Identifying Eucalypts
Words: Dr Jugo Ilic
Identification of timber species is carried out by examining samples using prior knowledge and experience, and also by viewing samples at low and high magnification and comparing them with known samples usually from a reference wood collection. In Australia the best known such collection with samples from some 13,000 species, the Australian Wood Collection, was described in AWR#50.
There are many sources of information on species identification, but among the best practical publications is Bruce Hoadley’s book Identifying Wood. This book describes a simplified approach for identification of hardwoods and softwoods, although it deals principally with woods from the USA, the northern temperate region and some tropical species as well. However, very little information is available for the many Australian species and notably the eucalypts.
It is often said that different ‘species’ of timbers exhibit a unique ‘finger-print-like’ structure, and by examining the structure it is usually relatively easy to place an unknown correctly into a genus. Often this is quite satisfactory: for example, an unknown might be oak (Quercus sp.), teak (Tectona sp.), pine (Pinus sp.), elm (Ulmus sp.) or spruce (Picea sp.). But, considering there are over 700 species of eucalypts in Australia, no one is satisfied when a particular sample is just called a eucalypt. The reaction is ‘What eucalypt’? To answer that question we need to know how species are determined in the first place.
The differentiation of species is based on the botanical characteristics of trees and not the structure of the wood – it is to do with the type, shape and size of the leaves, fruits, flowers, bark and, more recently, plant DNA. Unfortunately botanical variations between species are not always mirrored in the structure of the wood. This is because many timber species have numerous structural features in common, especially those from the same genus where hybridization may occur. So even for the botanist there is difficulty in classification. All these factors make the correct identification of the timber very difficult and at times impossible.
A lot of work was carried out by CSIRO researchers to develop wood identification methods by carefully describing the wood structure and characteristics of eucalypts, but no simple solution has been found. To this end, the aim of this article is to indicate what can be accomplished and what some of the limitations are for identification. The macro method is within the reach of most people interested in timber identification; other methods including microscopy are for use only in the well-equipped wood technology laboratory.
Fortunately, macroscopic timber characteristics are useful for grouping different timbers of the eucalypts. It has been found that to some degree timber groupings based on wood structure follow botanical groupings and also, to a lesser extent, groupings based on bark characteristics. The earliest classification of eucalypts introduced by European settlers developed from the varying bark appearances and has persisted. Baron von Mueller formalised six bark groupings for eucalypts:
Bark groupings of eucalypts
1. Gums: The largest group of all, giving rise to the common name for trees of the genus gums; the bark is smooth on trunk and limbs except for varying amounts of rough bark at the butt.
2. Stringybarks: Outer bark thick, brownish, fibrous and stringy, persistent on trunk and larger branches.
3. Ironbarks: Bark thick, hard, brittle and deeply furrowed, persistent on trunk and larger branches.
4. Boxes: Rough bark, sub-fibrous and interlaced extending over varying length of the trunk and branches, with usually smooth upper branches.
5. Peppermints: Bark sub-fibrous, somewhat resembling the box-bark, but more stringy and furrowed; the upper branches and sometimes a portion of the trunk are usually smooth.
6. Bloodwoods: The bark is rough, firm, reddish, friable, scaly and flakier than the other groups. Nowadays this group has been reclassified to the genus Corymbia.
These groupings were found to be useful but were modified by R. T. Baker in 1915, in his well-known publication The Hardwoods of Australia and their Economics. He classified eucalypts according to the six bark groups and added the following groups which included some wood types as well: woolly-butts, blackbutts, mahoganies, tallowwoods and ashes. Unfortunately the distinction between some of these groups is neither clear nor consistent for the bark types and wood types. Currently, only six fairly distinct groups are actually useful—ashes, stringybarks, ironbarks, boxes, gums and the bloodwoods (Corymbia). Even among these groups there is a lot of variation and similarity, for example, between ashes and stringybarks, ironbarks and boxes; the gums span them all. Only the bloodwoods (Corymbia) are distinctly different.
General appearance
Useful characteristics for indentification include general appearance, colour, weight and the results of the burning splinter test. Other than variations in colour and texture, eucalypts are very similar in general appearance. One particular feature, however, is the distinct greasiness or oiliness of tallowwood (E. microcoreys) and, to a less marked degree, of spotted gum (Corymbia maculata) and southern blue gum (E. globulus). The fineness of the texture of some boxes (red box, E. polyanthemos) is also an important feature associated with vessel (pore) size.
Colour
It is important to note if the sample is sapwood or heartwood. In timbers such as mountain ash (E. regnans) there is little difference between the colour of sapwood and heartwood. In other timbers the difference is very marked—the sapwood of redgum (E. camaldulensis) varies from straw to cream coloured, assuming it is not sap-stained. Colour descriptions should be based on heartwood, or in the case of sapwood trees such as white cheesewood (Alstonia scholaris), from the colour of the mature wood.
Tree age has an effect—wood from saplings is generally paler than timber from older trees of the same species. Surface condition or weathering will have an effect. True colour is revealed by making a fresh cut and exposing an underlying surface of the wood. Two main groups are common—firstly, those which are definitely coloured, being red as in Sydney blue gum (Eucalyptus saligna), red-brown as in red ironbark (E. sideroxylon) or brown as in spotted gum (Corymbia maculata) to chocolate-brown as in carbeen (C. tessellaris) and gimlet (Eucalyptus salubris).
Secondly, there are those which are not so definitely coloured, being pale as in mountain ash (E. regnans) to light-brown (E. globulus). Some of the pale coloured timbers often show pink tints as in manna gum (E. viminalis), especially when freshly sawn, where the colour of the material is determined from a dry longitudinal surface. Small specimens of green timber freshly cut from the tree can always be dried quickly by exposure in the sun or by placing near a heat source.
Weight
The air-dry density of various eucalypts differs considerably, the lightest being the ash group, e.g. E. regnans. At the extreme other end of the scale many of the ironbarks have density well over 1000kgm–3 (see AWR#74). The most commonly encountered dense eucalypts are the ironbarks and some of the boxes, grey ironbark and grey box being two well-known examples. Thus, on the basis of weight alone, there is little difficulty in distinguishing between, say, an ironbark and mountain ash. However, density is of no assistance in distinguishing closely related species, for example, members of the stringybark group.
It should be noted that differences in density are only useful if samples are air-dried; all eucalypt timbers when green are heavier than water, i.e. >1000kgm–3. Air-dry weight can also be influenced by collapse which, when severe, makes the timber harder and heavier. For example, the average air-dry density of mountain ash (E. regnans) before reconditioning is approximately 680kgm–3 compared with the average figure of 630kgm–3 for reconditioned material.
Burning splinter test
This test is useful for differentiating some closely related species. It can only be performed on wood from sound and dry heartwood. Sapwood or wood that has been subjected to weathering or decay gives misleading results. Match-sized samples are cut from the heartwood and burned in an area without draughts. Splinters from some timbers will burn to a definite ash; others will burn to a charcoal or to a fine thread of ash which slowly drifts away.
Only in those cases where a definite and comparatively complete ash is obtained is the result designated as a ‘full ash’. Very few timbers burn in this way, most notable are two red-coloured eucalypts that are otherwise indistinguishable. Karri (E. diversicolor) gives a full white ash and jarrah (E. marginata) produces a charcoal. Grey ironbark is another timber which burns to a definite ash, often buff in colour.
Anatomical characteristics of wood
Wood is not a simple homogeneous material but is made up of individual cells which may occur as isolated units among other cells or as a tissue composed of many similar units. These tissues form the vertical and radial conducting systems, the storage system and the ground mass which gives the tree trunk its strength to stay erect and provides the constructional material we call timber.
The main elements of which wood is composed are vessels (pores)—from the cross-section, circular or oval holes which may be empty or filled with deposits or tyloses. They are the cut ends of the vessels through which the nutrient solutions which form the food of the tree are carried vertically up the tree trunk. Wood rays are strips of tissue extending from the outer layers of the tree towards the pith. They conduct manufactured food materials from the periphery of the stem inwards and also store food. They are alive throughout the sapwood. On the cross surfaces they often appear as straight lines differing in shade from the background.
They may change direction slightly in their course through the wood and may vary considerably in width in different timbers, and sometimes even in different samples of the same timber. In eucalypts they are always narrow. On a radial (quartersawn) surface of the wood they appear as ribbons of horizontally running tissue which may differ considerably in colour from the rest of the wood. Soft tissue (parenchyma) usually appears somewhat lighter in colour than the rest of the wood, but may occasionally contain much resinous matter which makes it darker than the background. In most eucalypts the parenchyma can be seen to surround the vessels or as scattered cells.
Identifying from cross-sectional characteristics
The fibres or details of individual cells of which these tissues are composed can be seen when sections of wood are examined under a microscope. When an unknown timber has come to hand the first thing to do is to note all the things about it which can be seen with the naked eye and by feel, together with such details as where the tree grew.
The next thing to do is to prepare a clean cut cross surface, using a sharp knife or a razor blade. Radial and tangential (backsawn) surfaces can be split for further observation to show characteristics usually reserved for microscopy. This may be useful at this level only for non-eucalypt species. While physical features are seldom sufficient in themselves, they are useful. Colour is also useful but may vary considerably depending on the surface examined—end surfaces may appear different in colour from longitudinal surfaces.
Observation of a cleanly cut cross-section with a 10x hand lens will indicate quickly whether the unknown eucalypt timber belongs to the bloodwood group (Corymbia), which is characterised by radial arrangement of vessels and abundant parenchyma sometimes arranged in concentric bands. ‘Bloodwood’ refers specifically to the frequent kino exudations from the bark and in certain cases to the rather commonly occurring kino veins in the wood. Common bloodwoods are spotted gum, red bloodwood (C. gummifera), white bloodwood (C. trachyphloia), carbeen (C. tessellaris), yellow bloodwood (C. eximia), marri (C. calophylla) and even the well-known red-flowering gum (C. ficifolia). The structure of these woods is practically identical with that of the woods of the genus hard apple (Angophora).
In other eucalypt groups the vessels are solitary, that is, not in any radial or tangential rows, although at times some tendency to oblique arrangement is usually evident. Thus, in addition to the separation of the bloodwood group on vessel arrangement, the variation in size and number of vessels may be useful.
In mountain ash (E. regnans) and other species of the ash group, the vessels are large, often clearly visible to the naked eye and not particularly numerous; on the other hand, in the case of timbers of the box and ironbark groups the vessels are not visible to the naked eye and are more numerous. The parenchyma (soft tissue) arrangement should also be examined on the cleanly cut cross-section.
Here again bloodwoods are the most distinctive with parenchyma that are often arranged in bands spreading tangentially from the pores. In the eucalypt groups the parenchyma is either surrounding the vessels or diffuse or both. The amount of parenchyma tissue varies from species to species and within species. Generally speaking, however, it may be said to be quite sparse and paratracheal, surrounding the vessels only in the timbers of the stringybark group; quite abundant, paratracheal and diffuse in timbers of the gum group such as river redgum (E. camaldulensis); and present in jarrah (E. marginata) but not very marked. Thus all close relatives of river redgum have the same type of parenchyma.
To some degree, therefore, it is comparatively easy to place a eucalypt timber in a group, but to go past the grouping on the basis of macroscopic examination is impossible and at times this is still impossible after examination of structural details by means of the microscope.
Dr Jugo Ilic worked as a wood scientist at CSIRO for 36 years. As part of his research activities he authored the CSIRO Atlas of Hardwoods and curated the Australian Wood Collection. He has written several articles for Australian Wood Review magazine.