Tree bark is a vital protective layer to the inner tree, preventing moisture loss and acting as a barrier to pathogens, insects, and fungi. When most types of trees' bark are removed, even when only a thin ring of bark around the trunk, the tree will die. Tree bark also helps to insulate the tree against extreme temperatures. In most trees, the innermost bark also helps to move nutrients throughout the tree before becoming a part of the outer layer of the bark, thickening the protective barrier.
Cork bark is one of a few types of trees with a different bark structure. The bark on a cork oak tree comprises three separate and distinct layers, allowing the two outermost layers to be harvested while leaving the inner layer intact. Cork bark also has two unique growing phases for the two outer layers of the bark. The first phase occurs from when the tree is planted until after the second harvest when the tree is between 34 and 38 years old. This virgin phase features a rough and knotted outer layer of bark and a thinner and inconsistent middle layer.
After the initial phase, the bark enters its mature phases. Mature cork bark is often called corkwood, but it should be noted that this is still the outer two layers of bark, and the tree is not cut during harvesting. Once a cork tree reaches its mature phase, the bark can be harvested every 7 to 9 years.
The innermost layer of the cork oak tree's bark is responsible for moving nutrients and provides some protection from insects, fungi, and pathogens. This inner layer also regrows the bark's middle and outer layers after the harvest. The middle layer of cork bark truly makes it stand out from other tree barks. This distinctive layer looks incredibly different to the human eye, but you can see the differences at a cellular level under a microscope. Cork has a honeycomb cellular shape, making it more flexible than other types of wood. This middle layer is what most cork products are made from, including wine stoppers, flooring, and cork fabric. The outermost layer of the bark is what is exposed on the tree. This layer looks similar to other oak tree bark. This layer is rougher and darker in color.
Comparisons are often made between cork bark and birch bark, but there are a few key differences. Birch is still only a two-layered bark. The outer bark of the birch tree will peel and eventually fall off once the tree no longer needs the bark. This is quite different from cork bark, which must be harvested but provides a higher, thicker yield.
A frequent question we hear about cork is, is cork a hardwood or a softwood? Softwoods are fast-growing trees like pine, cedars, and spruces. Because they grow so quickly, the wood is softer and less durable. Hardwoods are slower-growing trees such as oak, maple, and walnut with a complex cellular structure. Cork is actually neither! While cork is a wood product, it is the bark of the tree rather than the wood itself!
Another unique property of cork bark is the high concentration of suberin. Suberin is a complex fatty acid that helps prevent water loss, and with self-healing properties, that is the reason cork boards will "heal" after a pushpin is pressed into the bulletin board! Suberin is frequently found in plant roots, but the high concentration found in the harvestable bark of the cork oak tree is unusual for tree bark. Suberin also acts as a moisture barrier and is one of the main reasons cork seals bottles so successfully!
Cork is frequently compared to bamboo as they have similar uses and are both renewable materials. Bamboo products are made from the cut stalks of bamboo, meaning the bamboo is felled to make the products where the cork tree continues to thrive while it regrows the bark and continues to remove CO2 from the atmosphere. In some instances, bamboo is more durable. However, it is also harder, making it less comfortable to stand on than cork when used in flooring or footwear. Cork can also be used in a wider variety of applications, including insulation, closures, and industrial applications.