Bark is a non-technical term for the layers of tissue outside the vascular cambium of a tree, woody vine or shrub.  The purpose of bark is to protect the tree against sun damage, fire, invertebrates, bacteria and fungi. Bark is made up of two components – the inner bark (living tissue composed of the innermost periderm) and the outer bark (dead tissue on the surface and the outer side of the periderm), also known as rhytidome. The rhytidome is what most people associate with bark, as it is the outer layer that covers the trees and can peel away easily. This outermost layer dies each year and will shed off the trunk in some species, but stay behind for others. This all depends on how the bark of each species responds to the pressure created from the wood on the inside growing and expanding against the outer layers.

There are several types of bark that can be recognised in Eucalyptus and closely related groups, which are loosely categorised into smooth bark and rough bark.

Smooth Bark

Species that lose their bark annually are the smooth barks, as the rough bark that sheds leaves behind a new layer of smooth living bark. The bark that is sheds comes off in slabs, ribbons, or small flakes.

A curious smooth bark is the minni ritchi, which is the reason I began looking into bark types! Minni ritchi is a bark that is reddish brown and peels in small curly flakes or fine strips, reminiscent of curly hair. The smooth outer bark splits longitudinally and horizontally, which allows the edges to curl outwards to expose new layers of bark underneath. The name likely derives from an Australian Indigenous language (possibly minariji) of unknown origin (Garuwali , on the north-east NT border, has been suggested). A number of species of Acacia and Eucalyptus have minni ritchi bark, of which five have been recorded in central Australia. These include Acacia cyperophylla, Acacia grasbyi, Acacia monticola, Acacia rhodophloia, and Eucalyptus orbifolia. The species that is well known around Alice Springs is the Red Mulga (Acacia cyperophylla), used in many town gardens for its fast-growing nature and ability to survive in many soil types.

Rough Bark

Of the rough bark types, there are a few that are classified based on their presence largely in Gums; however the types can extend to other species as well.

Ironbark is hard, and becomes compacted and furrowed with age, producing longitudinally furrowed and kino (plant gum) impregnated rough bark that exists over the whole trunk. Colour varies from grey to black and red-black. Local examples include Silver-leaved Ironbark (Eucalyptus melanophloia) slightly north-west and Wandi Ironbark (Eucalyptus jensenii) in the top end, however the majority of ironbark species are found on the eastern coast.

Tessellated bark types are those where the bark splits into flakes or tesserae, which are rough and somewhat corky. In some species, the upper trunk is often smooth, with tessellations becoming more apparent at the basal end of the tree trunk. The weathered tessellations of the Desert Bloodwood (Corymbia opaca) are often grey to brown but newly shed flakes leave a yellow to red under layer. While looking somewhat different, the Ghost Gums (Corymbia aparrerinja) have tessellated bark that isn’t rough in appearance but rather smooth.

Stringy (or fibrous) bark encompasses a large group of species that have longitudinally furrowed and fibrous bark that can be pulled off in strings. Some species may appear smooth along the stems and the stringy component found towards the base. While only Eucalyptus tetrodonta can be found in the top end, several other species can be found on the eastern coast.

Box bark has grey, white or brown short-fibered and tessellated or flaky sections. An example is one of the Coolabah species (Eucalyptus microtheca), which can be found locally.

Imperfectly shed ribbons, strips or curls can be found on trees where the bark sheds in either long but coarse ribbons, strips or irregular flakes to expose a smooth surface underneath. This is often confused with loose basal slabs, which is another group altogether (see below).

Loose basal slabs have rough bark, largely on the basal section of the trunk, which accumulates as the bark dies and is held in place for a period of time before shedding. This is an ambiguous bark type, as smooth bark on the upper most section of the tree and rough bark on the lower section causes confusion.

Compacted bark is similar to ironbark and varies from brown to black, but generally covers the basal region of the trunk, while branches remain smooth. They may often be known as blackbutts.

More Bark Beyond the Gums

Bark will differ in structure depending on the species, with many developing longitudinal cracks, others plates, flakes and a few will develop horizontal lenticels (which allow for gas exchange). Colouration of the bark will allow for varying degrees of light reflectance, where dark barks absorb light and creamy white bark will reflect it to protect it from sun damage. Some species will allow thick layers of cork to build-up in order to protect the tree from sun damage and fire, whereas thin bark that peels off regularly has the advantage of preventing the accumulation of lichen and other surface damage in moist environments. Bark can also contain chemicals or exude sap to protect the tree from invertebrates, bacteria and fungi. The differences in bark structure are one of the reasons I find them so amazing and wonderful to photograph!

There is little in the way of a standard identification key to bark types outside of the Eucalyptus genus, which can lead to confusion in describing a species, though the bark of many species can often be used as a visual cue to identification for those with a little experience or good observation skills. Here are a couple of additional identifiable bark types, but you may find more types when you are out exploring. Feel free to share them with us!

Corkwood has thick and heavily furrowed longitudinal cracks. The bark itself is quite light and corky and helps to protect the tree from fire damage. A local example is the Fork-leaved Corkwood (Hakea divaricata).

Paperbark has flaky bark that can be pulled off in thin papery strips or flakes. A local example is the Inland Tea-tree (Melaleuca glomerata) and others in the Melaleuca genus.

References:

Brooker, M.I.H. & Kleinig, D.A. (1990). Field Guide to Eucalypts, Volume 2: South-western and Southern Australia. Inkata Press, Melbourne

Australian National Botanic Garden (2018). Euclid. https://www.anbg.gov.au/cpbr/cd-keys/Euclid

American Forests (2013). The Language of Bark. http://www.americanforests.org/magazine/article/the-language-of-bark/