A simple start to learning these complex organisms we call ‘trees’, is knowing and understanding the functions of a tree’s 5 main wood layers. These layers are called The Outer Bark, The Inner Bark, Cambium, Sapwood and Heartwood.
The outer bark
Mostly dead tissue, the outer bark protects the tree and other wood layers from outside elements. It is the tree’s insulation from cold and heat. It keeps moisture out from the rain and as well keeps water in so the tree does not dry out. Even on the same species of tree, the outer bark will vary in shape and colour depending on the environment the tree is located in.
The inner bark
Also called the phloem, it is the inner soft bark under the outer bark. Only living for a short time, the inner bark transfers food to the rest of the tree. Eventually it dies and becomes part of the outer bark.
Cambium
The cambium is a growth layer of the tree making new cells during the growing season. Food and hormones called “auxins” are passed down from the leaves and through the phloem. This stimulates growth in cells and an annual production of new bark and new wood.
Sapwood
Sapwood is new wood that moves water and minerals up to the leaves. As newer rings of sapwood are laid down, inner cells turn to heartwood. It is easy to decipher the sapwood from the heartwood by its colour. Sapwood is usually white to light yellow and the heartwood is light to dark brown.
Heartwood
The heartwood takes up the largest area of the trunk. Unlike sapwood, heartwood is hard and durable, providing the tree with structural strength. It is composed of cellulose fibers that are bound together by a chemical called “lingin”. The heartwood is dead, but it will not decay or lose strength as long as the outer layers are intact. It is easily recognizable, as it is darker in colour from cells containing tannins and other substances.
Next time you see a wood pile or a tree stump, take a look and see if you can identify each layer beneath the bark!