Fiber volume, hardwoods

In hardwoods, morphological structural elements in longitudinal series comprise the segmented structure termed vessel . Vessels, which are exposed in transverse section, constitute about 10-46% of the stem volume in deciduous hardwoods and are cells of relatively large diameters (50-300 p.m). Vessels have in short the appearance of open vertical tubes within the wood structure because their end walls have partially dissolved. By comparison, the hardwood vessel diameter can be as much a 10 times the diameter of a softwood fiber. 

Figure 5. Cross-sectional and longitudinal surfaces of a ring-porous hardwood. In the cross-sectional view (X) the largest diameter cells are springwood vessels whereas the smaller cells with obvious openings are sum-merwood vessels. Smaller diameter thick-walled fibers constitute most of the remaining volume. Transversely oriented food-storing cells can be seen on the radial surface (arrow). 40X (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. College of Environmental Science and Forestry)...

However, specifically it refers only to those cell types found in hardwoods which meet the above definition. Fibers range in length from 0.7 mm to 3 mm with an average slightly less than 2 mm for domestic species. In diameter, an average of less than 20 ym can be expected. The percentage of the volume of wood occupied by fibers varies considerably. In sweetgum, fibers constitute only... 

Hardwoods contain several cell types, specialized for different functions (Fig. 1-9). The supporting tissue consists mainly of libriform cells, the conducting tissue of vessels with large cavities, and the storage tissue of ray parenchyma cells. In addition, hardwood contains hybrids of the above-mentioned cells which are classified as fiber tracheids. Although the term fiber is frequently used for any kind of wood cells, it more specifically denotes the supporting tissue, including both libriform cells and fiber tracheids. In birch these cells constitute 65 to 70% of the stem volume. 

Hardwoods contain a substantial volume of fiber cells, but the distinguishing feature of angiosperm xylem is the occurrence of vessels. The vessels are seen on the wood cross section as holes or pores... 

Hardwood fibers, because of the presence of vessels, occupy a proportionally smaller volume of wood tissue than softwood fibers... 

The volume ratio of vessels to fibers and fiber wall thickness are two important factors influencing the hardness and density of different hardwood species and the permeability of these woods to liquids and gases. Wood grain is also partly a function of these two parameters. 

In softwoods, basic density is strongly related to the volume proportion of latewood and its average fiber wall thickness. However, hardwood basic density depends not only on fiber wall thickness but also involves the volume ratio of fibers to vessels. Native commercial woods fall mostly in the basic density range of 0.35-0.65 g/cm, although native species can be as low as 0.21 g/cm (corkwood) and as high as 1.04 g/cm (black ironwood) (2). 

The reduced vessel volume of tension wood, together with thickened fiber walls, can lead to a higher than normal basic density. This general situation, coupled with a difference in wood chemistry, could cause a variable response of such tissue to both chemical and physical treatments or to microbial degradation when compared to normal hardwood xylem. 

Stamm and Loughborough (32) first reported that this relationship has been reported (32) to be approximately valid for woods of the continental United States. The mean value of the ratio Shf/G was 27 for 107 hardwood species and 26 for 52 softwood species of the United States. These ratios should be equivalent to the fiber-saturation point Mf if the green volume specific gravity Gg is taken to be... 

Woody biomass, either gymnosperms (softwoods) or angiosperms (hardwoods) is inherently anisotropic and hygroscopic it is porous material, with the porosity caused by the hollow fibers that make up the woody material. Any number of wood technology texts (see, for example, Haygreen and Bowyer [13]) describe the physical structure of woody biomass. The porosity or void volume of wood exists as macropores—tracheids, rays, resin canals and related structures useful in moving moisture and nutrients within the tree. Chemically, woody biomass is comprised of cellulose, the hemicelluloses, one or another type of lignin, and extractives such as pinoresinol, catechin, pinosylvin, and related compounds. 

---