We tend to compare tree bark to our skin, and in many ways they are very similar. Bark and skin are both made of several layers, with the toughest being on the outside. Young bark, like an infant is softer than old bark. And like skin, bark’s main purpose is to protect the more valuable innards.
A tree’s outer bark, sometimes called cork is the layer of wood whose purpose is to protect the tree from the outside world. The outer bark protects the inner bark from moisture, drought, wind and sun. In winter it insulates from the cold and in summer offers protection from heat.
The inner bark which scientists call phloem is vital to the growth of trees. Think of phloem as pipelines that carry food from the roots to the rest of the tree. Like skin, if you scrape or bruise your outer skin it will quickly heal with little or no consequences, but if you gouge beneath skin or bark, you will cause damage that might require attention. A tree’s inner bark is short lived and soon becomes part of the outer bark.
The cambium is a thin layer beneath the inner bark, and is most responsible for the tree’s growth. Activity in the cambium layer is rampant, particularly in spring, as the tree creates new cells to replenish the inner bark.
Sapwood is the fourth layer and acts as tree’s pipelines for transporting water to the leaves. Sapwood is always moist, usually yellow in colour, and is not considered part of the bark.
The central part of a tree is heartwood. Its purpose is less significant, but helps keep the tree upright. Heartwood is essentially dead but won’t rot because it receives protection from the outer layers.
Bark has a clever ability to adapt to its immediate environment. Some bark can protect trees from the heat of forest fires. The bark of a Sugar Maple for example will have tough bark when living on its own in the sunshine but will have softer bark if planted in the shade of a forest.
Bark can have a wide array of characteristics with varying textures, forms and colours. Sugar Maple bark is usually grey and smooth, while bark of Norway Maple is black and heavily fissured. Bark of a Shagbark Hickory is distinctive because of its dangling loose pieces sometimes more than a metre long.
Take a walk through Canatara Park and you will see that some Oaks (White Oak) have scaly mottled bark that is easy to flake off, while other Oaks (Red Oak) are firm and heavily furrowed. A Cork tree has rather normal looking bark but is very soft and pliable like cork.
Birch trees are highly prized for their ornamental value. Birch bark contains strong oils that preserve bark much longer than Birch’s heartwood. Paperbark Maple has unique burgundy coloured bark with layers that can be peeled off like an onion skin. Sycamore has bark that easily falls off exposing smooth thin bark underneath. Aspen has smooth grey bark for its first decade or so of life, becoming very rough with age.