Growth Habits of Eucalypts

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Based on Jacobs, MR (1955).  Growth Habits of the Eucalypts  as summarised by Florence RG, (1996). Ecology and Silviculture of Eucalypt Forests.   I have added comments in the text related to my years of observing growth in eucalypts, particularly in northern NSW.

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 eucalypts.  This document is a summary of the description provided by Jacobs on the bud system and the leaves of eucalypts and is presented to give you an understanding of the potential responses of trees to silvicultural manipulations and changing environmental conditions. 

The bud systems of eucalypts

The capacity to grow rapidly whenever environmental conditions are suitable, and to survive and recover rapidly from fire and other damaging agencies is largely due to the functioning of the bud systems found in the eucalypts. Four bud types may be recognised.

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 attach by leaf-eating insects or abrasion from neighbouring trees. In most parts of Australia the growth potential of trees is reduced by insect damage to the foliage.

Shoots from accessory buds

At the base of the naked bud there is a meristematic region which may or may not be organised into a recognisable growing tip. As long as naked buds and leaves of the crown are undisturbed, these meristematic regions are inhibited from developing. Where 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.

Epicormic  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 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 cotyledons and 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. It is these reserves together with the dormant buds that 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, where 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 and scarp, but only on relatively infertile siliceous soils and harsh exposed sites.

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 (red bloodwood) on sandstone soils in the high rainfall region near Sydney, and the multi-stemmed form of E. botryoides (bangaly or southern mahogany) 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. 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 form of  E. botryoides is an example of vegetative reproduction 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. 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. tetradonta (Darwin stringybark) from far northern Australia, and in E. pachycalyx (shiny bark gum) from north Queensland.

The dynamic nature of the annual shoot

Eucalypt buds do not have a resting stage or resting period (i.e. the buds are classified as having indeterminate growth). Where a tree (from another genus) has a resting bud, it will contain a complete annual shoot in embryonic form (i.e. the buds are classified as having determinate growth). Consequently, all components of the shoot which will develop in the next growing season are represented in the tissue of the bud that rests over winter.

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 generally related to favourable growing conditions. Seasonal conditions, the building up of nutrient reserves, flowering, attack by insects on foliage, and factors connected with the root system may also play a part in regulating crown expansion.

The leaves of the eucalypt

The leaves of most species of eucalypts 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' trees)

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 know 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 (Tasmanian  or southern blue gum) 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 damaging agencies.  While most apparent on damaged trees, these epicormic 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 epicormic shoots develop at positions back along the branch. They are soon replaced by intermediate and then by mature foliage.

Intermediate 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 speices are they opposite or sub-opposite. In general, adult leave 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 is eucalypts 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 characterises 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.

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.

Leaf longevity

Although eucalypts are evergreen trees, 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.  Eucalypt leaves on saplings and mature trees on the New England Tablelands average about 12 to18 months in age.  A significant proportion of the mature leaves are shed during spring flush the following year while some last until the end of that growing season.  Leaves severely damaged by insect attack generally have a much shorter life span and are shed much earlier.

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, (e.g. E. sieberi - silvertop ash) may contain 3-4 years worth of leaves in the crown unit. In marked contrast to this, an exceptionally fast growing species (E. grandis - flooded gum) 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. This is commonly seen in the characteristic burst of spring growth in temperate climates or in the burst of growth at the start of the wet season in tropical and sub-tropical climates. 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. This process is seen in E. populnea (poplar of bimble box) growing in north-western NSW where there is a distinct thinning of the crowns with prolonged dry conditions. When 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 attach may be less than 1 year.  Trees affected by New England dieback (commonly associated with insect defoliation and leaf damage related to increasing intensity of agricultural land use) exhibit the development of new secondary crowns from accessory and epicormic buds in the branches of the crown followed by the shedding of damaged leaves.

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