Department of Plant Director, Royal Botanic Gardens Kew, Richmond, Sciences, University of Oxford, Oxford, UK Surrey, UK NICK BROWN ERIKA PIGNATTI WIKUS Oxford Forestry Institute, Department of Plant Dipartimento di Biologia, Universita di Trieste, Sciences, University of Oxford, Oxford, UK Trieste, Italy Chapter 12 SANDRO PIGNATTI DAVID THORMAN Dipartimento di Biologia Vegetale, Rostrevor, Station Road, Whittington, Oswestry, Universita di Roma ‘La Sapienza’, Roma, Italy Shropshire, UK Chapters 5 and 14 Chapter 13 GHILLEAN T PRANCE BRENT H MCCOWN School of Plant Sciences, University of Reading, Professor, Department of Horticulture, Whiteknights, Reading, UK University of Wisconsin-Madison, Madison, Wisconsin, USA Chapter 7 CHRISTOPHER T BRETT THOMAS BEUCHEL Institute of Biomedical and Life Sciences, McKay Nursery, Waterloo, Wisconsin, USA University of Glasgow, Glasgow, UK

7 Abbreviations BP before present PGA polygalacturonic acid B.t. Bacillus thuringiensis PME pectin methyl esterase B.t.k Bacillus thuringiensis var. kurstaki PRPs pathogenesis-related proteins CWD coarse woody debris REGUA Reserva Ecologica Guapi-açú RG-I rhamnogalacturonan-I DED Dutch elm disease RG-II rhamnogalacturonan-II DON dissolved organic nitrogen SMS selective management system E east W west HRGPs hydroxyproline-rich glycoproteins IBA indolebutyric acid TDF temperate deciduous forest IUCN International Union for Conservation of TEF temperate evergreen forest Nature TSS tropical shelterwood system IPM integrated pest management Note: all sectioned specimens were photographed MUS Malayan Uniform System under the light microscope except where MVP minimum viable population designated TEM (transmission electron mya million years ago micrograph). Abbreviations: LS, longitudinal section; RLS, radial longitudinal section; NEP net ecosystem productivity TS, transverse section; TLS, tangential NPP net primary productivity longitudinal section.

8 Dedications To the Editor’s family With great love to my dear, supportive, and forbearing wife Diane; in warm memory of my late wife Ruth; with all love and affection for our children Tanya and Adrian; and to my six bright, creative, and affectionate grandchildren – Sean, Aidan, Declan, Cian, Myles, and Marcus. To William Shakespeare The quality of mercy is not strain’d; It droppeth as the gentle rain from heaven Upon the place beneath. It is twice blest: It blesseth him that gives and him that takes. ’Tis mightiest in the mightiest; it becomes The throned monarch better than his crown; His sceptre shows the force of temporal power, The attribute to awe and majesty, Wherein doth sit the dread and fear of kings; But mercy is above this sceptred sway, It is enthroned in the hearts of kings, It is an attribute to God himself (The Merchant of Venice)

9 SECTION 1 INTRODUCTION TO WOODY HABITAT CHAPTER 1 Survey of trees: their global significance, architecture, and early evolution Bryan G Bowes INTRODUCTION role of trees in the complex web of life. According to Jane (1970), there are some 20,000 Most trees – apart from those growing in plantations, horticulture, arboreta, parks, or urban species of woody plants but, more recently, Oldfield settings – are distributed in various biomes, or major et al. (1998) estimated that there are possibly up to regions, of the terrestrial world (1; Packham et al., 100,000 tree (arborescent) species: these were 1992). Each biome is characterized by its own range defined as single-stemmed species growing to at least of animal and plant life. Chapters 2–5 of this Guide 2 m in height at maturity. However, this definition consider the various natural regions populated by seems unduly restrictive, since it would apparently trees, while Chapters 10 and 14 emphasize the vital exclude some natural multi-stemmed species of 1 Overview of the Blue Mountains Eucalyptus 1 forest near Sydney, Australia. This forest is dominated by eucalypts, with some 90 arborescent species of Eucalyptus and other members of the family Myrtaceae occurring here.

10 2 Multi-stemmed 3 3 Multi-stemmed 2 specimen of specimen of Sorbus aucuparia Eucalyptus (mountain ash, gregsoniana rowan tree), growing in which shows no Australia. evidence of a coppice origin. 45 5 Conical canopy of Araucaria 4 Foliage of the recently discovered Wollemia nobilis araucana (Wollemi pine) growing in the Royal Botanic Gardens, (monkey puzzle) Sydney, Australia. bearing ripening female cones. This specimen is growing in Scotland, but the tree is endangered due to over-logging in its native southern Chile. common trees such as Salix (willow), Sorbus, and 1994; having survived until then, as a small Eucalyptus (2, 3), as well as various palms and other population of conifers growing within the protective taxa. walls of a narrow, deep canyon located only 150 km from Sydney, Australia. Various new Eucalyptus In any case, more species of tree are very likely to species (Chapter 4) were only recently recognized in be discovered, or sometimes just await accurate Western Australia. scientific recognition and description. For example, Wollemia nobilis (Wollemi pine, 4) is a relic of the Oldfield et al. (1998) concluded that, worldwide, extensive araucarian flora existing in the early some 8,500 tree species are classified as threatened Cretaceous period, 116 mya (million years ago). (4, 5). Of these, over 4,500 are endangered or However, the Wollemi pine was only discovered in critically endangered in their native habitats, due in

SURVEY OF TREES 11 7 6 Specimen of 6 7 Tall specimen of the tree fern Cyathea sp. the palm growing in its Woodyetia native temperate bifurcata growing rain forest in an Australian habitat in botanic garden. Queensland, Australia. large part to over-logging of natural forests, or their 8 Tall specimen of 8 clearance for cultivation (Chapters 2–5 and 14). Betula pendula 9 However, the continued widespread occurrence of (silver birch) tree cover was recently confirmed by the UN Food growing in the and Agriculture Organization, which estimated that wild in Scotland. about one third of the Earth’s land surface currently remains as forest (1; Lamb, 2002). Nevertheless, these calculations were probably overly optimistic, since the term ‘forest’ was applied to tree cover of as little as 10% on any land over half a hectare in area. TYPES OF TREES 9 Large cones of the conifer Abies To many European and North American observers, procera (noble it is perhaps surprising that some 300 species of fir) native to ferns, principally in the genera Cyathea and northwestern Dicksonia, are arborescent. Tree ferns mainly grow America but in tropical mountainous regions, but are also found growing in in temperate Australasia and elsewhere. Several Scotland. species of Cyathea may reach 20 m in height, while Dicksonia antartica grows up to 10 m tall. The normally unbranched trunk of the tree fern rarely exceeds 30–40 cm in diameter and bears a terminal crown of very large compound fronds (leaves), which may be several metres long (6). However, almost all tree species are angiosperms, commonly termed flowering plants (7, 8), or gymnosperms (4, 5). The seeds of gymnosperms are naked (not enclosed within a fruit) and are usually borne in obvious cones (9). By contrast, the seeds of angiosperms are enclosed

12 within a fruit (10), which develops from the carpel The latter are tropical/sub-tropical in distribution (pistil) on a flower (11). and bear large compound fronds (15). These xerophytic leaves have thick, waterproof cuticular Gymnosperms are almost entirely arborescent. coverings and sunken pores, or stomata, concerned Taxonomists give varying estimates of their present- with gaseous exchange from the internal leaf tissue day numbers, but it seems reasonable to conclude to the outside air (17). Cycads are generally rather that at least 500 conifer (12, 13) and 200 cycad squat (15), although Macrozamia hopei may attain species (14, 15) are still extant plus a single species of 20 m in height (Foster and Gifford, 1974). They bear Ginkgo biloba (maidenhair tree, 16). Of the three either male or female cones on separate individuals, gnetophyte genera, only Gnetum gnemon is a tree. and generally these cones are massive (14). The trees Pollination in the gymnosperms is mostly effected by undergo only very limited woody (secondary) wind, but beetle vectors may be involved in cycads. 10 11 11 LS 3 hermaphrodite flower of the 11 arborescent dicot 2 Magnolia. Note the numerous basal stamens, while abundant green carpels are inserted above on the flower axis. 10 TS of an immature fruit of the dicot tree Aesculus 13 13 Tall specimen hippocastanum (horse chestnut) showing two cotyledons of Pinus sylvestris (1) within the ovule (2). Fruit wall (3). (Scots pine) growing wild on 12 12 Tall specimen an exposed site in Scotland. Note of Sequoia the long trunk sempervirens (devoid of (coastal redwood branches) but and native to with a terminal California) but rounded canopy. planted in the mid-1800s in a botanic garden in Scotland.

SURVEY OF TREES 13 thickening but their trunks are strengthened by the (pines, 18). However, some have scale-like leaves, as in covering of persistent fibrous leaf bases (15). species of Cupressus (cypress) and Chamaecyparis lawsoniana (Lawson cypress), or spiny leaves, as in Conifers commonly have needle-like leaves as in Araucaria araucana (monkey puzzle). Like the cycads, Larix (larch), Abies (fir), Picea (spruces), and Pinus 14 Massive male 14 15 Large 15 cone of the tropical specimen of the gymnosperm African cycad Cycas circinalis, Encephalartos but growing in altseinii growing a botanic garden in an Australian in Scotland. botanic garden. 16 17 16 Newly expanding foliage of the deciduous Ginkgo 18 biloba (maidenhair tree) growing in an Australian botanic garden. 17 TS of a leaflet of Cycas revoluta showing its thick epidermal cuticle (stained dark blue) and sunken stomata confined to the lower epidermis. 18 Foliage of Pinus caribaea, which is native to the Caribbean region, but is here growing in an Australian botanic garden.

14 conifer leaves are generally xerophytic with several exceptions, such as Larix (larch) and Metasequoia modifications, such as thick, waxy cuticules and glyptostroboides (dawn redwood, 19, 20), most sunken stomata, to minimize water loss. Most conifers are evergreens. Their leaves are generally conifers occur in the boreal, montane, and temperate shed over only a few years, while the large spiny rain forests of the northern hemisphere (Chapters 2 leaves of Araucaria araucana (monkey puzzle, 5, 21) and 3). The numerous species of Pinus (pine, 13) are last up to 30 years. mostly indigenous to the northern hemisphere, but P. caribaea (Caribbean pine, 18) occurs in its extreme Conifers often grow into very large individuals south. Conifers such as species of Fitzroya, (Chapters 2 and 3) and many are extremely long- Podocarpus, and Araucaria (5) are indigenous to the lived, as is demonstrated in the famous Mariposa southern hemisphere (Chapter 4). With some Grove in the Sierra Nevada of California. Here, there are many giant trees of Sequoiadendron giganteum 19 19 A specimen of 20 Metasequoia glyptostroboides (dawn redwood), a deciduous conifer native to southwest China, in its winter aspect growing in England. 20 New foliage of the deciduous conifer Metasequoia glyptostroboides (dawn redwood). 21 21 Young 22 22 Massive specimen of the ancient conifer Araucaria specimen of araucana Sequoiadendron (monkey puzzle) giganteum (big showing its spiny, tree, Welligtonia) long-lived leaves growing in its on the main stem native habitat at and branches. Mariposa Grove, California, USA.

SURVEY OF TREES 15 (big tree, Wellingtonia, 22). A 3,500-year-old and the conifer forests (Chapter 2). Angiosperms are specimen of this tree (known as ‘General Sherman’) divided into two main taxa (groups), the monocots is ca. 83 m tall, with a basal diameter of some 11 m. and the many more numerous species of dicots, On the Californian coast, a specimen of Sequoia which display a number of different vegetative and sempervirens (coastal redwood) is 111 m in height, floral characteristics (23A, B). while other North American conifers such as Abies procera (Noble fir), Pseudotsuga menziesii (Douglas Monocots are mainly herbaceous and generally fir), and Pinus monticola (western white pine) may bear flower parts in threes or multiples of three. be 70 m or more tall. The oldest, accurately dated Their leaves are frequently long and narrow, with living tree is a nearly 4,770-year-old specimen of their main veins running lengthwise along the blade Pinus longaeva (the bristlecone pine ‘Methuselah’), (23A, 24). Although most species are not woody, growing at about 3,000 m elevation in the White Dracaena draco (dragon tree) and some other Mountains of California (Anonymous, 2003). A monocots (Chapter 6) undergo an unusual form of Scottish specimen of Taxus baccata (yew) is woody secondary thickening and may form large estimated to be 5,000 years or more old; but it is trees. Many palms may also become tall trees (7) and impossible to accurately date the relicts of such a some species live for more than a century large-diametered, but now much-fragmented (Tomlinson, 2003). Palms do not undergo secondary specimen, which has none of its heartwood thickening, but instead increase in thickness by a remaining (Lewington and Parker, 1999). process of diffuse secondary growth throughout the living tissues of the trunk (Esau, 1965). Flowering plants (trees, shrubs, and herbs) dominate most terrestrial habitats, except for the By contrast, in dicots the flower parts usually northern biomes of the moss- and lichen-rich tundra, occur in fives or fours, or are indefinite in number (11, 23B) and most species show some degree of 23 24 B1 1 A1 1 2 3 32 A2 B2 A3 B3 24 Leaf of a large-leaved bamboo showing the arrangement of parallel A4 B4 veins in its leaf, which is typical of the venation in a monocot. 23A–B Idealized diagrams illustrating the salient differences between mono- (series A) and dicotyledonous (series B) flowering plants. A1/B1: LS seeds; cotyledon (1), plumule (2), radicle (3). A2/B2: Leaf and floral differences. A3–4/B3–4: distribution of the primary xylem (red) and phloem (blue) in the young shoots (A3/B3) and roots (A4/B4).

16 26 25 25 Leaves of Firmiana malayana (family Sterculiacea); 26 Leaf of the dicot Acer pseudoplatanus (sycamore) although a tropical dicot tree, it is deciduous, shedding its showing its branched veins which terminate in a reticulum leaves after a dry period and remaining bare for up to three of fine veinlets. Note also the numerous insect galls. months. 27 27 Newly 28 expanding foliage from a breaking bud of Aesculus hippocastanum (horse chestnut) in spring. 28 Stump of the conifer Pinus sylvestris (Scots pine) clearly showing its annual rings. woody thickening (Chapter 6). The leaves are Temperate broadleaved trees are usually generally relatively wide, and hence a dicot tree is deciduous (Chapter 3), producing new leaves when commonly termed broadleaved (25). From the main the buds burst in spring (27). The foliage senesces vein(s) of the leaf, prominent side veins arise and and drops in autumn (the fall), with the further branch with the ultimate veinlets, forming a accompanying dormancy of the tree. In seasonally closed network in the leaf blade (26). On average, dry tropical climates, trees such as Firmiana some 25–30 different broadleaved species occur per malayana (25) may shed their leaves during the dry hectare of temperate forest, whereas the number of season. However, most tropical species are evergreen tropical tree species is much greater, with up to 300 (Chapter 5), and the woody thickening of their present per hectare (Thomas, 2000; see also Chapter trunks and branches usually proceeds without 5). Pollination of broadleaved trees is primarily abrupt interruption (Longman and Jenik, 1987). By effected by insects and, to a lesser extent, by birds contrast, in temperate broadleaved species, and also and mammals, but wind pollination is common in most coniferous trees, the annual growth rings mark species native to higher latitudes. the cessation of woody thickening at the end of the growing season in late summer/early autumn and its resumption in the following spring (28; see also Chapter 6).

SURVEY OF TREES 17 IMPORTANCE OF TREES FOR 29 CIVILIZATION 29 Reconstruction of wooden crannoch (communal Trees and timbers have been used all over the world, dwelling) built for the protection of its inhabitants over the both prehistorically and in historical times, in the margins of a loch, as used in the late Bronze/early Iron Age construction of shelter and housing (29, 30). Today of Scotland. timber – together with a large variety of wood products – is used worldwide, both for domestic 30 housing and for larger-scale constructions. Wooden- framed buildings (31) house more than 70% of the 30 The interior of the Great Hall of Stokesay Castle in populations of Australia, Japan, North America, and England, which still retains some of the original massive Scandinavia. Such construction is much less popular oak trusses holding up its roof. in the UK but, even here, in a typical brick-built modern family house, wood is used for the rafters, 31 trusses, joists, stairs, doors (and often the window frames), while the floors are either of timber or wood 31 Exterior of a modern wooden-framed bungalow in composites (Lewington, 2003). In developing Queensland, Australia (courtesy of Molly Miller). Despite countries of the tropics, smallholder farmers utilize the danger from termites, wood is still a very popular agroforestry and cultivate various species of trees, as building material for Australian housing. well as harvesting plants from the wild. These provide fruit, animal fodder, timber, poles, firewood, livestock fences, and shade, all in combination with the nurture of their livestock and food crops (Longman and Jenik, 1987). In Burkina Faso (West Africa) the tree Bombax costatum (locally known as li-suoli) provides a profusion of edible or other useful products for the villagers. The flowers attract bees, which provide honey, the dried outer parts of its flowers make a mineral-rich sauce, the fruit is used in brewing, the floss around the seeds gives kapok; the bark yields a brown dye, the leaves provide animal fodder; and the timber is used for the fishermen’s canoes. Among other species harvested from local trees for food are the leaves of Adansonia digitata, Balanites aegyptica, and Commiphora africana, and fruits from Tamarindus indica and Saba comorensis (Anony- mous, 2005). Many species of both conifers and broadleaved trees yield important commercial timbers. Conifer woods lack thick-walled hard fibres and are classed as softwoods. This term is somewhat misleading, since the water-conducting cells (tracheids) in the wood are often thick-walled and tough (Chapter 6). Taxus baccata (common yew), for instance, possesses a very strong, durable, and even-textured wood. Yew branches were used extensively in the Middle Ages for the production of the English longbow, while the hard wood is excellent for use in

18 turnery. Broadleaved trees usually contain a large a vast quantity of conifer wood pulp, mainly from proportion of tough, thick-walled wood and fibres Picea (spruces), is processed into newsprint, (Chapter 6), and hence are termed hardwoods. cardboard, fibreboard, and rayon. However, the soft, light, and easily carved wood of the tropical broadleaved Ochroma pyramidale Several broadleaf trees provide important (balsa wood) provides an obvious exception. temperate and tropical hardwood timbers (Chapters 3–5). Some may attain great heights and Eucalyptus In the boreal (taiga) forests of the northern diversicolor (Karri), formerly an extensively logged hemisphere across the far northwest of Europe, native of southwest Australia but now protected, Russia, and North America, huge areas of natural grows up to 87 m tall and 4 m in trunk diameter. In conifer forests still exist (Chapter 2) but many have Tasmania, a 350-year-old specimen of E. regnans been very heavily logged. In Scandinavia and Finland (called ‘El Grande’) was discovered in 2002. It was most of the original conifer trees are now replaced by 79 m tall, with a girth of 20 m and an estimated commercially managed secondary forests, and in volume of some 439 cubic metres (Anonymous, northern Europe large areas of conifers such as Picea 2004). The tree was accidentally fire-killed (but still abies (Norway spruce), P. sitchensis (Sitka spruce), remains standing) by a mismanaged forestry and Pseudotsuga menziesii (Douglas fir) have been regeneration burn, which spread from an adjacent planted. In the northwestern USA, plantations of logged area. In Tasmania this magnificent tree Abies (firs), Picea glauca (white spruce), Pinus species is still being cut down to provide woodchip (pines), Tsuga heterophylla (western hemlock), for the paper industries in Japan and elsewhere. Pseudotsuga menziesii (Douglas fir), and other conifers are widely established and provide very There are numerous other important hardwood valuable commercial timbers. In the southern trees, with some further examples being various hemisphere various conifers, including Araucaria species of Carya (hickories), Dryobalanops (kapur, araucana (5), Fitzroya cupressoides, and species of camphor wood), Fagus (beech), Fraxinus (ash), Agathis and Podocarpus, have been logged for Grevillea (silky oak), Juglans (walnut), Khaya timber, sometimes to near extinction in the wild (African mahogany), Liquidambar (satin walnut), (Chapter 3). In addition to timber for construction, Magnolia (magnolia), Populus (cotton wood, poplar), Quercus (oak), Swietenia (American 32 mahogany), Tectona grandis (Asian teak), Tilia (lime, linden), and Ulmus (elm). Such trees provide very valuable timber and are extensively used for buildings/joinery, furniture, and sculpturing (30, 32). Smaller trees and branches are also harvested to provide poles, fuel, and charcoal (33). Finer branches from species such as Betula (birch), Corylus (hazel), Salix (willow), and other genera, often from trees which have been pollarded or coppiced (34, 35), are used for brooms, baskets, and fencing (36). 32 16th century carved limewood (Tilia sp.) altar piece from southern Germany.

SURVEY OF TREES 19 33 34 33 Demonstration of a charcoal kiln in a national park in 34 Ancient pollard of the broadleaved Salix sp. (willow) southern Spain. growing along the river Cam in England. 35 36 35 Young coppice branches growing from a stool of the 36 Demonstration of a wicker fence, woven from broadleaved tree Acer pseudoplatanus (sycamore). coppice branches of Corylus avellana, at a crannoch reconstruction in Scotland (see also 29).

20 Many broadleaved trees provide valuable foods domestica (apples, in a large variety), Mangifera and spices for human consumption (37–42). Some indica (mango), Nephelium lapaceum (rambutan), important examples from temperate and tropical Olea europea (olive), Pistacia vera (pistachio), regions are Artocarpus spp. (breadfruit and Prunus spp. (apricot, cherry, damson, nectarine, jackfruit), Bertholletia excelsa (Brazil nut), Carica peach, and plum), Syzgium aromaticum (clove), and spp. (papaws, mountain papaw), Castanea spp. Theobroma cacao (cocoa). (sweet chestnut, Chinese chestnut), Cinnamomum verum (cinnamon), Coffea spp. (Arabica, robusta, In addition to such broadleaved trees, the and liberica coffees), Citrus spp. (grapefruit, lemon, monocot palm family, which is mainly confined to lime, tangerine, and orange), Ficus carica (edible fig), the tropics and sub-tropics, contains a number of Garcinia mangostana (mangosteen), Malus × trees which provide very important foods. Coconuts are gathered from Cocos nucifera (43), dates from 37 38 38 Tropical breadfruit on the small broadleaved tree Artocarpus. 37 Crop of Citrus aurantium (Seville orange) growing on a southern Spanish street. 39 39 Apple tree 40 40 Fruits of Juglans nigra (Malus x (black walnut) – a broadleaved tree domestica) native to North America but here bearing ripe fruit growing in England. in an English autumn.

SURVEY OF TREES 21 Phoenix dactylifera (44), and sago from the trunk of 41 the Metroxylon sagu. Metroxylon rumphii and the fruits of Elaeis guineensis yield both palm oil and 41 Fruits of the tropical broadleaved evergreen tree palm kernel oil. Various palm species are also tapped Garcinia mangostana (mangosteen) – some fruits have for their sugary sap, which may be used as a been opened to show their white arils containing the sweetener or fermented into toddy. Palm hearts edible seeds. (terminal buds) are harvested from various palms, and provide a vegetable delicacy. Many palms are also of importance for housing, with their trunks used for beams and flooring, and the leaves for roofing and weaving. Species of Pandanus (screw pines, another tropical family) yield large fruits (45), which are roasted and their seeds eaten by the Australian Aborigines. The long fibrous leaves of P. utilis are also used in thatching and to make Manila 42 43 Crop of 43 coconuts borne among the terminal crown of palm leaves on the arborescent monocot Cocos nucifera. 42 Fruits of the tropical broadleaved tree Nephelium 45 lapaceum (rambutan) with some fruits opened to show their edible arils. 44 44 Crop of edible date fruits from the tropical palm 45 Fruit of the arborescent monocot Pandanus sp., which Phoenix dactylifera. provides a food sometimes eaten by the native (aboriginal) Australians.

22 47 46 46 Edible seeds and fan-shaped foliage of the 47 Mature seeds of the shrubby cycad Cycas circinalis. The gymnosperm Ginkgo biloba (maidenhair tree). seeds are highly toxic (as are all cycad seeds) but after prolonged leaching in water they can be safely eaten. hats. The long, hollow, and very strong stems of fabric and rope from Adansonia spp., and rubber several tall tree-like bamboos, such as from Hevea brasiliensis. Various drugs and Dendrocalamus spp., are used extensively to build medicines are also sourced from trees. Quinine is houses and bridges, for scaffolding in building derived from the bark of Cinchona officinalis, while construction, and also as water pipes. The newly an extract from the bark of the Prunus africana is emerged shoots of bamboos, such as species of used to treat prostatic disorders. The latter species is Bambusa, Dendrocalamus, and Phyllostachys, are slow-growing and classified as endangered due to harvested when about 15 cm long and, after removal over-felling in the wild. In Kenya, a single tree may of their enclosing leaf sheaths, are widely cooked as fetch around £100 for its bark (Lewington, 2003). a vegetable in Asia. Many other trees yield a variety of medicines. Aspirin was originally isolated from Salix alba The seeds of a number of conifers and some other (white willow), while various antiseptics are also gymnosperms are edible. Examples include various derived from trees, for example tincture of witch species of Pinus, Araucaria (bunya-bunya pine, hazel from Hamamelis virginiana, tea tree oil from monkey puzzle, and parana pine), Gnetum gnemon Melaleuca alternifolia, and eucalyptus oil from (gnetum), and Ginkgo biloba (ginkgo, 46). In some species of Eucalyptus. Recently, the drug Taxol has Pacific Island countries, and also Australia, the been isolated from Taxus brevifolia and T. baccata indigenous people gather starchy seeds of Cycas (47) (Pacific yew and common yew, 49) and is used in the and other cycads such as Lepidozamia peoffsykana. treatment of breast and ovarian cancers. After prolonged leaching in water, the seeds can be ground into flour. However, the fresh untreated seeds Trees and forests are very important as amenities, are very poisonous for humans (Chapter 4). particularly in the developed world, for activities such as walking, hunting, riding, picnicking, and Several plant commodities are derived from the general relaxation. In the past, and still to some bark of trees; such as cork from Quercus suber (48),

SURVEY OF TREES 23 48 49 48 Stacked cork from the bark of the evergreen 49 Foliage and ‘berries’ of Taxus baccata (common yew). broadleaved tree Quercus suber (cork oak), after The foliage and red ‘berries’ are sometimes eaten by cattle harvesting in Spain. with fatal results but the foliage contains a potent anticancer compound which, when extracted, is marketed as Taxol. 50 Specimen of a large evergreen broadleaved tree Ficus 50 sp. (fig) – this is regarded by Buddhists as a sacred tree, as indicated by the ribbons tied around its trunk and small figures of Buddha embedded in its roots. extent today, trees have had deep spiritual/religious species, in a mountainous area of Zhejiang, China. significance for humans. In Europe, oaks (Quercus However, it has also survived in Chinese, Korean, robur and Q. petraea) were important in Germanic and Japanese temple gardens, having been planted and Norse mythology, while in Britain the Celts there and tended by Buddhist monks. In Chile, believed oak trees to be sacred. Also, throughout Araucaria araucana (monkey puzzle, 5) is a sacred Europe, the evergreen, long-lived, and highly tree for the Pehuenche Aborigines, with its nutritious poisonous common yew (Taxus baccata, 49) must seeds forming a staple of their diet. have been of great significance to our ancestors. Ancient specimens grow in many churchyards, and it is likely that some of these churches were established on what were originally pagan religious sites (Lewington and Parker, 1999). The relic yew, still just surviving in the church graveyard at Fortingall in Scotland, greatly pre-dates the coming of Christianity to Scotland. In Asia, several species of fig, in particular Ficus benghalensis (banyan tree) and F. religiosa (bodhi tree), are revered. Buddha is believed to have gained enlightenment while sitting under a specimen of the latter tree. Such figs still often have images of Buddha embedded in their spreading roots, while the trunk is bound round with yellow and red ribbons (50). Ginkgo biloba (ginkgo, 46) still exists in the wild, but as an endangered

24 TREE ARCHITECTURE variable, and their canopies may range from TRUNK AND CANOPY OF THE AERIAL TREE somewhat spherical to broadly spreading (1). The latter shape is commonly seen in European In most trees it is estimated that the trunk (51) makes temperate species (8). In marked contrast, some up 40–60% of their mass, while the aerial canopy broadleaved trees are narrowly or broadly columnar, (which bears the foliage, cones, flowers, and fruit; 5, with examples seen in Fagus sylvatica ‘Fastigiata’ 8, 25, 37) contributes another 20–30% (Thomas, (Dawyck beech, 54), Populus nigra ‘Italica’ 2000). Frequently, the height of a tree is (Lombardy poplar), Nothofagus cunninghamii approximately 100 times its trunk diameter, but there (myrtle beech), and Chrysolepis chrysophylla are various exceptions to this rule of thumb (52). (golden chestnut). Polyalthia longifolia (Indian mast Many conifers retain a single main trunk, from which tree) has a weeping habit. Various ‘architectural arise regular rings of branches (53). The mature models’ have been proposed to encompass the conifer tree canopy often retains a regular pyramidal extensive range of tree forms, but it seems that each form (12, 19), due to its continued growth from a species conforms to one of 25 possible characteristic dominant terminal bud. Boreal and montane conifers branching patterns (Bell, 1991). (Chapter 2) often have branches that slope downwards and from which the snow tends to slide According to Thomas (2000), the shape of a tree off, while some species of Abies (fir) and Picea is also a compromise between various factors, such (spruce) have broad canopies when growing in deep as the overall exposure of the foliage to optimize shade or at treeline level (Thomas, 2000). Tree photosynthesis, reproduction (the display of flowers architecture often changes with age. For example, for pollination and subsequent seed dispersal), the Pinus sylvestris (Scots pine) is more or less conic most ‘cost-effective investment’ by the tree in when young but, particularly in exposed sites, many growing its large wooden skeleton, and the response of its lower branches have been lost by the time it of the tree to its immediate physical surroundings. attains maximum height. Its long trunk is often then Hence, in addition to its basic ‘architectural model’, left bare, except for its terminal canopy (13). the shape of a tree is often greatly modified by external environmental factors, such as grazing and Palms and some other arborescent mono- wind (13, 55–57), disease (58; see also Chapters 8 cotyledons frequently show an unbranched trunk and 9), and pruning/tree surgery (34, 35, 51; see also bearing a terminal tuft of large, elongated leaves (7, Chapter 12). 43). The natural form of broadleaved trees is very 51 51 Massive 52 ancient pollarded specimen of the broadleaved deciduous tree Quercus robur (English oak) growing in the wood pasture of Hatfield Forest, England. 52 Huge swollen trunk of the broadleaved sub-tropical tree Brachychiton rupestris growing in an Australian botanic garden.

SURVEY OF TREES 25 54 53 Trunk of Pinus 53 54 Specimen of sylvestris (Scots pine) showing the broadleaved how several side Fagus sylvatica branches all ‘Fastigiata’, originate in which in 1860 whorls at the originated as a same level from sport in the the trunk. woodland at Dawyck, Scotland. 55 56 Grazed and 56 wind-trimmed 58 55 Group of wind-trimmed conifers (‘Krumholz’) miniature exposed to fierce gales on the tree line at Libby Flats, specimen of Wyoming, USA. Fraxinus excelsior (ash) growing in a gryke of the limestone pavement on the Burren Region of western Ireland. 57 Major branch 57 breakage on a large specimen of Quercus petraea (sessile oak) growing in native mixed woodland in Scotland. 58 Canopy of Pinus sylvestris (Scots pine) with a huge foliage gall probably caused by infection with the fungus Taphrina.

26 UNDERGROUND ROOT SYSTEM OF THE TREE in many tropical trees, this is much more marked and the root buttresses may extend several metres up the Tree roots generally lie underground and hidden trunk and slope for some distance obliquely into the from view, unless exposed by erosion in a wind- soil (63). blown tree or on a very shallow soil over bedrock (59, 60). Nevertheless, the complexly branched root According to Rackham (2004), most trees in system is a vital component of the tree. Water and England are shallow-rooted, with the tap root not nutrients are absorbed from the soil via the roots, persisting after early seedling growth. This is also and anchorage for the aerial trunk and canopy is also frequently likely to be the case in Europe and provided by the root system. In thickened roots, an elsewhere, with such trees very susceptible to gales. outer layer of impermeable cork is present, but the In the woodlands of western Scotland many wind- lenticels are channels through which gaseous thrown, prostrate, but living trees still survive after exchange with the soil atmosphere can occur (61). In gales from some decades earlier. These continue to many trees, the root system is estimated to make up grow actively but with only very limited root contact 20–30% of the tree mass, but the figure may be as with the soil (64). A similar situation occurs when a little as 15% in some tropical trees and up to 50% in standing tree is root pruned (as in site clearance for trees growing in very dry conditions (Thomas, construction or road building). It is estimated that in 2000). many cases a tree may survive when only a quarter (sometimes less) of its root system still remains A tree root system generally shows a central plate (Rackham, 2004). However, although such a of framework roots (60), which then spread laterally restricted area of roots may be sufficient for the as far as the margins of the aerial leaf canopy, where water requirements of the tree, it will often be they are much narrower and are primarily concerned insufficient for its role as a stabilizing support for the with the absorption of water and its dissolved aerial tree. In areas with only a thin soil cover, the minerals. At their origin from the trunk, the root plate lies superficially (59), and the expansion framework roots may be 30 cm or so thick, and of these roots that accompanies secondary frequently graft together where they cross each other, thickening helps to fragment and weather the surface so producing a rigid root plate which binds the of the underlying bedrock (65). In deeper soils the associated soil and stones together (59, 60). The root plate may be situated a metre or more beneath swollen root bases often produce a prominent collar the surface, while sinker roots may further grow at the base of the trunk which, even in temperate down to a deeper lying water table. trees, may flare into small buttresses (62). However, 59 60 59 Underside of the root plate of a wind-felled specimen of 60 The root plate of Quercus petraea (sessile oak) felled by Fraxinus excelsior (ash) growing in mixed native woodland wind-blow, and then exposed after flooding by the waters in Scotland. of Loch Lomond, Scotland.

SURVEY OF TREES 27 62 61 64 61 Detail of a main root of Quercus petraea (sessile oak), 65 exposed after flooding/soil erosion, showing numerous lenticels in its bark. 62 Young specimen of the deciduous broadleaved tree Fagus sylvatica (beech), demonstrating that even temperate trees may develop root buttresses. 63 63 Massive root buttresses on the tropical broadleaved tree Parkia javanica. 64 Large wind-blown specimen of Quercus petraea (sessile oak) in Scotland, which was probably felled by the devastating gales of 1987/1990, now showing the transformation of a side branch into a large vertical new trunk despite its meagre root connection to the soil via the original root plate. 65 Part of the exposed root system of Fraxinus excelsior (ash) growing in rock fissures and helping to break up the bedrock.

28 ACCESSORY ROOTS ON THE AERIAL TREE shell of fused Ficus spp. roots remains (71) and these thicken to form an independent hollow-trunked fig Some trees develop trunk- or branch-borne tree. adventitious roots. As well as providing an increased aerating and absorbing surface (66, 67), these may Many old trees are hollow, and very commonly serve as an additional anchorage/support to the the growth of internal adventitious roots within the trunk, as in species of Ficus and Pandanus (68, 69), trunk allows recycling of nutrients released from the and also facilitate the spread of the tree following rotting heartwood. In the UK this is especially branch rooting (70). common in ancient pollards of tree such as Fagus sylvatica (beech, 72), Fraxinus excelsior (ash), and Tidal mangroves have various anchorage and Carpinus betulus (hornbeam). In other trees, the aeration devices, such as the complex stilt (prop) rooting of low branches (layering) may eventually roots of Rhizophora spp., the upright peg roots of Avicennia spp., and the knee roots of Bruguieria spp. 66 (67). At low tide, atmospheric oxygen diffuses through the lenticels into the well-developed cortical 66 Knee roots of the conifer Taxodium distichum native to intercellular space system of the subterranean roots. swampy areas of the southern USA. Knee roots also develop in the gymnosperm Taxodium distichum (swamp cypress, 66). Pillar and strangler root systems occur in a number of species of fig (Ficus, 69, 71). A specimen of F. benghalensis (banyan tree), planted in 1782 in Calcutta, now forms a tree grove comprised of some 1800 separate pillars (trunks), which originated as branch-borne adventitious roots (Thomas, 2000). The roots of the strangler figs originate from a seed(s) germinating in the canopy of a host tree. These roots repeatedly branch and anastomose as they grow down the trunk and eventually reach the soil. The host trunk often becomes completely enclosed (‘strangled’), dies, and then rots away. The 67 68 68 Prop roots of the arborescent monocot Pandanus pedunculatus growing along the sandy coast of New South Wales, Australia. 67 Breathing roots in tropical broadleaved trees. Knee roots of Bruguieria gymnorhiza and upright air roots of Avicennia marina exposed at low tide in the coastal mud flats of Queensland, Australia.

SURVEY OF TREES 29 lead to the propagation of a new individual (70). 69 Root suckers (shoots forming on roots) occur on many species of broadleaved tree, such as Ilex 69 Specimen of the evergreen broadleaved tree Ficus (holly), Ulmus (elm), and Robinia pseudoacacia benghalensis (fig) growing in an Australian botanic (false acacia). Clonal stands of thousands of poplar garden, showing its numerous pillar roots. and aspen (Populus) have originated in this manner in North America. Some conifers, such as species of Abies (fir), Picea (spruce), and Juniperus (juniper), also propagate by root suckers, and Sequoia sempervirens (coastal redwood, 73) also propagates vegetatively in this way. 70 71 The fused 71 73 roots of a Ficus (strangler fig) in Queensland, Australia. Note that the trunk of the original ‘strangled’ host tree has rotted away. 70 Large layered branch of Fagus sylvatica ‘Purpurea’. The branch has rooted and is considerably thickened compared to its diameter (arrow) near its origin from the tree trunk. 72 Top of a hollow 72 trunk of an ancient pollarded specimen of Fagus sylvatica (beech), showing large adventitious roots growing down its hollow core to tap nutrients released from the rotted heartwood. 73 Adventitious shoots sprouting from roots of the conifer Sequoia sempervirens (coastal redwood).

30 THE ROOT SYSTEM AND inhabited by the actinomycete Frankia (77), while in MICRO-ORGANISM SYMBIONTS the cycad Macrozamia riedlei, the nitrogen-fixing blue-green alga Nostoc is present in its surface roots. In many plants, the roots develop fine hairs just These varied micro-organisms utilize part of the behind their growing tips (74). These greatly increase fixed nitrogen for their own growth, but some is also the absorptive surface of the root. However, root available to the growing tree roots. Correspondingly, hairs are frequently sparse in trees and may be absent some of the sugars synthesized by the tree are used from some gymnosperms. Nevertheless, most trees by the micro-organisms. develop accessory root/micro-organism symbiotic systems of nutrient supply and absorption. These Symbiotic endomycorrhizae are commonly take the form either of root nodules (75–77) or of located within the fine roots of a wide variety of endo- and ectomycorrhizae (78–80). In Alnus rubra trees, including various species of broadleaved (red alder) and some legumes, nodules and trees such as Acer (maple, sycamore), Fraxinus mycorrhizae can occur on the same root system. (ash), Juglans (walnut), Magnolia, and Olea Some trees can form either endo- or ectomycorrhizae (olive), as well as species of conifers such as and, occasionally, as in Eucalyptus, Juniperus Juniperus (juniper), Sequoia sempervirens (coastal (juniper), and Salix (willow), both types may be redwood), and Sequoiadendron giganteum (giant present on the same tree. sequoia). The root tips may develop some root hairs and usually there is little external evidence of Root nodules are inhabited by symbiotic micro- the associated fungus; but within the root the fungi organisms (77), which fix or transform atmospheric penetrate the cell walls of the outer paren- nitrogen in the soil into nitrogenous compounds chymatous tissue (cortex) to produce highly utilizable by the tree. In leguminous trees, such as branched arbuscules (78). The thread-like species of Acacia and Laburnum, and also Ulmus mycorrhizae ramify through the soil and provide (elm) and other members of the elm family, the the tree with additional access to essential bacterium Rhizobium occurs in the nodules. In inorganic ions (especially phosphate) and water Alnus (belonging to the birch family, in which other located in the soil some distance away from the nodulated species also occur) the nodules may live nearest young roots. up to 10 years and grow large (75), sometimes to tennis ball size. Nodules in Alnus spp. (75) and Ectomycorrhizae are found in about 90% of Comptonia peregrina (bog-myrtle family, 76) are northern hemisphere temperate broadleaved tree 74 75 75 Large warty nodules of the broadleaved tree Alnus glutinosa exposed by flooding from Loch Lomond, Scotland. 74 Profuse development of root hairs on a leaf cutting of Saintpaulia.

SURVEY OF TREES 31 roots, and also in most conifers (Thomas, 2000). hemisphere and some tropical dipterocarps. Infected These mycorrhizae also occur in Eucalyptus and roots typically lack root hairs, are distorted, and Nothofagus (southern beech) of the southern may be coralloid-like (79). The fungus is visible 76 77 1 2 2 1 76 Large complexly lobed root nodule of the broadleaved 77 TEM showing vesicles (1) of the symbiotic actinomycete shrub Comptonia peregrina grown in aeroponic culture. Frankia enclosed within the walls (2) of Alnus glutinosa root nodule cortical cells. 78 79 1 7 2 3 4 1 2 3 56 22 78 Diagram illustrating the internal structure of a tree root with 79 Diagram illustrating ectomy- endomycorrhizae. External hyphae in the soil (1), fungal arbuscle and corrhizae on a Pinus root (1), and the vesicle (2, 3), fungal hypha (4) penetrating between root cortical cells (5), clumpy coralloid roots (2) which have root endodermis (6), and root epidermis (7). developed in association with the investing symbiotic fungal hyphae (3).

32 externally on the root as a smooth sheath (79, 80), 80 while internally its hyphae (fungal threads) ramify 1 within the intercellular spaces of the outer cortical cells, but do not penetrate the cell walls. In late summer/autumn the fruiting bodies of many common mushrooms and toadstools represent the aerial extension of various ectomycorrhizal fungi (81). These are usually associated with a broad range of host conifers/broadleaved trees (Dix and Webster, 1995). THE EVOLUTION OF LAND PLANTS 80 TS of a fine root of Fagus sylvatica (beech) invested by AND TREES ectomycorrhizal hyphae (1). The Earth is estimated to be some 4,600 million 81 years old, with its crust beginning to develop about 4,200 mya (Willis and McElwain, 2002). Later, 81 Autumn fruiting bodies of the fungus Amanita around 3,900 mya, as atmospheric water vapour muscaria (fly agaric), the underground mycelium of which began to condense, the earliest oceans appeared. forms an ectotrophic mycorrhizal association with tree Photosynthetic organisms first evolved in these seas roots, especially those of Betula spp. (birch). (Photo about 3,500 mya, in the form of stromatolites, which copyright of Norman Tait.) are symbiotic mat-like communities of prokaryotic algae and bacteria. Their fossilized remains are abundant, and even today, living stromatolites occur in the shallow sea off the western coast of Australia (82), and in several locations elsewhere. Prokaryotes lack discrete nuclei and other membrane-bound organelles, but these organelles occur in the later- evolved eukaryotic organisms (83). The oldest known eukaryotic plant fossil is a large single-celled alga dating from about 2,100 mya. Filamentous red and green algae first appeared in the oceans about 1,200 and 800 mya, respectively (Willis and McElwain, 2002). THE FIRST LAND PLANTS 82 SILURIAN PERIOD (443–417 MYA) 82 Stromatolites exposed in the shallow coastal waters Soils had begun to form on the continental surfaces off the coast of Western Australia. (Photo copyright of during the early Silurian period, due to the combined Stephen D Hopper.) action of atmospheric weathering and the organic acids produced by lichens (symbiotic associations of fungi and algae) and micro-organisms growing on the exposed rock. During this period the first vascular plants began to occupy the margins of the Continental Shelf. Cooksonia was an early colonizer, and its fossil remains occur in various European and North American sites. This leafless spore-bearing plant (84) grew only about 6 cm tall and bore sporangia on the

SURVEY OF TREES 33 tips of an equally branched (dichotomous) stem. an epidermis containing stomata (pores for gaseous Each sporangium contained many uniformly-sized exchange) is evident in some. The underlying cortex, spores (homospory), with each spore bearing three presumably photosynthetic, enclosed a core of radiating ridges (trilete markings, 85). Most elongated water-conducting cells (tracheids, specimens of Cooksonia are poorly preserved, but Chapter 6). 83 84 3 2 4 1 2 4 3 83 TEM of a green leaf cell of the moss Polytrichum 84 Reconstruction of the late Silurian vascular plant commune (hair moss) showing the membrane-bound Cooksonia (ca. 6 cm tall), a dichotomously branched and organelles characteristic of eukaryotic plants. Nucleus (1), leafless early land plant with terminal sporangia. chloroplasts (2), cell wall lined by plasmalemma (3), and ground cytoplasm (4). 85 85 Diagram of a spore tetrad (group of four haploid spores derived from the meiosis of a diploid spore mother cell) in an early vascular plant such as Rhynia and showing the trilete scars on a detached spore (top right).

34 DEVONIAN PERIOD (417–354 MYA) up to 2 cm wide, with Rhynia reaching about 20 cm in height. Both plants arose from a basal rhizome A variety of somewhat larger homosporous land bearing absorptive rhizoids (87), and their plants had evolved by the early-to-mid Devonian photosynthetic stems bore terminal sporangia (88). period. The Rhynie Chert from Scotland (86) In Rhynia, the cylindrical core of the stem contained contains the abundant silicified fossils of several such spirally thickened tracheids, but these were plants. Aglaophyton and Rhynia both formed apparently absent in Aglaophyton (89). upright, dichotomously-branched, and leafless stems 86 87 87 Reconstruction of the leafless 86 Polished face of a block of Rhynie Chert, from which Devonian vascular various primitive land plant remains may be identified in plant Rhynia (ca. thinly sectioned material examined under the microscope. 50 cm tall). Note the terminal sporangia on the dichotomously branched shoots which arise from a rhizome. 88 89 90 88 LS of the sporangium of a Rhynie 89 LS of the stem of a Rhynie fossil 90 Reconstruction of the leafy fossil leafless plant, probably plant, probably Aglaophyton. Note Devonian vascular plant Aglaophyton. the central strand of conducting cells. Asteroxylon.

SURVEY OF TREES 35 By contrast to these leafless forms, fossil lycopods, herbaceous lycopod Huperzia selago (91) is a distant such as Asteroxylon (from the Rhynie Chert, 90) and relative of such ancient plants. Baragwanathia, were densely covered with spiny leaves. Their stems contained a fluted central In the warm and humid climate of the mid-to-late vascular (conducting) system, and in Baragwanathia Devonian period, extensive forests developed. These this linked to veins in the leaves. The stem was up to contained cone-bearing lycopods, such as the very 1 m tall and 1–2 cm wide in Baragwanathia, but was common tree Lepidodendron (92), together with somewhat shorter in Asteroxylon. The present-day Archaeopteris (93, 94) and other progymnosperms. 91 92 91 Specimen of the extant herbaceous lycopod Huperzia 93 94 selago (club moss). 92 Reconstruction of the Devonian/Carboniferous leafy arborescent lycopod Lepidodendron with its root system known as Stigmaria. 93 Reconstruction of the Devonian progymnospermous tree Archaeopteris. 94 Fossil specimen of Archaeopteris showing numerous leaves on thin side branches coming off from a main branch.

36 96 95 95 LS of a mixed cone of the extant lycopod 96 Leafy twigs from a fine branch of Selaginella. Note the numerous small spores in the Lepidodendron. microsporangia and the few larger spores within the megasporangia. As in the present-day herbaceous lycopod 97 97 Coaly remains Selaginella (95), many of these forms were heterosporous (with each sporangium bearing a few of a large branch large spores or many smaller ones), while some of Lepidodendron reproduced by seed. showing the spiral, diamond-shaped Lepidodendron was heterosporous, and some pattern of its species apparently grew up to 35 m tall, with trunks persistent leaf 1 m in diameter (92). The upper trunk was bases. repeatedly divided into finer branches to form a conspicuous crown, bearing simple, spiny leaves probably heterosporous. Its trunk reached up to 8 m (96). After leaf abscission, the swollen corky leaf in height by 1.5 m wide (93), and its roots penetrated bases formed a characteristic interlinked diamond down to 1 m beneath the soil surface. The upper pattern on the trunk (97). These persistent leaf bases trunk bore spirally arranged branches with abundant gave mechanical support to the tall Lepidodendron small leaves (94) and its wood resembled that of trunk. Internally, only scanty secondary wood present-day conifers. developed, but the thick-walled outer cortex provided extensive additional support. The root system (often known as Stigmaria) was shallow but extended sideways up to 12 m from the base of the trunk (98), with the four primary branches dividing dichotomously and bristling with numerous side roots (99). The mid-Devonian group of plants known as the progymnosperms are considered likely progenitors of modern seed plants (Willis and McElwain, 2002). Archaeopteris is a common fossil example and was

SURVEY OF TREES 37 98 99 99 Root of Stigmaria (Lepidodendron) showing numerous lateral roots emanating from it in a bottle-brush arrangement. 98 A specimen of Stigmaria (the stump of Lepidodendron); this is one of a number of in situ stumps revealed by excavations on the Carboniferous site at Fossil Grove in Glasgow, Scotland. 100 Reconstruction of 100 101 Fossil frond 101 the Carboniferous of Alethopteris, arborescent horsetail a Carboniferous Calamites. seed fern. CARBONIFEROUS PERIOD (354–290 MYA) giant horsetail trees, such as Calamites (100), and some tree ferns (101), which bore seeds rather than Further tree groups had evolved by the early spores. In more temperate regions, smaller Carboniferous period, but the warm and wet arborescent lycopods and horsetails predominated, equatorial forests were still dominated by while ferns and seed ferns were common. Lepidodendron (92) and various other tree lycopods. The other major components of these forests were

38 103 104 102 102 Specimens of the extant herbaceous pteridophyte Equisetum (horsetail). (Photo copyright of Norman Tait.) 103 Fossil stem of the Carboniferous arborescent horsetail Calamites. 104 Fossil of the leafy twig of Annularia, possibly belonging to Calamites. Calamites was a common tree, with the main 105 trunk growing up to 18 m high (100). The present- day herbaceous horsetails (102) are its distant 4 relatives. The sporangia of Calamites were borne in 12 cones, and some species were heterosporous. Calamites and other similar horsetails bore 3 conspicuous rings of side branches with prominent longitudinal ribs (103), while their leaves were small 105 LS of an ovule from the Carboniferous seed fern and fused (104). Within the trunk and branches, a Lagenostoma ovoides. Micropylar chamber (1), wide cylinder of wood surrounded a ring of vascular integument (2), female gametophyte (3), and nucellar strands (veins) and central pith. beak (4). Various homosporous tree ferns inhabited these Cordaites (106) had extensive woody thickening and forests and were often very similar in appearance to grew up to 30 m tall, with a trunk of 1 m in width. present-day tree ferns such as Dicksonia and Cyathea The trunk branched frequently and bore spirally (6). Psaronius was the largest Carboniferous tree fern, arranged, strap-shaped leaves up to 1 m long by 15 and grew up to 10 m high with an unbranched trunk cm wide. The male and female cones were about 1 cm bearing a terminal tuft of fronds several metres long. long but, unlike in modern conifers, the naked ovules No woody thickening occurred in the trunk, but (immature seeds) and pollen sacs were situated at the mechanical support was provided by its persistent tips of the scale leaves. In other cordaitales, the leaves woody leaf bases and the profuse development of were much smaller and needle-like. prop roots. Seed ferns such as Medullosa, which grew up to 4 m tall with a trunk up to 50 cm wide, had a similar fern-like habit and foliage. Nevertheless, they reproduced by seeds rather than spores and their seeds are common Carboniferous fossils (105). A number of conifer-like shrubs and trees bearing seeds evolved during the Carboniferous period.

SURVEY OF TREES 39 107 106 Reconstruction of the 106 Carboniferous/Permian tree Cordaites. 107 Fossil specimen of the frond of Nilssonia, an arborescent Jurassic cycad. 108 PERMIAN AND PERIODS TO THE PRESENT DAY 108 Late Triassic fossil trunk of the conifer Araucarioxylon revealed by erosion of softer In the succeeding colder Permian period (290–248 rocks of the Petrified Forest in the USA. mya), the tree lycopods and horsetails were no longer dominant (Willis and McElwain, 2002). However, 109 there was an increase in various cordaitales, seed ferns, and primitive conifers, together with the newly 109 A fossilized dicotyledonous tree leaf from the late evolved groups such as the glossopterids, bennettites, Cretaceous period, from Mull, Scotland. ginkgos, and cycads (107). The latter two groups persist into the flora of today (15, 16, 46, 47). During already become the dominant vascular plant group the subsequent Triassic period (248–206 mya), in most habitats (109), as they have remained to the extensive evolution of new conifer lines occurred present day (1). (108). Araucaria (5, 21), Pinus (13, 18), and Taxus (49) are some of their present-day relatives. The earliest traces of flowering plants are found as pollen grains in the Cretaceous period (144–65 mya) in deposits about 130 million years old. The nature and form of these first flowering plants (angiosperms) is disputed, but they were probably herbs or small shrubs rather than trees (Willis and McElwain, 2002). By the late Cretaceous period, various present- day dicotyledonous tree genera such as Ulmus (elm), Nothofagus (southern beech), Alnus (alder), and Betula (birch, 8) had evolved. Monocotyledons are rare in early Cretaceous deposits, but by about 110–100 mya, palms had evolved, and screw pines appeared early in the Paleocene period (65–55 mya). By the later Paleocene period, flowering plants had

40 SECTION 2 WORLD DISTRIBUTION OF FORESTS CHAPTER 2 Northern boreal and montane coniferous forests Aljos Farjon INTRODUCTION ability to colonize vast areas while maintaining their dominance under these adverse conditions. Despite The boreal climatic zone occurs in the northern the numerous ice ages pushing all arborescent species hemisphere and lies roughly in the latitudinal zone far to the south, some trees were able to migrate again between the Arctic tundra to the north and temperate in warmer periods, such as exist at present, to reforest broadleaved forests and grasslands in the south. Its areas further north. However, many tree species natural vegetation is dominated by forests of needle- remained at more southern latitudes or along ocean leaved conifers, but various species of broadleaved coasts, while some became extinct when retreat was trees such as Betula (birch), Populus (poplar), Salix not possible, as was the case in much of Europe. (willow), and Alnus (alder) often occur in wetter areas. Here, the nitrogen-fixing actinomycete Only a limited number of species had the Frankia occurs in the roots of alder (Chapter 1). necessary properties to enable them to re-colonize Under natural conditions, these boreal forests cover the boreal zone. In larger mountain systems (such the region except where interrupted by swamps, as the Alps, Caucasus, Tien Shan, and Rocky lakes, river flood plains, high mountains rising above Mountains), coniferous forests developed which the climatic tree line, or by the oceans. Despite were very similar to the boreal forests, but due to human impacts through logging and conversion, their isolation a number of different species could much of this broad forest belt still remains intact evolve in each of these mountain ranges. The Pacific across North America (Canada and Alaska), coast has long functioned as both a refuge from the northern Scandinavia, and Siberia. ice ages and a corridor for plant migration, with the result that its coniferous species differ from those The northern boreal forests have no counterparts in the interior of North America and Eurasia. in the southern hemisphere, not only due to the lack of great expanses of land at the appropriate latitudes, In this chapter a few examples of some important but also because the southern conifers belong to and characteristic species of the northern coniferous different taxa, which have different ecological forests are described. These have been chosen to strategies for survival. The northern conifer species illustrate conifer diversity both taxonomically and have adapted to an extreme climate and have the ecologically.

NORTHERN BOREAL AND MONTANE CONIFEROUS FORESTS 41 CONIFERS OF THE BOREAL or in water-logged soils. During the brief but hot and FOREST BELT dry summers, it is subject to frequent forest fires, which may burn vast areas. Fires are difficult to control in the The coniferous forests of the interior continental vast expanses of the largely road-less taiga; they boreal zone, known by the Russian term ‘taiga’, are determine the patterns of regeneration and forest dominated by a few genera and species in the Pinaceae composition almost as much as the limited growing (pine family). The most widespread of these are Larix season, and are responsible for a dynamic ecosystem (larch), Picea (spruce), and Pinus (pine), while Abies with rapid biomass turnover. These conditions are, (fir) is more localized and usually montane. Others are however, also the reasons why humans have largely left essentially coastal conifers, or occur south of this zone. the northern boreal forests undisturbed. Habitation is In both Eurasia and North America, each of these difficult and agriculture impossible. Tree growth is genera is represented in the boreal forests with slow, especially on poor soils away from river flood generally no more than two, or at most three, species. plains, resulting in thin, even stunted, trees that are Larches, the only deciduous conifers in this region, uneconomical for man to exploit. comprise two species (Larix gmelinii and L. sibirica) in boreal Eurasia and one (L. laricina) in North In the following sections some of the more America. In mountains to the south of the taiga zone, important conifer species will be described briefly. other species such as L. decidua (European larch) and L. occidentalis (American larch) occur. The same LARIX GMELINII paucity of species is seen in Picea (spruce), with both continents having only two. Picea abies (Eurasian L. gmelinii (Dahurian larch) occupies all of eastern Norway spruce) and P. obovata (Siberian spruce) Siberia, east of a line from Lake Baikal to the mouth actually integrate in the northeast of Russia. However, of the Yenisei River. Varieties of this larch described in North America, P. mariana (black spruce) and by various authors as different species occur, P. glauca are genetically and ecologically more distinct especially along the Pacific coast, and one variety but occupy virtually the same geographical range. forms an isolated population in central Kamchatka. The differences are mainly in the seed cones, all of There are a few more species of Pinus in the boreal which are smaller and have thinner scales than those zone, especially in North America, but no more than of European or Siberian larch. Under favourable six in total. The most widespread of these is Pinus climatic and soil conditions this larch can reach a sylvestris (Scots pine), of which the Scottish height of 35 m and a diameter of 1.5 m, and form a populations are a tiny western outlier of a range that straight bole. However, it is often much shorter and reaches from western Europe to the Pacific. In both curved with its branches spreading horizontally. As continents, species belonging to soft pines (subgenus in all larches (110), the deciduous leaves grow on Strobus) occur along with the more common and widespread hard pines (subgenus Pinus), of which 110 Scots pine is a representative. In North America, Pinus banksiana (Jack pine) is the most widespread of the 110 Cones and foliage of Larix decidua (European larch). pines, but in regions of the Rocky Mountains it is (Photo copyright of Bryan Bowes.) replaced by P. contorta (lodgepole pine). Whereas all conifers of the boreal zone become habitually stunted at the climatic limits of trees, certain species of pine are genetically determined to be shrubby, such as P. pumila (a soft pine with edible seeds) in eastern Siberia and P. banksiana in Canada, although the latter may grow to a small tree in more favourable soil conditions. All these species of conifers must cope with a very short growing season and a prolonged, extremely cold winter. The taiga forest is under snow for up to eight months of the year, and often grows over permafrost

42 short spur shoots, which increase only a few mm in 111 length each year, and in autumn the leaves become bright yellow before falling. Among the boreal 111 Picea abies (Norway spruce) growing in the Alps of conifers, only larches are deciduous, but the fossil Switzerland. record shows that several other deciduous conifers, now confined to southern latitudes, once grew north and the Alps. In the history of the ice ages, when trees of the Arctic Circle. Throwing off leaves in winter repeatedly retreated south and west over a time span may have evolved in conifers in response to of perhaps two million years, separation into varieties prolonged darkness, as a means of saving energy (perhaps even species) occurred. Hence, P. abies is when photosynthesis has come to a standstill. much more variable morphologically than P. obovata, due to its more complicated history of retreat and LARIX LARICINA separation. Growth forms of P. abies that were adapted to different climatic conditions evolved. In the L. laricina (tamarack) in North America is the interior of Eurasia, far from the ameliorating influence vicariant (i.e. a closely related sister species separated of oceans, the long winters are extremely cold and by a geographical barrier) of L. gmelinii. It is produce moderate quantities of dry snow. These distributed across Canada, and in interior Alaska conditions select for trees with slender, columnar also has a large but disjunct population that crowns, and short, horizontally spreading branches. some botanists consider as taxonomically distinct. In much of Europe, winters are milder and produce The seed cones of L. laricina are even smaller large quantities of wet snow in the mountains. Here, than those of its Siberian counterpart (L. gmelinii), spruce trees have longer, drooping, or even pendulous but otherwise they are very similar species. branches, allowing the snow to fall through. At times Morphologically, L. laricina is much more constant of maximum glaciation, P. abies was restricted to across its vast range than L. gmelinii, but it can grow scattered refuges in southern and eastern Europe. to the same dimensions. Especially in the eastern When the climate warmed in interglacial times, the parts of its range, it often occurs on peaty soils in remnants spread out and often met. The resulting swamps and muskegs, where it remains more genetic diversity of P. abies is reflected in the extremely stunted and is usually accompanied by Pinus variable morphology of its seed cones. This variation banksiana and/or Picea mariana. occurs in both growth forms, as they were not subject to the same adaptive selection. PICEA OBOVATA PICEA GLAUCA P. obovata (Siberian spruce) is closely related to P. abies (Norway spruce), with the main morphological P. glauca (Canadian or white spruce, 112) extends all differences being the smaller seed cones and rounded the way from Newfoundland to the coast of the scales in P. obovata. Intermediate forms are found in Bering Sea in Alaska and, together with P. mariana, northeastern Russia, where the two taxa meet, and is one of the two spruces of the Canadian taiga. these are interpreted as natural (introgressive) hybrids. P. glauca occupies the better drained, often more Siberian spruce extends from the Ural Mountains to fertile soils away from swamps and muskeg and, the Sea of Okhotsk, but is absent from northeastern especially on river flood plains, it can attain a height Siberia. Spruce stands tend to be more monospecific than larch stands, unless edaphic (soil) factors, such as on peaty soils, retard their growth. After fires, pioneer trees of Salix (willow), Betula (birch), and Populus (poplar) invade burnt areas, and are later succeeded by Picea oborata. PICEA ABIES P. abies (Norway spruce, 111) occurs from Scandinavia and northern Russia to central Europe

NORTHERN BOREAL AND MONTANE CONIFEROUS FORESTS 43 112 113 112 Forest of Picea glauca (white spruce) on the bank of 113 Natural stand of Pinus sylvestris (Scots pine) growing the Porcupine River in the Yukon Flats, Alaska. in the Scottish Highlands. (Photo copyright of Bryan Bowes.) of 40–50 m with a trunk diameter of over 1 m. 114 Upper trunk 114 It develops the same narrow, columnar habit as of Pinus sylvestris P. obovata, and has small seed cones with thin, light- (Scots pine) brown scales. showing its orange bark. PICEA MARIANA (Photo copyright of Bryan Bowes.) P. mariana (black spruce) and P. glauca virtually overlap in their ranges geographically, but are of the taller Scots pine. P. sylvestris is easily recognized seldom found growing together. In boggy areas and by its orange, thin, papery bark, which occurs higher where permafrost is prevalent, P. mariana replaces up the trunk (114) and on the branches of larger trees, P. glauca, unlike in Siberia where there is only one its short, glaucous-green needles in fascicles of two, species (P. obovata). Under such conditions, and its small cones with thin, dull-brown scales. In the P. mariana forms an extremely slender tree, growing Far East, some closely related species with similar up to 30 m tall in the southeast of its range. characteristics occur. These may be only subspecies or Elsewhere, however, it is much smaller and is often varieties, a somewhat similar situation to that dwarfed to a few metres. The small seed cones often occurring with Larix gmelinii. The habit of Pinus aggregate at the top of the tree, where squirrels gnaw sylvestris is dependent on its growth conditions. In off shoots with cones and cause reiteration (i.e. dense stands of more or less even age, it becomes (like secondary growth of twigs by activation of dormant all northern conifers) slender with a straight bole, and buds). Hybrids between the two species, and with such conditions are imitated in plantation forestry. another spruce (P. rubens), have been reported, but In old, open stands it is more picturesque, as in the two species seem genetically well separated some old forest remnants in eastern Norway and through most of their joint ranges. Scotland (113). PINUS SYLVESTRIS P. sylvestris (Scots pine) has a very wide distribution (113) that reaches well beyond the boreal forest zone. Inside that zone, it is more restricted, but occurs in habitats ranging from dry sandy soils and rocky slopes to margins of moist bogs and lakes. It is found from Portugal to the Sea of Okhotsk in eastern Siberia. Beyond that line it is replaced by the shrubby species P. pumila (dwarf pine), although the two ranges partly overlap, with the dwarf pine forming the understorey

44 MONTANE CONIFERS green and erect, and bear protruding (exserted) bracts. The cones turn brown and disintegrate in Conifer diversity increases substantially, especially in autumn. In this feature the genus Abies differs the uplands, south of the boreal zone and this trend markedly from Picea, where the seed cones are continues down to subtropical latitudes. In this pendulous and remain intact. chapter, however, only conifers in montane regions, roughly down to 40° North, will be discussed. (This ABIES LASIOCARPA therefore excludes consideration of Mexico, the Himalayas, and most of China and Japan, all regions A. lasiocarpa (subalpine fir) occurs throughout the which are very rich in conifer species.) The montane Rocky Mountains from the Yukon Territory to regions have a more or less continuous contact with New Mexico, from the subalpine zone to a the boreal forests and served as refuges for many maximum elevation of 3,500 m. Its slender, conical northern conifers during the ice ages. In these shape (116) is a characteristic sight in the western mountains there is a climatically determined zonation of vegetation that mimics the continental 115 115 Abies alba zonation from warm temperate to Arctic, but over (white fir) in the greatly compressed distances. Vosges Mountains of This accounts for a substantial increase in France. diversity in these mountains. One of the most spectacular examples for conifers is the Sierra 116 116 Abies Nevada of California, rising from only a few lasiocarpa (alpine hundred metres in the Central Valley of California to fir) in the Rocky 4,418 m at Mt Whitney. The Eurasian mountains Mountains north of 40° are less species diverse, largely due to a (Sawtooth less favourable E–W orientation, which blocked tree Mountains) of migration to the south in advance of the ice age Idaho, USA. winters. Species in the Pinaceae (pine family) generally dominate the forest but other conifers are important, with trees of the Cupressaceae (cypress family, including the formerly recognized family Taxodiaceae, redwoods) and, to a lesser extent the Taxaceae (yew family), enriching the forests. Among them are species with great tolerance to extreme situations and others that grow so well that they are the biggest tree species in the world. ABIES ALBA A. alba (white fir, 115) is widespread in the mountains of central Europe, from the Pyrenees to the Carpathians. It is the only common species of fir in Europe, with other species only existing in relict populations in countries around the Mediterranean. On well-drained soils with abundant precipitation, A. alba grows as a tall tree (55–60 m) in a belt between the broadleaved and coniferous forests. Either it may form pure stands, or it is mixed with Picea and Pinus species, or often Fagus sylvatica (beech). In early summer, high in the crown of mature fir trees, seed cones are produced. These are

NORTHERN BOREAL AND MONTANE CONIFEROUS FORESTS 45 mountains of North America, where it grows in resulted in monospecific stands of this species – more or less open stands interspersed with alpine resembling managed plantations – in many meadows. The densely set foliage branches and mountain forests of western Canada and the USA. conical shape of A. lasiocarpa help to prevent The seed cones of Pseudotsuga (118) differ greatly breakage from the heavy snowfall the tree endures from those of Abies, and are of the pendulous, in much of its range. It produces erect, purple cones intact type (as in Picea) but with long three- with hidden bracts. pronged (trident) bracts above the rounded seed scales. PSEUDOTSUGA MENZIESII PINUS PONDEROSA P. menziesii (Douglas fir, which is not a true fir) occupies a range roughly similar to that of P. ponderosa (Ponderosa pine) is one of the most subalpine fir, but generally occurs at lower widely distributed pines in western North America. elevations. Two varieties of P. menziesii are It occurs both in the Cascades and Sierra Nevada commonly recognized, a coastal form and the ranges, and throughout the Rocky Mountains, inland form (var. glauca). The species extends into but not further north than 52° in British Columbia. the mountains of Mexico but, being moisture- Several forms or varieties are recognized and dependent, it becomes increasingly restricted some botanists (but not the present author) include southwards to favourable sites. On the coast in the the mostly Mexican P. arizonica in it. There is Pacific Northwest, this conifer attains great size, to also confusion about the ‘typical’ variety around 100 m tall and 4–5 m diameter. It is (var. ponderosa), which David Douglas apparently naturally mixed with other conifers in old-growth originally collected on the Columbia River in the forest (117) but, unlike many other species of these north of its range. However, most foresters still forests, it has a capacity to colonize after believe that the magnificent trees found further south disturbance in great numbers. After logging in Oregon and California constitute this typical operations on old-growth stands, foresters have variety. P. ponderosa is a ‘sun-loving’ pine, which preferred to replant with Douglas fir. This has forms open stands or may occur mixed with other 117 Large trunks 117 118 of Pseudotsuga menziesii (Douglas fir) on the Pacific coast of Washington, USA. 118 Female cones of Pseudotsuga menziesii. (Photo copyright of Bryan Bowes.)

46 conifer species, as in the Sierra Nevada of California. America (all those belonging to the subgenus Pinus), Its bark forms characteristic large plates (119) and its with two-needled pines in Canada and numbers of egg-shaped seed cones are up to 15 cm long. These up to eight in Mexico. cones are generally smaller than those of its near relative P. jeffreyi, with which it can grow together in JUNIPERUS OCCIDENTALIS California. The clustered pollen cones of Ponderosa pine are strikingly coloured (120). There is a gradual J. occidentalis (western juniper) occurs from shift in P. ponderosa from two-needled pines in the Washington to California in the high mountains of north, via two-to-three needled, intermediately the Cascade and Sierra Nevada ranges, although located pines, to predominantly three-needled pines most of the tree-forming juniper species have a in the south. In general, needle numbers increase from more southerly distribution. J. occidentalis is a north to south among many species of pine in North magnificent tree (121), which grows extremely slowly but reaches 20 m in height and 2.5 m 119 119 Trunk of diameter. Its orange-red, stringy bark contrasts with Pinus ponderosa dark-green, dense foliage of the minute scale leaves. (Ponderosa pine) Most trees are dioecious (bearing reproductive in California, USA. organs of one sex only), but exceptions occur. Western juniper is capable of survival under very extreme conditions, growing in cracks in bare granite rock with no soil content, and tapping seepage water that runs off the rocks into the fissure. As with all junipers, the wood is hard and extremely durable, but its slow growth makes commercial growing impossible. For this author, they are the most beautiful of all the conifers in the Sierra Nevada of California, which also has arguably the most beautiful conifer forests in the world. COASTAL CONIFERS At relatively northern latitudes, coniferous forests reach the coasts of both the Atlantic and Pacific 120 121 121 Juniperus occidentalis (western juniper) in the Sierra Nevada of California, USA. 120 Pollen cones of Pinus ponderosa (Ponderosa pine) shortly before pollen dispersal.

NORTHERN BOREAL AND MONTANE CONIFEROUS FORESTS 47 Oceans. However, it is only on the Pacific coast that numerous and prominent, both on the forest floor many conifer species are more or less restricted to a and right up into the tops of the trees. Forest fires coastal strip, or to islands such as Japan. The coast and storm damage are relatively rare and localized. of the North Pacific is a major refuge for ancient conifers that have become extinct elsewhere, their On the Asian side of the Pacific, conifers tend to demise being due to geological history and to climate grow more as solitary emergents above broadleaved changes during the last few million years. Conditions trees, although conifer-dominated forests and groves for growth of conifers are probably nowhere better occur on a smaller scale. Much of the old-growth than in the Pacific Northwest of North America, but forests have been replaced with other types of planted trees also thrive in Ireland and Scotland at vegetation, as an agriculture-based civilization has had the same latitudes, which indicates the similar a much longer history here than in North America. climatic conditions in parts of Atlantic Europe. TSUGA HETEROPHYLLA The coniferous forests that extend from California to Alaska along the coast of the Pacific T. heterophylla (western hemlock, 122, 123) is one of Ocean can be classified as temperate rain forest. two species of Tsuga in North America (there are The climate is continually moist or wet, with seven species in Asia, but none in Europe). The other precipitation evenly distributed throughout the year species is T. canadensis (eastern hemlock), which is as fog, rain, or wet snow, while temperatures are less coastal in its distribution. Hemlocks are extremely cool without extremes in summer or winter. These shade-tolerant, and are also the conifers that grow conditions prove to be particularly favourable for closest to the sea-coast in many areas. Their small evergreen conifers, which attain extraordinary sizes. leaves and numerous slender branches form dense The forest begins directly above the line of highest horizontal sprays, blocking the sunlight from reaching ocean tides and climbs up to about 1,500 m in the the forest floor, which is usually covered in 20–50 cm south and only a few hundred metres in the north. of mosses, and hence is inaccessible. The tiny There is usually a mixture of conifer species, with pendulous cones of the hemlocks disperse numerous some conifers growing in groves as well as small, winged seeds that can germinate on fallen trees individually dispersed. Although some broadleaved where some litter has accumulated. Growing on these (flowering plant) trees are common, especially along ‘nurse logs’ the hemlock seedlings start to compete for streams, they do not reach the canopies of the tall light, water, and nutrients. Eventually, a colonnade of conifers. Ferns, bryophytes, and lichens are trees will indicate where a long since decayed conifer tree once fell to the ground (123). 122 123 122 A ‘nurse log’ with numerous seedlings of Tsuga 123 Column of Tsuga heterophylla (western hemlock) trees heterophylla (western hemlock) on the Pacific coast of standing on an old ‘nurse log’ in the rain forest of Washington, USA. Washington, USA.

48 THUJA PLICATA This allowed the native coastal Americans to build their large houses, as well as sea-going whaling boats. T. plicata (family Cupressaceae, 124, 125) is termed The wood is also rot-resistant, which is an important by American foresters, western red cedar. However, it quality in a continuously wet climate. However, this is not a true cedar (Cedrus) as it does not belong to property now works against long-term conservation the Pinaceae. T. plicata has an eastern sister species in of the species because it is still logged from old- North America and a few relatives in eastern Asia, growth forests to supply manufacturers of garden but the genus has become extinct in Europe. T. plicata furniture, sheds, and greenhouses. is the largest species and has foliage with glossy green scale leaves, which are arranged decussately (i.e. in SEQUOIA SEMPERVIRENS AND opposite pairs that are alternately shifted about 90º SEQUOIADENDRON GIGANTEUM on the shoot) and diversified into laterals and facials. It bears erect, small, thin, and oblong-to-ovoid Sequoia sempervirens (coast redwood, 127) is woody cones with spreading scales (126). The thin, famous for being the tallest species of all trees, with stringy bark covers wood that can be split into large one specimen currently having reached 112 m in planks by a simple technique without the use of saws. height. The species is now restricted to a narrow 124 124 Thuja plicata 125 (western red cedar) on the Pacific coast of Washington, USA. 125 Three massive trunks of Thuja plicata (western red cedar). 127 127 Sequoia sempervirens 126 (coast redwood) on the Pacific coast of California, USA. 126 Foliage and seed cones of Thuja plicata (western red cedar).

NORTHERN BOREAL AND MONTANE CONIFEROUS FORESTS 49 128 129 128 Sequoiadendron giganteum (giant sequoia), the champion ‘General Sherman Tree’ in Sequoia National Park, Sierra Nevada, USA. 129 Seed cone and foliage of Sequoiadendron giganteum (giant sequoia). (Photo copyright of Bryan Bowes.) 130 Bark of Sequoia 130 sempervirens (coast strip along the foggy Pacific coast, but before the redwood). (Photo ice ages it was once a common component of copyright of Bryan the northern coniferous forests of America and Bowes.) Eurasia. European settlers nearly logged it to oblivion, but it is one of the great achievements of 131 the North American conservation movement that this did not happen, and that most of the remaining 131 Clonal group of Sequoia sempervirens (coast redwood) stands of redwoods are now protected. The closely growing in a Californian redwood forest. (Photo copyright related Sequoiadendron giganteum (giant sequoia, of Bryan Bowes.) 128) occurs in the Sierra Nevada of California, and was at one time considered as another species of the coast redwood. S. giganteum can grow enormous. One specimen, known as ‘General Sherman’, is thought to be the largest tree in the world based on its volume, and is 83 m tall with an estimated weight of 6,000 tonnes. Its congeners, or close relatives, are known from fossils as far away as Australia, which demonstrates an ancient lineage dating back to the early Cretaceous period more than 100 million years ago. S. sempervirens has both scale and needle leaves, and bears small seed cones. S. giganteum has only scale leaves, but has larger cones (129) with more seeds in two rows on each scale. Both species have very thick, stringy, and fire-resistant bark (130), but persistent hot fires, caused by the ignition of accumulated dead wood, can creep inside hollow trunks and kill even the largest trees. S. sempervirens readily sprouts from the base (131) or from a large underground lignotuber, but this capacity for vegetative regeneration is relatively rare in conifers and absent in S. giganteum.