Bark Codes; or Barking Up the Right Tree

The oozing of sap from the trunk is not the first - or probably 10th - thing I'd think of when asked to characterise eucalypts, but early Europeans certainly did. Abel Tasman back in 1642 in van Diemen's Land (later to be renamed Tasmania for him) was intrigued by it, and collected samples. This may have been a hint as to the nature of their interest; he probably had hopes of economic applications. Forty-six years later the English pirate-naturalist William Dampier reported that "the Gum distils out of the knots or cracks that are in the bodies of  the trees". Governor Arthur Phillip, who commanded the first British colony on what is now Sydney Harbour, first used the term 'gum-tree' in 1770; he too collected this gum, and send samples back home.

Now, most people probably use Phillip's term for all eucalypts (ie in excess of 700 of them) but in an attempt to make such a huge number of species manageable, and in part because such characteristics do largely reflect relationships within the genus, we divide eucalypts up according to their bark characteristics and use descriptive terms to reflect it. In this classification 'gums' are actually smooth-barked eucalypts.

All eucalypts shed their bark - the question as to why they do does not have a generally agreed answer, but perhaps it makes life harder for bark-burrowing insects. However, gums shed it all at once, leaving a clear smooth surface. At least, that's the theory, but nature being nature the rule is immediately broken. For instance in many gums patches of bark remain on the tree, giving darker or differently-coloured patches.

Yet other gums retain the shed bark, still attached to the branches.

Most eucalypts however retain their dead bark, which comes off in bits over time, giving a rough surface to trunk and often branches too. We classify these rough-barks too according to the surface type. Boxes - nothing to do with containers, but named for the European Buxus, which also produces hard wood used for instance in making mallet heads - have bark which tends to be in plates or narrow blocks. 

Peppermints have short-fibred, crumbly bark; they also have chemicals called pipiterones in the foliage which have a strong peppermint aroma and have been used to make menthol. Eucalypt - including peppermint - oil extraction has been a major industry in Australia; in South Africa it still is!

By contrast, some eucalypts have very long-fibred bark, which when dead can be pulled off in long strips.

Ironbarks can look superficially similar, but the bark is deeply fissured, generally impregnated with tannin-bearing kino, beads of which can be seen in the bark, and which help to make the bark literally iron-hard; you can easily bounce an axe off it.

Bloodwoods have tessellated bark, often quite soft, which 'bleed' non-viscous kino from damage to the trunk. Some mammals, especially Sugar Gliders Petaurus brevipes, exploit this by chewing the bark and harvesting the flow.

As I suggested above, it's never straight-forward; members of the ash group (again named for the resemblance of the timber, this time to Northern Hemisphere Fraxinus) have a rough stocking on the lower trunk, but bare upper limbs.

Such classifications are ultimately just human constructs but when dealing with so many species, any help is worth trying! Anyway, any excuse for talking about the wonderful eucalypts is worth seizing on.