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- U.S. Department of Agriculture - Forest Service - Structure and Function of Wood (Nov. 13, 2024)
Bark constitutes, on the average, about 10 percent of the volume of a tree, but the figure varies depending on tree species and age. Although inner and outer bark can be distinguished by eye, growth rings are not macroscopically distinct. The cellular composition of bark has certain similarities to that of wood but also important differences. Nutrient-conducting cells—sieve cells and sieve tube members (which in wood correspond to the axial tracheids of softwoods and the vessel members of hardwoods, respectively)—are thin-walled and nonlignified, serving usually only one season of growth. Fibres and parenchyma cells also are contained in bark, as well as some specialized cells, and the bark of some softwood species possesses resin canals. Annual growth increments are very thin, and cells are deformed a short distance from the cambium, because of the pressure exercised by the growing wood. Consequently, growth rings are generally difficult to distinguish even under a microscope. Content of cellulose and hemicelluloses is lower in bark than in wood, and the properties of lignin are different. Differences also exist in physical and mechanical properties—density, hygroscopicity, dimensional stability (shrinkage and swelling), and others. Like wood, bark is anisotropic with regard to dimensional stability and strength. Thermal properties and heating value are similar to those of wood.
Bark can be used in products such as particleboard and fibreboard, and small amounts are tolerated in semichemical and kraft pulping processes. Bark also has utility as a source of adhesives and other chemicals (e.g., tannins, dyes, and waxes), in environmental protection applications (e.g., absorption of spilled oil, industrial wastes, and sewage effluents), and as a fuel. Nevertheless, differences in the structure and chemical composition of different barks, difficulties of separating bark from wood when in the form of particles, and differences in the properties of inner and outer bark have impeded fuller utilization.
Cork is also bark—the outer bark of a western Mediterranean oak, Quercus suber. It is stripped without adverse effects to the trees and grows again; it can be collected every 8–10 years. Aside from its use as bottle stoppers, cork is incorporated in life preservers and fishing equipment. As a reconstituted material (after chipping, grinding, and gluing), it finds uses as thermal and sound insulation, gaskets, flooring, bulletin boards, and various other products.
George Thomas Tsoumis