Bark

The outer part of woody stems and branches is comprised of bark and typically comprises 9 to 15 percent of a log's volume (Chang 1954). There are large differences in the types and amounts of various chemical components that comprise bark, even within a single species which depends on growing conditions, age, and various disturbances (Harkin and Rowe 1971). The volume of bark and coarse materials that are available annually in the United States approaches 83 million dry tonnes. Historically, this bark has been used to produce tannins, dyes, resins, flavorings, and medicinal products. As utilization techniques have improved, more and more chemical extracts have become commercially available for utilization. However, bark can be difficult to utilize due to soil contamination during harvesting (Harkin and Rowe 1971).

Bark Pile

The most common use of bark is in mulching or soil amendment (Mater 1969). When worked directly into the soil, bark’s decomposition rate is considerably slower than wood, thus lasting longer; its lower consumption of nitrogen also results in less nitrogen-availability related stress affecting planted vegetation (Allison 1965).

Bark has also been commonly used as a fuel source. On average, 9 tonnes of completely dry bark is the equivalent of nearly 6 tonnes of coal with an effective heating value of 2.34 to 2.93 kWh per pound of bone dry material (Harkin and Rowe 1971). However, these values dynamically decrease as percent moisture content rises. Bark can be processed to create a higher value fuel by manufacturing briquettes but still is limited due to its higher moisture values (Dingwall 1969).

Discarded bark has found an outlet in building materials such as fiber and particleboard (Murphey 1969) and because bark conducts heat less readily than wood, it is a component of insulation board (Martin 1970).

The chemical utilization of bark is still in its infancy, mainly due to the economic expense of transportation, storage, and volume of the material. Very few pure organic compounds have been isolated on a grand scale, such as Salicin from aspen bark but, with advanced fractionation techniques emerging, more compounds are on the horizon (Pearl 1968). More crude fractions such as tannins have been used in larger quantities in the fields of tanning leather, development of drilling muds, adhesives for plywood and particleboard, and taxol for research into cancer treatments (Hathway 1962, Hergert et.al 1965, MacLean and Gardner 1952, Minore and Weatherly 1996).