Tuesday, January 27, 2009
This week it is the letter 'B' for ABC Wednesday so I am going with B is for Bark. I am not talking about the noise that a dog makes but rather the covering on a tree. I have always loved the huge variety of patterns and textures that bark comes in so I figured why not?
Bark serves as a waterproof, defensive layer for the tree, helps prevent loss of water from the tree by evaporation, acts as a barrier against attacks by insects and diseases, insulates the tree from drastic temperature changes, and in some instances, protects the tree from fire damage. It also serves as a shield to protect a very important part of the tree—the cambium layer.
The cambium (a watery layer only a few cells thick) is the generative layer, giving rise to both xylem and phloem. Xylem carries water and minerals from the roots to the leaves. Phloem carries manufactured food (sugars) from the leaves to the roots.
Bark also rids the tree of wastes by absorbing and locking them into its dead cells and resins.
Thickness of the bark varies with the different tree species. In some, the protective armor is quite thin, but in others it can be very thick. For example, the bark on a few of the giant sequoia trees in the Sierra Nevada Mountains is two feet thick.
As the growing tree expands by adding a new sapwood layer, its protective armor becomes too tight, and the outer bark splits and cracks. Definite bark patterns are produced by each species as a result of this cracking, and these patterns are so distinct that some trees can be identified by them. The outer bark layers also are shed in many different ways. Birth bark peels off in paper-thin strips; sycamore bark flakes off in large, thin, brittle plates; cedar bark peels off in long, fibrous strips; and the cinnamon-red plates of the ponderosa pine flake off in bits like jigsaw puzzle pieces.
So there is plenty going on under this tranquil exterior! Because bark does serve so many purposes, there are very few trees that can survive being ring-barked. Ring-barking is when a horizontal strip of bark is removed all the way around the circumference of the tree either by animals or man, thus essentially cutting of its 'circulation'. The tree will then slowly die.
There is one big exception to this. The mighty Baobab Adonsonia digitata. The debate continues to rumble as to whether this can truly be called a tree or rather the worlds biggest succulent. The bark on the lower part of the trunk often bears scars caused by local people who harvest and pound it to retrieve the strong fibre.
Although the bark is often heavily stripped by people and elephants, these trees do not suffer as a normal tree would from ring-barking. Baobabs have the ability to simply continue growing and produce a new layer of bark. The wood of the baobab is soft, light yellow and spongy.
You may have worked out by now, I love trees, their shapes and structures and their shade but I have to confess, although I would be able to tell you the names of many trees in the area of Africa where I used to live, I have been very lazy about learning North American trees, so please don't ask me to identify all the trees I have photographed above!
To see how B inspires other people go to ABC Wednesday.
All Bark Pictures - CJT
Baobab - Robin Pope Safaris
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