Ecology and Silviculture of Eucalypt Forests

By RG Florence

This classic forest management text examines the ecology and silviculture of eucalypts in forests and plantations in Australia and overseas. The book presents approaches to the formulation of ecologically sustainable forest practices through a more fundamental understanding of Eucalyptus.

The 14 chapters of the book are divided into three sections covering: the ecological background to silvicultural practice; the regeneration and continuing development of the forests; and silvicultural practice, including the current practices within the eucalypt forests.

• Language: English
• Publisher: CSIRO PUBLISHING
• Release date: Mar 30, 2004
• ISBN: 9780643102521

Book preview

CHAPTER

1

Introduction to the eucalypts

Introduction

Eucalyptus occurs within all but the driest parts of an entire continent—hence it is a component of a number of distinctive climatic zones and vegetational formations. Reasons for the outstanding environmental and vegetational range of the eucalypt may be embedded in the selection pressures and evolutionary responses which followed the break-up of the Gondwanan land mass, and subsequent continental drift. These matters are addressed in Chapter 2. But first it is useful to appreciate the broad structural components which make up the Australian vegetation, the taxonomic diversity within Eucalyptus and ways of classifying it, and the common attributes or growth habits of the species which have helped contribute to its dominance of the Australian landscape.

Structural components of the Australian vegetation

The Australian continent can be divided into a number of vegetation zones which closely follow climatic patterns, especially rainfall (Figure 1.1). There is a very marked summer distribution of rainfall in the north, a uniform to winter-dominant rainfall in the south, and a tapering off of rainfall from coastal areas to the arid interior.

This book focuses on the Australian ‘forests’ which occur only in relatively narrow coastal zones where rainfall is relatively high and fairly reliable, that is, along the eastern and south-eastern coasts (including Tasmania), and in the far south-west of Western Australia (Figure 1.2). The forests are described as being ‘closed’ where the leafy crowns of the trees in the upper canopy touch or intermingle (represented by rainforest), and ‘open’ where they do not (mostly dominated by Eucalyptus). If the outlines of all the tree crowns within the open forest were to be projected to the ground, they would cover only about 30–70% of the ground area.

Forest begins to give way to ‘woodland’ where the average annual rainfall begins to fall below 900 mm. In places there is a broad overlap zone with a mosaic pattern of woodland and forest. The trees are more widely spaced in woodlands than in forests, with a projective cover of 10-30% (Figure 1.3). Woodland and ‘open-woodland’ types are usually dominated by eucalypts, and occasionally by trees of other genera, such as Callitris, Casuarina, Allocasuarina, Melaleuca and Acacia. The woodland trees have more rounded crowns than those of the forest, and the length of the tree bole may be less than the depth of the crown. The woodland floor is grassy rather than shrubby, although it may be generally more shrubby on soils which are infertile and affected by repeated burning.

Figure 1.1

Broad vegetation zones within the Australian continent (modified from Specht 1970).

The vegetation of the woodland zone has been greatly altered for this is a major agricultural resource. Trees have been cleared or partly cleared for cereal growing and sown pastures, and to improve the grazing potential of native pastures. Largely as a result of grazing pressures, there has been a high incidence of decline and death of woodland trees in recent decades.

The woodland zone gives way. in turn, to shrublands and grasslands as annual rainfall drops below about 400 mm and becomes increasingly erratic. The wattle (Acacia spp.) now replaces the eucalypt as the dominant genus, with Acacia shrubland replacing eucalypt woodland. However, again, a large zone of overlap occurs, containing widely spaced, low eucalypts and a tall shrubland of wattles. Finally, where the average rainfall is below about 200 mm, the Acacia shrubland is replaced by low open shrubland known as ‘shrub steppe’, by ‘hummock grassland’ dominated by spinifex (Triodia spp. and Plectracll1le spp.) and, ultimately, by large areas of desert.

This book is concerned only with those tree communities which are a commercial wood resource. Logging of the closed forest has generally ceased in Australia, and the eucalypt woodlands do not normally support forest industries. The open forests constitute the major wood resource. and may be placed in one of a number of categories. based on the quality of the site. species composition, productivity and structural attributes.

Figure 1.2

Typical eucalypt sclerophyll forest. This is east coast E. pilularis forest. Left: mature forest. Right: pole regrowth forest.

Figure 1.3

Typical eucalypt woodland; the species are E. blakelyi and E. melliodora.

Forest formations

Marginal rainforest

Tall-boled eucalypts and other myrtaceous tree species (notably Lophostemon confertus and Syncarpia glomulifera) may occur as emergents from a secondary rainforest stratum. This forest can be a transition between rainforest and open sclerophyll forest, or it may occur more extensively wherever environmental conditions are suitable for it (Chapter 2).

Tall open forest

Tall open forest constitutes the most productive part of the commercial forest resource. The trees of the forest have a dominant height of at least 30 m, and frequently 60 m, with boles being at least half the tree height. There may be an understorey of ferns, tree-ferns, and small trees and shrubs which are mesophytic rather than xerophytic or sclerophytic in character. The term ‘wet sclerophyll forest’ has traditionally been used to describe this type, and will continue to be used in this book. It seems to describe very aptly the situation where a canopy of tall sclerophyll-type trees dominates a secondary stratum of plants which may be largely of rainforest origin. This type of forest is found on better quality soils, and particularly on those of higher nutrient status..

Open forest

The open eucalypt forest has a dominant height of between 10 and 30 m. The understorey is likely to be shrubby or grassy. The shrubs may be more xerophytic in character (small and often sharp leaved), and vary in frequency from sparse to dense. The ground surface on some of the poorer soils may be bare or litter covered in patches. The term ‘dry sclerophyll forest’ is frequently used to describe this type and, again, will be used in this book.

Low open forest

Low open forest has a dominant height of only 5-10 m, and is not usually considered as a wood resource.

Taxonomic status of Eucalyptus

About 500-600 eucalypt species are currently recognized. Some are widely though discontinuously distributed, others are narrowly restricted to localized niches. Many species contain a high level of genetic variation—expressed through distinctive ecotypes displaying disjunct and clinal variation, and variability within local populations. Superimposed on this, hybridization and introgression between interbreeding species may occur where their distributions overlap, or have overlapped in the past.

Against this background, an hierarchal classification of the eucalypts was never going to be easy. After 200 years of trying there is still much to be learnt about the genus, much to be done to resolve just what should be a species and what should not, and where individual taxa might be positioned within the classificatory structure. As L. D. Pryor has aptly put it, ‘the definition of a species as that which a competent taxonomist considers a species is not only a jest, but it has some aspect of truth in it’.

The first thorough classification of Eucalyptus was by George Bentham (1863-78) in the Flora Australiensis. This classification placed primary emphasis on the morphology of the anther. Subsequently, Ferdinand von Mueller produced many papers and a major book, Eucalyptographia (1879-84), but was unable to devise a better classification than the antheral classification of Bentham. J. H. Maiden (1903-31, A Critical Revision of the Genus Eucalyptus) and W F. Blakely (1934, A Key to the Eucalypts) extended Bentham’s antheral classification, but did not significantly advance the orderly classification of species.

The most recent classification of the eucalypt has been that of L. D. Pryor and L. A. S. Johnson in 1971 (A Classification of the Eucalypts). This is based primarily on an appreciation of genetic relationships between the species. It was becoming apparent from field observations and from attempts to create artificial hybrids, that some species hybridize readily while others do not. The classification of Pryor and Johnson (1971, 1981) creates ten subgenera which are genetically isolated from each other. Each of the subgenera is divided into sections, series, subseries, superspecies, species and subspecies. Each species is given a code consisting of up to six capital letters, the first three of which indicate the subgenus, section and series. If a taxon is placed in a subseries, this is indicated by the fourth letter; if a subseries is not used, the fourth letter is replaced by a colon (:). The fifth letter indicates the specific status, and a sixth is added when the taxon is regarded as a subspecies. Examples of coding for two species, E. pilularis (subgenus Monocalyptus) and E. diversicolor (subgenus Symphyomyrtus) follow:

Figure 1.4

A suggested general phylogeny of suballiances, genera, subgenera and sections in the Eucalyptus alliance. Width of band ends are in order of, but not proportional to, the number of species. Sections are designated by their two letter codes (from Pryor and Johnson 1981).

Eucalyptus pilularis (MAIAAA): M (Monocalyptus); A (Section Renantheria); I (Series Pilularis); A (Subseries Pilularinae); A (Species pilularis); A (Subspecies pilularis).

Eucalyptus diversicolor (SEB:A): S (Symphyomyrtus); E (Section Transversaria); B (Series Diversicolores);: (no subseries-a single species within the series); A (Species diversicolor).

The prospect for hybridization between eucalypt species depends on their genetic relationship, as indicated by the Pryor and Johnson classification. Interbreeding between species from different subgenera does not occur, either under natural conditions or in a controlled-pollination program. Interbreeding can usually occur between species within the one subgenus, although there are exceptions in the large subgenus Symphyomyrtus. For example, species of the section Adnataria (SU) which includes the boxes and ironbarks may interbreed, but are largely isolated from the rest of the subgenus Symphyomyrtus. Part of the Pryor and Johnson classification is given in Figure 1.4 (from Pryor and Johnson 1981).

Modem classification is based on a much wider range of attributes than was considered by the early taxonomists. A decision on the position of a taxon within the classification might take account of one or more of the following (Pryor 1976):

1. Genetic and anatomical criteria: where a species is known to hybridize with another, it must be placed in the same subgenus. Pryor cites the case of E. megacarpa which was placed by Bentham in what is now the section Maidenaria of subgenus Symphyomyrtus. However, as it is known to hybridize with E. marginata, and the anatomical attributes of its ovules and seed are more like those of Morwcalyptus than Symphyomyrtus, it has been reallocated to Monocalyptus.

2. Chemical substances: specific phenolic extracts are often produced very consistently in any one species or group of species. For example, Hillis (1966) found the substance ‘renantherin’ in most of the species of Morwcalyptus he examined. The presence of renantherin in E. megacarpa supported the reclassification of this species, and in the absence of renantherin, the relocation of E. deglupta and E. ganophylla from Monocalyptus to Symphyomyrtus and Eudesmia, respectively. Other supporting evidence for classificatory placements comes from isoenzyme studies and the use of gas chromatography in the assay of essential oils.

3. Characteristics of leaf surfaces: minor sculpturing and excrescences on the leaf surfaces of eucalypts, as shown in electron micrographs, are consistent for one species, and different but consistent in other species. These patterns have been used to help designate subspecies and to separate species.

4. Insects: eucalypts, like many other plants have a large suite of insect species in the natural environment, endemic to Australia, and specifically attached to them. The association of a specific group of insects with a taxon may assist its placement in a subgenus, or even confirm its identity at the species level; for example, each eucalypt species has more or less its own psyllid species.

The status of Eucalyptus as a single genus remains uncertain. There is apparently a good case, based on conventional botanical criteria, for reclassifying Eucalyptus into a number of genera, based largely on the present subgenera (e.g. Pryor and Johnson 1981). While this case is not challenged in scientific terms, there are many who believe that the reclassification could cause an unacceptable level of confusion. This stems from the fact that there are no readily discernible field characteristics which might be reliably used to place a eucalypt within its correct genus. Each of the larger subgenera contain the range of bark forms (complete fibrous, half-barked, smooth barked), and there are no consistent attributes of leaves, buds or fruits which would indicate the genus to a casual though botanically informed observer. Indeed, some of the species which may be most difficult to distinguish on general appearance, could belong to different genera.

It can be argued on this basis that a reclassification might not be in the best interests of the ‘users’ of the eucalypt, and could be particularly problematic in overseas countries. Thus there may be advantages in retaining and formalizing the present subgeneric classification and, indeed, the subgeneric status of the species is now widely used in botanical and ecological literature. Scientists will appreciate the botanical distinctiveness and attributes of eucalypts at the subgeneric level, and the significance this has in ecological terms. Attention is drawn to this in Chapter 2 and elsewhere in this book.

Growth habits of the eucalypt

An understanding of ecological and silvicultural principles within the eucalypt forests must be based on an appreciation of the basic growth processes or ‘growth habits’ of the eucalypt.

Growth Habits of the Eucalypt is the title of the book written by M. R. Jacobs, formerly Principal of the Australian Forestry School and subsequently Director General of Forests within the Commonwealth Government. The work was published in 1955. It was the first comprehensive text on the attributes of the eucalypt and the eucalypt forests, and will remain a primary source of reference on basic growth processes. Jacobs was a critical observer of nature, and had a great facility for descriptive and scientific writing. Much of the account of the growth habits of the eucalypts given here is drawn from Jacobs’s 1955 work, and from his later account of the role and performance of the eucalypt in a world setting (FAO 1979).

This section provides background to the bud systems, leaves, inflorescence and bark of the eucalypts. It also describes the developmental stages in the growth of the eucalypts, their efficient branch-shedding habit, and the growth stresses which can develop within the bole.

The bud systems of the eucalypt

The capacity to grow rapidly whenever environmental conditions are suitable, and to survive and recover rapidly from fire and other damaging agencies is, in large measure, a function of the bud systems found in the eucalypts. Four bud types may be recognized.

Naked buds

A bud on a fairly thin stalk may be observed in the axil of every eucalypt leaf as the leaf unfolds from its parent growing tip. These are called ‘naked buds’. In most cases there is only one naked bud in a leaf axil, but occasionally there are as many as three. The naked buds are inherently capable of rapid development as soon as a parent leaf unfolds. Those situated near the apex of the major branches of the tree tend to develop concurrently with their parent shoot, and to produce new leaves until conditions are unsuitable for growth. The eucalypt crown may develop very rapidly in this way where it is free from attack by leaf-eating insects. In most parts of Australia the growth potential of the tree is reduced by these insects.

Shoots from accessory buds

At the base of the naked bud there is a meristematic region which may or may not be organized into a recognizable growing tip. As long as the naked buds and the leaves of the crown are undisturbed, these meristematic regions are inhibited from developing. Where the leaves and naked buds are destroyed, one or more of the accessory growing tips will appear in some or all of the leaf axils. Should the new shoots be destroyed, further shoots will develop, and the replacement process may be repeated several times in a growing season. The accessory shoots are one of the reasons for the persistence of the trees, despite repeated grazing by insects and other unfavourable factors.

Epiconnic bud strands

When a parent leaf falls, the accessory bud-producing tissue in the leaf axil is not occluded by the diameter growth of the stem on which it lies. A small shaft of tissue with bud-producing properties grows radially outwards from the old leaf axil at a rate which corresponds almost exactly with the growth in diameter of the mother stem. Sometimes two or three of these shafts may develop, with termini at the wood surface or in the live bark. These shafts of bud-producing tissue are capable of producing leafy shoots, but are normally held in check by growth substances produced in the leaves and shoots above them. Should these leaves and shoots be lost through insect attack or fire, this check is removed, and one or several shoots may develop from the shafts. The shafts are called ‘epicormic bud strands’ or ‘dormant bud strands’, and the buds and shoots developing from them ‘epicormic’, ‘proventitious’ or ‘dormant’ buds and shoots.

Lignotubers and related structures

Lignotubers are of great significance in determining the persistence of eucalypts in a rather harsh environment. Most eucalypts develop lignotubers. These structures commence as swellings in the axils of the first few pairs of leaves formed on a seedling. As the seedling ages the swellings in the individual leaf axils fuse and increase in size, forming a bulbous mass given the name ‘lignotuber’.

As the young seedling lignotuber continues to increase in size there is a proliferation of dormant bud strands within the woody mass, originating from the axillary meristems. The storage tissue of the woody mass contains nutrient and starch reserves (Chattaway 1958; Bamber and Mullette 1978). It is these reserves and the dormant buds which facilitate vegetative recovery following damage to the stem.

In most species, lignotubers merge gradually into the main stem after the tree attains the young sapling stage. In other species the lignotuber persists throughout the life of the tree, when it may attain a very large size and give rise to a number of stems with distinctly separated bases. Such eucalypts are known as mallees and characteristically occur over large areas of alkaline soils in semi-arid regions of southern Australia. They also occur within high rainfall zones on the coast, but only on relatively infertile siliceous soils.

There are some eucalypt species which are found in a number of growth forms ranging from tall or short mallees to tall forest or woodland trees. Examples include the dimorphism expressed in E. gummifera on sandstone soils in the high rainfall region near Sydney (Mullette 1978), and the multi-stemmed form of E. botryoides with a large plate-like lignotuber (up to 6 m wide) on infertile siliceous sands, again in the high rainfall coastal zone of south-east Australia (Lacey et al. 1982a; Lacey 1983). In the latter case the lignotuber gives rise to many generations of stems, mainly from the periphery of the plate. As new roots are also formed in association with the formation of the lignotuber, the mallee E. botryoides is an example of vegetative reprodution within the genus.

Some woody perennials in the northern monsoonal zone of the continent, including a number of eucalypts (e.g. E. porrecta, E. ptychocarpa, E. jacobsiana) produce rhizomes (Lacey 1974; Lacey and Whelan 1976; Lacey et al. 1982b). Rhizomes originate from lignotubers of seedlings, and from mature trees when their aerial parts are destroyed by fire or severed mechanically. Under repeated burning, the branching and elongation of rhizomes may result in a network of underground stems. Substantial radial growth of rhizomes, associated with continuing tree development, can result in large underground stems which resemble massive lignotubers.

Where rhizomatous shoots emerge above ground level or are otherwise exposed to light, they undergo morphological changes and become vertical and generally unbranched aerial stems. These stems may form dense clonal patches or thickets. The production of shoots from adventitious buds in roots can also give rise to extensive patches of short stems similar in appearance to those formed from rhizomes. ‘Root suckering’ in this way is common in E. tetrodonta from far northern Australia, and in E. pachycalyx from north Queensland.

The dynamic nature of the annual shoot

Eucalypt buds do not have a resting stage or resting period. Where a tree (from another genus) has a resting bud, it will contain a complete annual shoot in embryonic form; that is, all components of the shoot which will develop in the next growing season are represented by primordia which can be recognized under a microscope.

The development of the annual shoot of a eucalypt is quite a different matter. The number of leaves which can separate from the growing tip is indefinite, and the naked buds can expand simultaneously with the mother shoot. Even the accessory and proventitious buds do not need a resting stage before they can form shoots.

Although the expansion of the crown can proceed in this way for as long as favourable conditions exist, rapid expansion seems to occur in waves or bursts of growth which are not always related to the best growing conditions. Seasonal conditions, the building up of nutrient reserves, flowering, attack by insects on foliage, and factors connected with the root system may all play a part in regulating crown expansion.

The leaves of the eucalypt

The leaves of most species of Eucalyptus change, sometimes markedly, during the development of the seedling into the mature tree. The differences between ‘juvenile’, ‘intermediate’ and ‘adult’ leaves can be important in identifying species.

Juvenile (including ‘seedling’) leaves

During the first year, pairs of leaves develop from the growing tip on opposite sides of the stem, and successive pairs are arranged at right angles to each other, an arrangement known as ‘decussate’. By the time four to six pairs of leaves have developed on a seedling or lignotuberous shoot, they may be spectacularly different from adult leaves. For example, those" of E. globulus are opposite, sessile, highly glaucous, oblong-acuminate in shape, and dorsiventral, while those of the adult are alternate, petiolate, non-glaucous, falcate-lanceolate and isobilateral. It is widely believed that juvenile leaves reproduce ancestral characters of the species. Juvenile leaves may also develop from epicormic bud strands along the bole and branches of a mature tree where it has been damaged by fire or other agency. These are known as ‘reversion shoots’. While most apparent on damaged trees, reversion shoots may be important in enabling a large tree to maintain its crown when the branches grow too long and become mechanically unstable. The ends of the branches die off and reversion shoots develop at positions back along the branch. They are soon replaced by mature foliage.

Interrnediate leaves

Intermediate leaves are frequently larger than juvenile or adult leaves, and many pairs of them may be produced by the growing tip after the juvenile stage, and before the more or less stable adult foliage is produced.

Adult (or mature) leaves

The final or adult form of the eucalypt leaf is usually coriaceous, thick, stiff, highly cutinized and rich in sclerenchyma. In this sense they are typical ‘sclerophyll’ leaves—a term widely used in a general way in describing the Australian eucalypt vegetation. The adult leaves are usually alternate, only in a few species are they opposite or sub-opposite. In general, adult leaves are petiolate, and falcate-lanceolate in shape. They vary, however, according to species, from almost linear, to narrowly lanceolate, to broadly lanceolate, elliptical, oblong, or even oval and orbicular. In the same species, and sometimes on the same tree, there can be an appreciable variation in the shape and dimensions of the leaves.

Leaf arrangement

Leaf arrangement in Eucalyptus follows an interesting pattern. In the adult stage eucalypt leaves are alternate, and might be thought of as having one of the spiral phylotactic arrangements common in many broad-leaved plants. However, the alternate condition in the genus has developed as a modification of the basic phyllotactic arrangement displayed by juvenile leaves in which the placement is opposite and decussate.

The basic leaf arrangement of a tree is brought about by happenings in the growing tip of the shoot. In normal eucalypt shoots, the leaves separate from the growing tips in clearly defined pairs at points which are referred to as leaf nodes. The leaves of each pair are, at this early stage, either opposite or sub-opposite, and the successive pairs are disposed at right angles to each other. This characterizes the basic leaf arrangement of eucalypts.

The disposition of leaves on mature eucalypt shoots varies considerably. In some species and in juvenile leaves there is little change from the arrangement seen in the growing tip, and sessile leaves remain in opposite pairs decussately arranged. The sub-opposite pairs may remain sub-opposite or become alternate at an early stage of development. The alternate pattern is created where twisting takes place in the internode between the leaf pairs, rather than in the internode within the leaf pairs. Where leaves are arranged alternately, there may be an appreciable amount of stem between the leaves of each pair, but there is little movement in this portion, and because of this, leaves of each pair remain on opposite sides of the stem, irrespective of the nature or orientation of the shoot or the length of stem between them.

The effect of this twisting will vary from species to species. Successive internodes usually, but not always, twist in opposite directions, reflecting the way leaves unfold from the growing tip. The most common arrangement of alternate leaves is that where successive twists of the internodes (in opposite directions) have brought the top leaf of one pair in line with the bottom leaf of the next pair, and the spaces between the leaves are alternately long and short. Sometimes, the alternate leaves are evenly spaced on either side of the stem. This is less common. It means that the twisting of the internodes has brought all the top leaves of successive pairs on one side of the stem and all bottom leaves on the other. A number of other patterns are described by Jacobs (1955).

The characteristic ‘hanging leaf habit of the eucalypt may be a result of the twisting of the petiole during leaf development. This habit seems to be connected with the development of the falcate-lanceolate or oblique leaf shapes. Embryonic eucalypt leaves are not falcate-lanceolate or oblique, rather the development of the falcate-lanceolate shape, and an exaggeration of an early tendency to obliqueness, takes place during the rapid, early growth of the leaves after they separate from the growing tip. As the developing leaf loses its symmetry, the petioles are subject to an increasing turning movement, causing, in turn, the leaves to hang following the direction of this movement. The hanging leaves may indicate a higher stage of evolution within those species which have developed the habit, perhaps associated with shedding of the ‘heat load’ and reducing transpiration during the hottest part of the day (Chapter 4).

Leaf longevity

Although the eucalypt is an evergreen tree, the period a leaf remains on the tree before it is shed is highly variable, and generally short. For example, Jacobs (1955) found the average leaf-life of eucalypts on dry sclerophyll sites was no longer than 18 months. Some leaves remained for 2 or 3 years, a few longer, but the average was surprisingly short. The actual life of any one leaf will be affected by species, position in the crown, bursts of growth, insect attack, and flowering.

The tendency of some species to hold leaves longer than others is probably partly a function of growth rate, and partly a genetic character. The sapling crown of a species of average vigour, for example, E. sieben, may contain 3-4 years’ worth of leaves, from tip to base. A slower growing species, E. marginata, may have 4 or more years’ worth of leaves in the crown unit. In marked contrast to this, an exceptionally fast growing species (E. grandis) on a very good site, may produce leaves which perform their function and are shed in a matter of months. Nevertheless, the crown structure of E. grandis will be similar to that of the other species.

Leaf-fall in eucalypts will be associated with a number of normal growth processes. Many leaves will fall from branches competing for leadership of a sapling crown or mature crown unit during the first year, while leaves on lateral branches will generally be more stable. A burst of growth will normally bring with it an increase in leaf-fall of older leaves from the rapidly extending leading shoots. Jacobs (1955) cites the case where an autumnal burst of growth and leaf-fall affected the whole of the tree, leaving it well crowned with leaves less than 6 months old. Accelerated leaf-fall will also occur in parts of the crown where fruits are forming after a heavy flowering.

A large loss of leaves may also be associated with climatic stresses, notably prolonged drought, or more simply, seasonally dry conditions. Indeed, leaf-fall in this way appears to be one of the processes of adaptation of the eucalypt to a dry climate. Where favourable conditions recur, crowns may experience a rapid development through the accessory buds. Similarly, vigorous extension of the crown may follow defoliation by insects. The burst of new growth is likely to be followed by severe leaf-fall among the chewed leaves, and because of this, the average life of leaves in places exposed to severe insect attack may be less than 1 year.

The eucalypt inflorescence

The individual flowers of Eucalyptus are mostly seen as a cluster of flowers in an inflorescence in the 10 axil of a leaf in regular numbers of 3, 7, or 15 or more flowers (Pryor 1954, 1976). It is apparent that the Eucalyptus inflorescence is a cyme, very considerably reduced, but still in most cases with the branching structure characteristic of the regular cymose dichasium.

When the inflorescence is first discernible, it is enveloped by bracts which completely enclose it. With growth and expansion of the developing flower cluster, the bracts are shed and the separate buds then appear. Depending on the particular subgeneric group the time between the very first appearance of the complete inflorescence in the leaf axil, and the shedding of the bracts, may be a matter of a month—or a year or more.

While each inflorescence is characteristically located in a leaf axil, there is, in some groups of species, a compound inflorescence made up of individual inflorescences in a manner more or less characteristic of Eucalyptus. The process by which a ‘eucalyptoid compound inflorescence’ is formed is described by Pryor (1976). Those species which have a compound inflorescence tend to fall into discrete groups and this feature is useful for morphological characterization and diagnosis.

There are a number of other features of the Eucalyptus inflorescence which have been of diagnostic value in classifying species. These are drawn mainly from Pryor (1976).

1. Most eucalypt species have a double operculum, that is, the cap which covers the reproductive organs before anthesis. The four separate petals of the calyx are seen to be united into a single outer cap, and the separate petals of the corolla into a second inner cap. Within a relatively few species the calyx remains composed of separate sepals which appear as teeth at the top of the receptacle (hypanthium), and the single operculum is composed evidently of four fused petals. In still another group there are no sepals to be seen as separate teeth, and there is only one operculum evidently visible (Carr and Carr 1959a, 1959b; Pryor 1976).

2. There are differences between species in anther shape and in the attachment of the filament and anther. Bentham (1867) used these characters in developing his classification of the genus. The generalized anther shape in Eucalyptus is that a pair of anther ‘cells’ are placed closely together with a narrow connective, and dehisce by means of longitudinal slits. A more specialized type is that with dehiscence by terminal pores.

3. The ovary of Eucalyptus characteristically has a number of locules within which the ovules are located before fertilization. There may be three to about ten locules, but four or five is most common. In addition to the ovules there are non-functioning ovules or ‘ovulodes’ within the locules. After the fertilization of the ovules, the ovulodes remain as small structures forming ‘chaff’. The seeds are sometimes morphologically very distinct from the chaff and at other times not readily distinguishable, although they are generally bigger and heavier.

4. There is much variation in Eucalyptus in the size and shape of buds and fruits which, although relatively constant in a given species, are substantially different between many pairs of species. As these organs are usually found on any specimen, and because of the differences between species, much consideration has been given to them in any system of classification.

The bark of the eucalypt

A high proportion of eucalypts have decorticating or deciduous bark resulting in smooth stems, often white or light coloured in appearance. Bark shed in decorticating species is seasonal and often associated with a colour change, as in E. rubida and E. deglupta. A similar habit is found in unrelated species of broad-leaved trees, but seldom are so many species in a single genus found with this feature (Pryor 1976). Most tree genera have persistent, dead outer bark, as do the remaining eucalypts.

The bark of decorticating species may peel off in different ways; for example, it may peel off in long strips (as in E. globulus and E. viminalis), in rather broad plates (as in E. camaldulensis) or in small flakes or scales (as in E. citriodora or E. astringens). It is often difficult to define the colour and surface texture of such barks; while the newly exposed patches may be shiny and of fine texture with comparatively bright and varied colouring, the old patches, ready to fall, are comparatively dull grey and less smooth.

Where species have persistent bark, that bark may be placed in one of four broad categories:

1. The ‘ironbark’ type is hard, with extremely short fibres, or is non-fibrous. It has deep longitudinal furrows, and breaks up into very small polyhedrons of hard, corky texture when crumbled. It is usually dark in colour, and sometimes contains inclusions of small masses of gum (kino) rich in tannin. Examples include E. paniculata and E. crebra.

2. The ‘box-type’ bark is shortly fibrous, pale grey and finely furrowed or reticulated obliquely on the surface (e.g. E. moluccana).

3. The ‘long-fibred’ or ‘stringybark-type’ is usually more or less dark brown, with long or very long fibres, and is deeply furrowed longitudinally. Where the outer layers have peeled off, the long, fibrous, often laminated texture is revealed. This category includes species such as E. robusta and E. botryoides, and the true ‘stringybarks’ (e.g. E. obliqua).

4. The ‘bloodwood’ -type bark may be dull grey to black, hard, with shallow, irregular furrowing chiefly in two directions, creating an effect of scales, more or less oblong in shape. Eucalyptus gummifera is a typical example. The basal bark of the tropical species E. tesselaris has a particularly distinctive ‘tesselated’ bark.

Developmental stages: seedling to mature tree

The eucalypt may develop through a number of ‘growth’ or ‘developmental stages’. It is usual to recognize the ‘juvenile’, ‘sapling’, ‘mature’ and ‘overmature senescent’ stages, and their main characteristics are described below. However, not all trees in the forest are readily classified in this way (Chapter 8). At any developmental stage, a tree may be affected by competition to the extent that it enters a ‘growth-restricted’ condition. Where it remains in that condition for some time, resumption of growth through the normal sequence of development may not be possible.

Juvenile stage

A fast growing, non-lignotuberous eucalypt will produce a single mainstem from the time it germinates. Alternatively, a large number of forest eucalypts have an early juvenile stage where several shoots with a plagiotropic habit develop, that is, they dispose themselves horizontally on the ground, or at an obtuse angle to the vertical. After a period which may be months or years, the plant strengthens and one shoot assumes an erect habit.

Sapling stage

Once vigorous height growth of a mainstem has continued for 3-4 years, the young tree enters the ‘sapling stage’. The stage is characterized by a crown of small branches, all of which should be shed as the tree gains height. The boundary between the juvenile and sapling forms may be said to be the stage at which the branches start to be shed from the base of the crown, and a clear bole begins to form.

The living part of the sapling crown usually consists of three or four annual extensions. Where a crown is growing vigorously in height (e.g. 1 m/year), its shape may be long and pointed. A weaker crown growing, say, 0.3 m in height will have a more rounded appearance.

Within the region of rapid vertical extension of the crown, there is not a well-defined leading shoot. Several shoots vie for dominance and any small event may decide which will become the mainstem; the branches which vie for dominance are called ‘competing branches’. The persistent and semi-persistent branches which form the main crown skeleton of later stages develop from them.

Pole stage

After it has gained a certain height which varies with the quality of the site, a young eucalypt enters the pole stage. The tree now has a strongly developed mainstem and a crown outline like that of a sapling, but the larger, lower branches are no longer quickly and cleanly shed.

The semi-permanent lower branches of a pole crown come from competing branches of an earlier sapling stage. They push upwards and, in some ways, are like modified saplings growing outwards from the mainstem. They form leaf-bearing units which give the crowns of eucalypts their characteristic appearance, the mature crowns being made up of a number of such units arising from the mainstem or main branches.

The mature stage

Where height growth is nearly complete and the crown begins to expand more in a lateral direction, the tree may be said to be entering the mature stage. It is at this point that the eucalypt loses its pole form and develops large, persistent branches. The difference between the pole and the mature tree is that in the pole, the semi-permanent branch units grow from the mainstem. In the mature tree they grow also from the large, persistent branches forming the framework of the crown. These persistent branches may be called the ‘shaping branches’ because they decide the outline of the crown.

The eucalypt may remain in the mature stage for decades, even a century or more. What is happening to the leaf-bearing units of the crown during this time? Eucalypt leaves do not have a long life. A branch of the ‘primary crown’, that is, one which has developed directly from a naked or accessory bud in a leaf axil, must always push outwards to retain a tuft of leaves at its end. As it pushes outwards it is weighed downwards. There is a limit to the distance from the trunk it can grow as a primary branch. It may grow outwards 6-9 m, but somewhere about this distance the leafy portion at the end becomes inefficient, and the leaves and axillary buds no longer grow vigorously. Epicormic shoots then develop from dormant buds on the top and sides of the branch, and develop into the leaf-bearing units of the mature crown. The epicormic unit nearest the end of the branch usually continues the outward extension of the branch. The epicormic leaf-bearing units (the ‘secondary crown’) contribute to the diameter growth of the parent branch and it becomes stiffer and more stable than it was under the influence of its own primary leafy shoots. This process may be repeated several times during the formation of a large shaping branch in a fully mature eucalypt crown. The enlargement and stiffening of the branch, and its extension to a distance of perhaps 12-15 m from the trunk, may be the work of epicormic units developing from the dormant buds.

During its period in the mature stage, the height and spread of the crown may change very little. Both may fluctuate as the extremities die and are replaced by new crown units. The mature crown of a eucalypt maintains itself by the continual production of new crown units, which die in turn. Thus there will always be some dead branchlets in a healthy mature crown. An undue proportion of dead branches or ‘stagheadedness’ is an unhealthy sign, but the death of a reasonable proportion of the crown units should be accepted as normal.

In the formation of woodland trees, the leaf-bearing units of the crowns of mature eucalypts are not usually as clearly defined as they are in forest trees. One reason may be the greater frequency of insect attack, with consequent recovery by the crowns by the development of large numbers of small epicormic branches. If an open crown eucalypt is not attacked by insects, units similar to those seen in forest trees develop after a few years.

The overmature stage

The patching up of a mature eucalypt crown by the development of dormant buds from the shaping branches may go on for a long period in the life span of the tree. All this time fungal attack is weakening the inside of trunk and branches alike. The shaping branches are usually the first to fail because their horizontal position makes them more liable to break. They break from the trunk and their place is taken by branches which develop from dormant buds on the trunk. These new branches are never as efficient as the branches of the primary crown. They may live for a few years, or even a decade or two, break and be replaced. This process may be repeated several times as the tree becomes old and decrepit.

Bole and wood quality

The well-grown eucalypt on a good quality site can produce a long, clear (branch-free) bole as it develops through the sapling and pole stages to maturity, and this is of considerable advantage in harvesting and processing the wood. However, because growth stresses develop within the wood, there may be difficulties in sawing tree boles, particularly where they have been grown quickly or are small.

Bole quality

An efficient branch-shedding mechanism contributes to the development of the eucalypt’s clear bole. As the crown of the young tree grows upwards, the lower branches become moribund and die. At a later stage they become brittle and break off, and the stubs are then occluded by the diameter growth of the tree. Successful occlusion of the branch stubs can take place only if the stubs do not become centres of fungal infection. Protection from infection is usually obtained during the moribund period when a protective layer develops in that part of the branch already included in the trunk. Substances such as tannins, latex or resins may be deposited in this layer, or the cells may become blocked with tyloses.

Jacobs (1955) recognized three phases in the shedding of eucalypt branches. The first phase is the development of a brittle zone near the base of the branch. In the second phase the main part of the branch breaks away from the trunk by a fracture across the outer part of this brittle zone. The third phase is the ejection of part of the remaining branch stub after a further fracture of the stub across the lower part of the brittle zone. This phase is a very favourable growth habit of eucalypts, as under suitable conditions it removes the branch down to the solid wood of the trunk. The efficient branch shed mechanism functions for nearly all branches up to 1-2 cm in diameter. In some species branches up to 3 cm may be shed efficiently, but beyond this are not shed effectively.

Growth stresses

Despite the clear, straight bole, many eucalypt species will not make good sawlogs until they have reached a relatively large diameter. The reasons for this lie in the cellular structure and orientation of fibres of the different species, and in the strain gradients which develop in a eucalypt bole.

The outer section of any green eucalypt bole is in a state of tension along its longitudinal axis, and the inner wood is in a corresponding state of compression. The forces involved in the longitudinal strain gradient across any diameter are considerable, with a number of consequences:

1. The cells of the inner wood may fail in the course of time and develop large numbers of small compression failures in the cell walls. The phenomenon is known as ‘brittle heart’; an allowance is usually made for this in determining the price payable for eucalypt logs.

2. Where the eucalypt stem is cut lengthwise, the release of inner compression can cause curvature in the outer pieces. Where these are straightened out artificially, the release of tension in the outer pieces causes a decrease in length, whereas the release of compression in the inner pieces causes an increase in length. Where large eucalypt logs are sawn lengthwise, the large radius of curvature of pieces cut from them is a nuisance, but not a serious one; it becomes more serious where end splitting of sawn pieces occurs as growth stresses are released. The conversion of smaller eucalypt logs to sawn boards presents a bigger problem and may have to be restricted to short lengths.

3. Where the trees are harvested for poles, the effect of longitudinal forces depends upon the wood structure and arrangement of fibres if the different species. Species which have straight fibres are likely to split at the ends of the poles, particularly where they have been grown quickly; species with an interlocked grain may split infrequently and may be preferable for the pole industry.

In addition to the longitudinal forces in a eucalypt bole, there are lateral stresses and strains which manifest themselves on the cross-section. For example, a tangential compression imposes a radial tension on the inner wood resulting in the star-shaped heart shakes which radiate out from the pith and which are a common sight on eucalypt logs when they are cross-cut.

CHAPTER

2

Species and community patterns

Introdution

There may be as many as 1000 eucalypt species and forms contributing to the vegetative cover of all but the driest parts of Australia. In seeking to understand the nature of and to develop management strategies for the forests and woodlands, it is pertinent to ask at the outset why Eucalyptus dominates the forests and woodlands from the wet, cool temperate areas of southern Australia, to the dry regions of the monsoonal tropics.

It is also pertinent to ask why there are so many species. It is logical enough to find different suites of species within the distinctive geographic zones of the continent. However, it is less clear why there can be as many as ten eucalypt species within a few hectares of forest, all competing for site occupancy, sometimes in single species stands, but more commonly in stands with mixtures of two or more species.

In developing a silvicultural policy for a forest, it will be useful to ask a number of questions about the nature of the species pattern in the forest. Is there a consistent pattern in the way species occur within the forest, or is this largely due to chance? for example, through the interposition of historic events such as fire. If there is a consistent pattern in the distribution and association of species, to what environmental factors are the species responding? And finally, what bearing will this have on silvicultural practice where an objective of management is to maintain stable and healthy forest ecosystems?

There has been, and undoubtedly always will be argument about the constraints the ‘ecological factor’ should place on silvicultural practice. It is the nicely expressed philosophy of Lutz (1963) which underpins the approach to silvicultural practice in this book. Emphasis on the ecological factor does not imply advocacy of any ‘naturalistic doctrine’ of forest management. Rather, between the two extremes (‘any disturbance will court disaster,’ and ‘natural tendencies can be safely ignored’), there is a wide opportunity for applying the basic philosophy of working in harmony with nature. This means, in effect, that silvicultural practice must be developed on a firm ecological base, requiring an appreciation of the ways in which environmental factors influence the distributions of species, and the ways in which biological processes may help maintain the stability and productivity of the forest communities. This present chapter is concerned with ‘pattern’ in the Australian forests, and Chapter 3 with the ‘processes’ of the forests.

Evolution of the Australian flora

In examining the nature of pattern in the eucalypt forests it is useful to start by exploring historical influences on the Australian vegetation in order to establish the environmental changes and selection pressures which may have moulded the present-day vegetation.

Theories of the development of the present Australian vegetation have long referred to the presence of elements with distinctive origins (Crocker and Wood 1947; Burbidge 1960). Botanists have traditionally identified (1) an Indo-Melanesian element making up much of the rainforest of semi-tropical Australia, and an important component of the flora of arid and semi-arid regions, (2) an Antarctic element, for example, the Nothofagus forests of southern Australia which have affinities with the floras of South America, New Zealand, and to a lesser extent South Africa, and (3) a distinctively Australian element reaching maximum development in southern Australia. However, there has been disagreement about whether all elements do in fact have distinct origins, or whether the differentiation of these elements has resulted from a very sensitive sifting of a single flora by ecological factors.

Concept and consequences of continental drift

In recent years it seems to have become accepted that the nature of the Australian vegetation is best interpreted in terms of continental drift; that is, the consequences of the separation of the Australian continent from Gondwana some 60 million years ago (see relevant chapters in Keast 1981). At that time the Australian continent was humid and cool, and largely covered by cool temperate rainforest dominated by gymnosperms, notably the conifers Podocarpus, Dacrydium and Araucaria.

During the early stages of continental drift there was a progressive change in climate; temperatures increased and rainfall became more seasonal—sufficient to create widespread vegetational instability. Massive erosion reduced the continent to a vast plain, and when this was nearly complete, laterization of the soil started to take place over much of the continent. By 20 million years ago, deeply weathered landscapes and soils were widespread throughout the continent.

Around this time there was a more fundamental change in climate, leading to increasing drying of the continent and continuing retraction of the rainforest. Eucalyptus now becomes more common in the pollen record together with cypress pine (Callitris), Casuarina, Allocasuarina and Acacia. The Kosciusko uplift and extensive basaltic flows along the eastern seaboard some 10-15 million years ago would have reversed the general trend of decline in soils and vegetation within this region. There would have been a wide range of new environments such as fertile basaltic soils, newly exposed parent materials and higher rainfall, and opportunities for vegetational expansion rather than contraction. This undoubtedly contributed to the conservation of some elements of the Australian flora, notably the rainforests, and the evolutionary expansion of others.

Despite these events, the gradual deterioration in climate continued over much of the continent. The replacement of moist, cool Nothofagus-gymnosperm forest by sclerophyll forests, woodlands and deserts was largely complete a million years ago. Moreover, there has been no specific end-point to climatic change—the Australian climate has been far from stable in the last 100000 years. A series of glacial and interglacial phases has caused continuing and widespread instability in the vegetation, soils and landscapes, with continuing selection pressures on the Australian flora (Crocker 1959; Butler 1967; Costin 1971).

Against this background it is possible to conceive the progenitor(s) of Eucalyptus as having outstanding evolutionary capacity to keep pace with environmental change. Thus Eucalyptus may have been a response primarily to declining soil fertility associated with eroding but ultimately stable landscapes, and the processes of weathering and laterization which followed. In most discussions of the evolution of the Australian flora, emphasis has been invariably placed on the selection pressures of changing climate. However, as long ago as 1913, Andrews suggested that Eucalyptus was primarily a response by ancestral Myrtaceous stock to poor soils and, secondarily, after a long interval, to a drying climate. It is in this way that we might now attribute much of the diversity in the Australian vegetation to the physiological responses of species to sites varying in the availability of nutrients and water, and other ecological attributes.

Adaptation of the eucalypt to low nutrient soils

Beadle (1966) has argued that the Australian sclerophyll flora represents an adaptation to low nutrients rather than to a dry climate, and that rainforest genera are more effectively excluded from an area by low nutrients than by a dry climate. If this is a valid hypothesis it should be possible to identify in xeromorphic and sclerophyllous vegetation, characteristics which enhance survival and growth on nutrient-deficient soils, and particularly, phosphorus-deficient soils. Specht and Groves (1966) suggest adaptive mechanisms of coastal heath vegetation to highly deficient sites. Within Eucalyptus, it is also necessary to seek those attributes which permit the development of a large-boled, long-lived tree on sites which are only marginally more fertile.

The lignotuberous habit of the eucalypt seedling may play many roles in its adaptation to a site; for example, the development of a relatively strong root system during a prolonged lignotuber phase could be important in maintaining nutrient supply during a subsequent dynamic growth phase. Again, while there is little direct evidence to confirm such an hypothesis, the eucalypt root system may gain access to phosphorus in forms in the soil not readily available to many plants. This may happen through phosphatase enzymes on roots, or the release of organic exudates able to react with aluminium or iron compounds, and release phosphorus within the rhizosphere. There are many examples in Australia where eucalypt forests growing on lateritic or other highly weathered soils have been replaced by Pinus plantations-but the pine has failed to grow or required large inputs of phosphate fertilizer for growth (Kessel and Stoate 1938; Gentle and Humphreys 1968). Another possible process by which the eucalypt gains access to phosphorus involves the complexing and dissolving of aluminium and iron compounds by organic constituents of the litter. This attribute may be most highly developed in eucalypt species occurring on more infertile soils (Hingston 1963; Ellis 1971a; Enright 1978).

A capacity to conserve and utilize nutrients efficiently within the biomass seems to be well developed within the eucalypt. A high proportion of foliar phosphorus is withdrawn from leaves which are about to fall as litter; for example, 70% of the phosphorus in the case of E. ohliqua (Attiwill et al. 1978). Moreover, soil nutrients taken up following a fire may be stored in the sapwood and phloem, and drawn on by the tree to maintain a stimulated level of growth over a period of years (Banks 1982). Some 70% of this phosphorus may be withdrawn from sapwood cells before they are converted to heartwood, accounting for 46% of the phosphorus requirement for net growth of E. ohliqua (Attiwill 1980). The proportion of sapwood nutrient which is withdrawn in this way is greater than that in Pinus radiata (Banks 1982) and rainforest species (Lambert et al. 1983). This is illustrated for E. pauciflora and P radiata in Figure 2.1. In this way much of the biomass nutrient is maintained in the eucalypt’s mobile sapwood pool. As the band of sapwood in the eucalypt represents only the past 7-10 years of growth, there will be in large eucalypt boles only a ‘thin shell’ of biologically active tissue, but this is able to maintain dynamic crown processes well into overmature and senescent growth stages. The mobilization of nutrients within a relatively small part of the tree biomass may explain why Attiwill (1972) found only 0.09 kg of phosphorus/t of dry matter within the biomass of a good quality E. ohliqua forest, compared with an average of 0.29 kg/t in forests of comparable productivity in North America, Africa and New Zealand. This must be a powerful factor contributing to the development of long-lived, large-boled eucalypt trees on poor sites.

It should not be concluded from this, however, that the ‘nutrient cost’ of growing any eucalypt forest is invariably low. Where a eucalypt plantation is grown and harvested on a short rotation regime (e.g. 10-1.5 years), up to half of the wood may be present as sapwood, and the amount of nutrient removed from the site may be similar to that of other plantation species producing similar wood volumes (Chapter 14). Moreover, not all eucalypts are uniformly adapted to, or compete equally well on, low nutrient soils. The vegetational gradient from dry sclerophyll forest through wet sclerophyll forest to rainforest is now seen to be largely associated with a gradient in soil fertility. The delimitation of eucalypt species along this gradient will reflect, in part, differences in the capacity of species to respond to increasing availability of nutrients and, perhaps, an increasing nutritional cost of biomass production.

Figure 2.1

The pattern of phosphorus concentration within tree rings of 60-year-old Pinus radiata and Eucalyptus pauciflora trees (from Banks 1982).

Adaptation of the eucalypt to a drying climate

As the climate began to deteriorate more dramatically in the mid- to late-Tertiary, the eucalypt would have been subject to a powerful new selection pressure, increasing drought. Today, eucalypt species are associated with a very wide range of environments, from the most favourable of climatic and soil conditions, to the shrublands and desert formations of the interior. Despite this range, limited physiological evidence points to what may be a very significant characteristic of the eucalypt, that is, no eucalypt can be regarded as a true ‘drought evader’—a plant which minimizes water stress in its leaves by closing stomata early and reducing transpiration and metabolism. Rather, the eucalypt might be generally characterized as being a ‘drought-tolerant mesophyte’ (Florence 1981); that is, it tends to maintain transpiration and cell metabolism under conditions of developing drought.

During the mid- to late-Tertiary we might think of the eucalypt progressively evolving characteristics enhancing its tolerance of dry environments. Thus, present day eucalypts may be placed in a number of categories denoting distinctly different drought tolerances, and reflecting the water-use attributes and growth rates of the species. These categories are:

1. coastal zone species with high rates of growth and water use, but with a limited capacity to regulate transpiration and tolerate drought stress (e.g. E. regnans);

2. coastal zone species with high rates of growth and water use, but with somewhat wider environmental tolerance (e.g. E. grandis, E. globulus);

3. more specialized dry region species which, again, are capable of rapid early growth, but have at the same time, a considerable capacity to regulate water use and tolerate more prolonged periods of drought (e.g. E. camaldulensis, E. tereticomis);

4. slow-growing species of the coastal zones which occur where the water status of a site would be limiting for species in Categories 1 and 2 (e.g. E. radiata, E. acmenoides);

5. slow-growing species of the woodland zones which are able to tolerate extensive periods without rain (e.g. E. melliodora, E. microtheca).

A more complete