Barking up the right tree

Bark worse than it's bite?

Hansel McDonald (he's so hot right now) famously said "I wasn't like every other kid, you know, who dreams of being an astronaut, I was more interested in what bark was made of on a tree." As moronic as this statement may sound it isn't too outlandish, for a child that is, not a model/actor. Come on, as a kid you probably had some wonderfully silly ideas about how the world worked or burning questions that just had to be answered - right?

We're not talking about the sound a dog makes, we are looking at the out most layer of stems, trunks and roots of plants. Although I imagine many of us rarely think about what bark is, it is certainly very important to the trees, shrubs and other woody plants it appears on.

Simply put, bark is the skin of woody plants. Much like our own skin, it is a defensive layer, protecting the important parts underneath the surface. The term bark applies to several layers of external plant tissue. Bark is really a non-technical term (but the most referred to) for the collective layers of tissue know as the periderm - see 'bark' is much easier to remember.

Like an onion, plants have many layers.

Everything outside the cambium layer (see above) can be considered bark. The cambium layer is extremely important to the survival of plants as it houses two crucial systems:

*The Xylem - which conducts water and dissolved nutrients from the roots to the leaves and also aid the development of woody stems; and

* The Phloem - which transports converted energy down to be stored in the roots.

In general bark does not photosynthesise, which is why most bark colour are shades of grey, brown, reds and oranges and rarely green. Although, as new green shoots develop, before stems become woody, many plants have the ability to photosynthesise in these early stages.

Bark has evolved to protect its plant through centuries of climatic and geographical conditions (and associated changes). For example, in fire prone areas, trees and shrubs have developed thicker bark to insulate from flames or shinier bark to reflect heat away from the important tissue underneath.

New life via Epicormic growth of a Eucalyptus.

Of course, as we know in Australia, there is no fool proof way to beat fires but many of our local species have developed methods to lessen the impact fires have.

Even in the most intense fires, where Gum trees (Eucalyptus sp.) are stripped of their foliage, waiting underneath for the danger to pass and conditions to improve, are nodes where stems will emerge. This is know as epicormic growth and it can activate quickly, covering the trunk in fresh new leaves ready to start photosynthesising.

Trees also have other defence mechanisms against those creepy, crawly inspects that chew and suck. The epidermis or trunk layer in many trees contain chemicals or naturally occurring insecticides that deter pest attack. It is very important that trees can safeguard from these initial attacks because secondary infections brought on by bacteria and fungus can cause a much more serious outcome for the species.

In cases where the trunk is penetrated by insect (or animal) attack the tree floods the area with moisture or sap to flush out any nasties and to seal the wound - these secondary attacks can happen very quickly and the tree is smart enough to identify this and divert precious energy to preserving it's health.

Other defences are a lot more obvious. Some species develop spikes or spines on their trunk which makes animals and birds work a bit harder to get to the trunk beneath or to climb into the luscious canopy above.

Ceiba pentandra - Kapok Trees are native to
Mexico, Central America and the Caribbean.

What a prick! The spikes of the Honey Locust or
Gleditsia tricanthos can reach up to 15cm long.

Every Pachira quinata has it's thorn..... 

But not all trunks are obnoxious, there are some very ornamental bark out there, some rough, some smooth, some colourful and some shiny, here are some of my favourites:

Mr Scribble - Eucalyptus haemastoma.
The zig-zag scribbles are made my the
larvae of the Scribbly Gum Moth

Crepe's anyone? The mottled bark of the
Lagerstroemia indica can take a few years
to "show off" but is worth the wait.

Angophora costata goes by many common names
Rose Apple & Sydney Red Gum to name a few
but it's bark has colour and character in spades.

Betula utilis 'Jacquemontii' has a lovely
flaking paper bark with age - colour and
contrast even without leaves.

Eucalyptus deglupta or Rainbow Gum for
obvious reasons - the only Eucalyptus sp. with
a natural range extending to the Northern Hemisphere.

The Aussie native Broad-leaved Paperbark,
Melaleuca quinquinervia was a very important tree
species to the Aboriginal people.

Generally most trunks are divided into smooth or rough bark types, an example of a smooth bark would be the beautiful Tibetan Cherry Tree (Prunus serrula) on the left

And a good example of a rough park would be the Eucalyptus crebra or Narrow Leaf Iron Bark on the right, which you'll commonly find in the Sydney basin & Cumberland Plain:


Each type has developed for specific reasons, evolving to handle the climatic conditions better. Though each also has their pitfalls. For instance, the smooth bark trunks have the ability to reflect heat but are much more susceptible to sunburn in a container grown situation (by changing the aspect they grow - this is generally a problem for all deciduous trees). In the event of lighting, smooth bark trees have a better chance of being unscathed, the lightning charge has a more direct path from the tree to the earth.

'Slip, slop, slap' it is important to plant trees facing
the same way they are grown in the field.

Rough barked trees general developed furrowed bark that helps direct and dissipate rain as it runs down the tree, to the root zone below. The downside of rough barked trees is that they generally come off second best in the event of a lightening strike, as the path of the electrical charge is disrupted and the lightening, like Zorro, leaves it's mark:

Lightening strikes can be devastating to trees (and surrounding property), worst case killing the tree or opening them up for secondary attacks from inspects or pathogens. Best case the tree gets a permanent reminder of the event but who knows, female trees may dig scars.

Trunks of trees also play host to much life, with many other organism's having symbiotic relationships with the tree, Moss & Lichen are the most common forms of organisms you'll find on trees, especially those with rough bark. But these relationships are for the long term, as it can take decades for moss and lichen to develop on trees.

A rolling tree gathers no moss?

Studies have discovered moss occurring on old trees trap enough water for blue-green algae to colonise. The algae in turn captures nitrogen, some of this nitrogen is released (via rain and run-off) to be used by the tree.

So I hope you've discovered there's a bit more about bark, remember don't judge a book by it's cover! We know, like our skin, bark protects the important bits and pieces below the surface, so let's help protect this important layer.

Reducing mechanical wounds from such activities as mowing & pruning. Keeping equipment sharp and clean, not piling up mulch or lawn clippings around the base of the trunk are all things we can do to help out trees happy and in turn they'll help us keep cool.